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Exercise preserves neuromuscular function in aging through unknown mechanisms. Skeletal muscle fibroblasts (FIB) and stem cells (MuSC) are abundant in skeletal muscle and reside close to neuromuscular junctions, but their relative roles in motor neuron maintenance remain undescribed. Using direct cocultures of embryonic rat motor neurons with either human MuSC or FIB, RNA sequencing revealed profound differential regulation of the motor neuron transcriptome, with FIB generally favoring neuron growth and cell migration and MuSC favoring production of ribosomes and translational machinery. Conditioned medium from FIB was superior to MuSC in preserving motor neurons and increasing their maturity. Lastly, we established the importance of donor age and exercise status and found an age-related distortion of motor neuron and muscle cell interaction that was fully mitigated by lifelong physical activity. In conclusion, we show that human muscle FIB and MuSC synergistically stimulate the growth and viability of motor neurons, which is further amplified by regular exercise., (© 2024 The Author(s). Aging Cell published by Anatomical Society and John Wiley & Sons Ltd.)

To investigate the effect of moderate or heavy slow resistance training on structural and mechanical properties of patellar and Achilles tendons in older men, in vivo. Healthy older men (n = 27) undertook a 12-week resistance training program (3 times/week) of triceps surae and quadriceps muscle-tendon complexes. Participants were randomly assigned to either a moderate load of 55% 1RM (O55, n = 13, age: 70.0 ± 4.6 years) or a progressively heavier load from 55% up to 80% 1RM (O80, n = 14, age: 69.8 ± 4.4 years). Tendon morphology was investigated using a 1.5 T MRI scanner. Mechanical properties of the free Achilles tendon (AT) and medial gastrocnemius (MG) aponeurosis, patellar tendon (PT), and deep aponeurosis of the vastus lateralis (VL) muscle were assessed using ultrasound imaging. Both training programs led to an increase in mean CSA of the PT (O55; +2.7%, O80; +1.7%, p = 0.002) and AT (O55; +2.8%, O80; +2.5%, p < 0.001). Force and stress were increased for all the four regions investigated, while stiffness and Young's modulus assessed at a common force level were only increased for the AT and VL aponeurosis. No interaction between group and time was demonstrated for any of the outcomes. No interaction between time and the four tendon sites was observed for any of the outcomes at common force. A supervised short-term heavy or moderate slow resistance training of equal volume yielded an increase in tendon size and altered mechanical properties of tendinous tissue., (© 2024 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Background: The long-term recovery (in years) of patellar tendinopathy treated with loading-based rehabilitation remains largely unknown., Purpose: To examine the clinical outcome and tendon structure years after exercise-based treatment of chronic patellar tendinopathy., Study Design: Case series; Level of evidence, 4., Methods: This was a 3- to 4-year follow-up evaluation of participants (N = 28) from a previous randomized clinical trial by the author group. All participants received loading-based rehabilitation for 12 weeks with either moderate-slow resistance (55% of 1 repetition maximum) or heavy-slow resistance (up to 90% of 1 repetition maximum). Both groups showed similar improvements after 3 and 12 months and were therefore collapsed in the present analysis. Function and symptoms (the Victorian Institute of Sport Assessment-Patella [VISA-P] questionnaire), tendon pain (numeric rating scale [NRS] during activity and during a single-leg decline squat test), and tendon structure (tendon vascularization and thickness on ultrasound) were assessed., Results: The mean follow-up was 3.6 ± 0.4 years after the baseline assessment in the original clinical trial. The VISA-P score was 83.9 ± 11.9 (95% CI, 79.3-88.5) at the latest follow-up and did not differ from the 1-year follow-up score ( P = .54). Similarly, NRS score during preferred sport (1.6 ± 1.7; 95% CI, 0.9-2.2) and single-leg decline squat (1.0 ± 1.8; 95% CI, 0.3-1.7) did not differ from the 1-year values and remained elevated. Power Doppler area and tendon thickness decreased significantly from 1 year to latest follow-up ( P < .0001 and P = .02, respectively), but power Doppler area >1 mm 2 was still present in 43% of the participants after 3 to 4 years, and the tendon thickness was still mildly elevated (6.4 ± 1.8 mm; 95% CI, 5.7-7.1 mm). Sports participation after 3 to 4 years (3.9 ± 2.7 h/wk; 95% CI, 2.9-7.1 h/wk) was significantly lower compared with preinjury levels ( P < .0001)., Conclusion: Clinical symptoms remained even years after loading-based treatment for patellar tendinopathy, whereas some but not all tendon structures normalized in this longer term follow-up., Competing Interests: The authors have declared that there are no conflicts of interest in the authorship and publication of this contribution. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto., (© The Author(s) 2024.)

Knowledge of how to treat chronic tendinopathy has advanced in recent years, but the treatment of early tendinopathy is not well understood. The main purpose of this prospective observational study was to investigate if changes occur in clinical and imaging outcomes over 12 weeks in elite athletes with recent debut of tendinopathy. Sixty-five elite adult athletes (24 ± 5 years) with early Achilles or patellar tendinopathy (symptoms < 3 months) were examined at baseline and after 12 weeks. Patients were divided into groups based on the duration of symptoms at the time of inclusion: (T1): 0-1 month, (T2): 1-2 months, or (T3): 2-3 months. Pain-guided activity modification was the only intervention. We assessed the following clinical outcomes: Questionnaires (Victorian Institute of Sports Assessment (VISA)) and pain scores (0-10 numeric rating scale, NRS), structural outcomes from ultrasonography: Thickness, echogenicity, and Doppler flow, and from magnetic resonance imaging: Cross-sectional area (CSA), thickness and length. Tendinopathic Achilles and patellar tendons displayed no significant differences on imaging tendon structural outcomes between T1 (n = 19), T2 (n = 23), and T3 (n = 20) at baseline or after 12 weeks, with one exception: Patellar tendons in T1 were thicker than T2 and T3 at baseline. Although athletes improved clinically on VISA and most NRS scores after 12 weeks, affected tendons with greater thickness, CSA and Doppler flow than contralateral tendons at baseline remained unchanged after 12 weeks. In conclusion, these data suggest that early tendinopathy in elite athletes can improve clinically after 12 weeks while morphology remains unchanged., (© 2024 The Author(s). Scandinavian Journal of Medicine & Science In Sports published by John Wiley & Sons Ltd.)

The aim of this study was to investigate the effect of aging and resistance training with a moderate load on the size and mechanical properties of the patellar (PT) and Achilles tendon (AT) and their associated aponeuroses; medial gastrocnemius (MG) and vastus lateralis (VL). Young (Y55; 24.8 ± 3.8 yrs, n = 11) and old men (O55; 70.0 ± 4.6 yrs, n = 13) were assigned to undergo a training program (12 weeks; 3 times/week) of moderate slow resistance training [55% of one repetition maximum (RM)] of the triceps surae and quadriceps muscles. Tendon dimensions were assessed using 1.5 T magnetic resonance imaging before and after 12 weeks. AT and PT cross sectional area (CSA) were determined every 10% of tendon length. Mechanical properties of the free AT, MG aponeurosis, PT, and VL aponeurosis were assessed using ultrasonography (deformation) and tendon force measurements. CSA of the AT but not PT was greater in O55 compared with Y55. At baseline, mechanical properties were generally lower in O55 than Y55 for AT, MG aponeurosis and VL aponeurosis (Young's modulus) but not for PT. CSA of the AT and PT increased equally in both groups following training. Further, for a given force, stiffness and Young's modulus also increased equally for VL aponeurosis and AT, for boths groups. The present study highlights that except for the PT, older men have lower tendon (AT, MG aponeurosis, and VL aponeurosis) mechanical properties than young men and 12-weeks of moderate slow resistance training appears sufficient to improve tendon size and mechanical adaptations in both young and older men. New and Noteworthy: These novel findings suggest that short-term moderate slow resistance training induces equal improvements in tendon size and mechanics regardless of age., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Prematurity has physical consequences, such as lower birth weight, decreased muscle mass and increased risk of adult-onset metabolic disease. Insulin-like growth factor 1 (IGF-1) has therapeutic potential to improve the growth and quality of muscle and tendon in premature births, and thus attenuate some of these sequalae. We investigated the effect of IGF-1 on extensor carpi radialis muscle and biceps brachii tendon of preterm piglets. The preterm group consisted of 19-day-old preterm (10 days early) piglets, treated with either IGF-1 or vehicle. Term controls consisted of groups of 9-day-old piglets (D9) and 19-day-old piglets (D19). Muscle samples were analysed by immunofluorescence to determine the cross-sectional area (CSA) of muscle fibres, fibre type composition, satellite cell content and central nuclei-containing fibres in the muscle. Tendon samples were analysed for CSA, collagen content and maturation, and vascularization. Gene expression of the tendon was measured by RT-qPCR. Across all endpoints, we found no significant effect of IGF-1 treatment on preterm piglets. Preterm piglets had smaller muscle fibre CSA compared to D9 and D19 control group. Satellite cell content was similar across all groups. For tendon, we found an effect of age on tendon CSA, and mRNA levels of COL1A1, tenomodulin and scleraxis. Immunoreactivity for elastin and CD31, and several markers of tendon maturation, were increased in D9 compared to the preterm piglets. Collagen content was similar across groups. IGF-1 treatment of preterm-born piglets does not influence the growth and maturation of skeletal muscle and tendon., (© 2024 The Authors. Experimental Physiology published by John Wiley & Sons Ltd on behalf of The Physiological Society.)

Background: It is necessary to better understand the structural characteristics of the supraspinatus tendon and associated muscle after rotator cuff repair and in the event of retear., Purpose: To study structural differences between the repaired and contralateral shoulders 1 year after rotator cuff repair in patients who received either progressive exercise therapy (PR) or usual care (UC) in a randomized controlled trial and to investigate whether there was interaction with tendon retear and limb dominance., Study Design: Cohort study; Level of evidence, 2., Methods: Patients with surgically repaired traumatic full-thickness rotator cuff tears involving the supraspinatus tendon were included. After surgery, they were randomized to PR or UC (active from postoperative week 2 or 6, respectively). The subacromial structures (acromiohumeral distance, supraspinatus tendon thickness, and vascularity) and the supraspinatus muscle thickness were examined with ultrasound at the 1-year follow-up., Results: A total of 79 patients were included. The characteristics of the 2 intervention groups (PR and UC) were comparable, including the Western Ontario Rotator Cuff Index score and number of retears. The authors found significantly thinner supraspinatus tendon (PR, P < .001; UC, P = .003) and reduced acromiohumeral distance (PR, P = .023; UC, P = .025) in the repaired versus the contralateral shoulders in both intervention groups. For neovascularization, there was no interlimb difference in either of the groups or between groups (PR vs UC). In patients with intact tendons, there was no interlimb difference in the muscle thickness, but in patients with tendon retear the muscle was significantly thinner on the repaired side ( P = .024 and P < .001, respectively). When the dominant supraspinatus tendon was repaired (both groups), it was significantly thinner than the nondominant healthy tendon, but this difference was not seen when the nondominant supraspinatus tendon was repaired ( P = .006)., Conclusion: One year after rotator cuff surgery, the repaired supraspinatus tendon was significantly thinner and the corresponding acromiohumeral distance was reduced. In patients with retear, the supraspinatus muscle was significantly thinner on the repaired side and early initiation of tendon-loading exercises did not affect these findings., Registration: NCT02969135 (ClinicalTrials.gov identifier)., Competing Interests: The authors declared that they have no conflicts of interest in the authorship and publication of this contribution. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

Background: Plantar fasciitis is a painful tendinous condition (tendinopathy) with a high prevalence in athletes. While a healthy tendon has limited blood flow, ultrasound has indicated elevated blood flow in tendinopathy, but it is unknown if this is related to a de facto increase in the tendon vasculature. Likewise, an accumulation of glycosaminoglycans (GAGs) is observed in tendinopathy, but its relationship to clinical pain is unknown., Purpose: To explore to what extent vascularization, inflammation, and fat infiltration were present in patients with plantar fasciitis and if they were related to clinical symptoms., Study Design: Descriptive laboratory study., Methods: Biopsy specimens from tendinopathic plantar fascia tissue were obtained per-operatively from both the primary site of tendon pain and tissue swelling ("proximal") and a region that appeared macroscopically healthy at 1 to 2 cm away from the primary site ("distal") in 22 patients. Biopsy specimens were examined with immunofluorescence for markers of blood vessels, tissue cell density, fat infiltration, and macrophage level. In addition, pain during the first step in the morning (registered during an earlier study) was correlated with the content of collagen and GAGs in tissue., Results: High vascularization (and cellularity) was present in both the proximal (0.89%) and the distal (0.96%) plantar fascia samples, whereas inconsistent but not significantly different fat infiltration and macrophage levels were observed. The collagen content was similar in the 2 plantar fascia regions, whereas the GAG content was higher in the proximal region (3.2% in proximal and 2.8% in distal; P = .027). The GAG content in the proximal region was positively correlated with the subjective morning pain score in the patients with tendinopathy (n = 17)., Conclusion: In patients with plantar fasciitis, marked tissue vascularization was present in both the painful focal region and a neighboring nonsymptomatic area. In contrast, the accumulation of hydrophilic GAGs was greater in the symptomatic region and was positively correlated with increased clinical pain levels in daily life., Clinical Relevance: The accumulation of GAGs in tissue rather than the extent of vascularization appears to be linked with the clinical degree of pain symptoms of the disease., Competing Interests: The authors declared that they have no conflicts of interest in the authorship and publication of this contribution. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

Purpose: To investigate power Doppler (PD) activity and tendon structure (between the injured and contralateral limb) in patients with unilateral patellar tendinopathy (PT) using ultrasonography (US). Secondly, the aim was to determine the intra-rater reliability of the PD activity and tendon structure., Methods: This study analyzed US baseline data from 57 male participants with symptomatic unilateral PT who had been enrolled in one of two randomized clinical trials. Data were analyzed to examine if systematic differences existed between injured and contralateral limbs using Fiji ImageJ., Results: The PD activity of the symptomatic tendon was larger 25.6 (Q1 = 14.9; Q3 = 41.6) mm 2 than the asymptomatic 0 (Q1 = 0.0; Q3 = 0.0) mm 2 (p < 0.001). There was a significantly greater tendon thickness at the proximal (2.5 mm 95% CI [2.0; 3.0]), mid (0.8 mm 95% CI [0.5; 1.1]), and distal (0.2 mm 95% CI [0.1; 0.4]) part of the tendon for the symptomatic compared to the asymptomatic tendon. Intra-rater reliability for PD activity and tendon structure ranged from moderate-to-excellent (0.74; 0.99)., Conclusion: These results provide mean estimates for tendon thickness of symptomatic and asymptomatic tendons, that can be used for clinicians to reliably estimate pathological tendon thickness., (© 2024 The Authors. Journal of Clinical Ultrasound published by Wiley Periodicals LLC.)

Physical activity can activate extracellular matrix (ECM) protein synthesis and influence the size and mechanical properties of tendon. In this study, we aimed to investigate whether different training histories of horses would influence the synthesis of collagen and other matrix proteins and alter the mechanical properties of tendon. Samples from superficial digital flexor tendon (SDFT) from horses that were either (a) currently race trained (n = 5), (b) previously race trained (n = 5) or (c) untrained (n = 4) were analysed for matrix protein abundance (mass spectrometry), collagen and glycosaminoglycan (GAG) content, ECM gene expression and mechanical properties. It was found that ECM synthesis by tendon fibroblasts in vitro varied depending upon the previous training history. In contrast, fascicle morphology, collagen and GAG content, mechanical properties and ECM gene expression of the tendon did not reveal any significant differences between groups. In conclusion, although we could not identify any direct impact of the physical training history on the mechanical properties or major ECM components of the tendon, it is evident that horse tendon cells are responsive to loading in vivo, and the training background may lead to a modification in the composition of newly synthesised matrix., (© 2024 Anatomical Society.)

Background: Achilles tendon ruptures often result in long-term functional deficits despite accelerated (standard) rehabilitation., Purpose/hypothesis: The purpose of this study was to investigate if delayed loading would influence functional, clinical, and structural outcomes of the muscles and tendon 1 year after a surgical repair. It was hypothesized that delaying the loading would reduce the heel-rise height deficit 1 year after Achilles tendon rupture., Study Design: Randomized controlled trial; Level of evidence, 1., Methods: In total, 48 patients with a surgically repaired Achilles tendon rupture were randomized to 2 groups: the standard group received the currently accepted rehabilitation, and the delayed group received the same rehabilitation except that initial loading was delayed by 6 weeks. The primary outcome was the heel-rise height difference between the injured and uninjured sides at 1 year. The secondary outcomes were (1) tendon length measured with magnetic resonance imaging, (2) muscle fascicle length and pennation angle of the gastrocnemius medialis muscle, (3) Doppler activity measured with ultrasonography, (4) Achilles tendon Total Rupture Score (ATRS), and (5) isometric muscle strength., Results: The mean heel-rise height deficits for the standard and delayed groups were -2.2 cm and -2.1 cm, respectively ( P = .719). The soleus part of the tendon was already elongated 1 week after surgery in both groups without a between-group difference (side-to-side difference: standard, 16.3 mm; delayed, 17.5 mm; P = .997) and did not change over 52 weeks. The gastrocnemius tendon length was unchanged at 1 week but elongated over time without a between-group difference (side-to-side difference at 52 weeks: standard, 10.5 mm; delayed, 13.0 mm; P = .899). The delayed group had less Doppler activity at 12 weeks ( P = .006) and a better ATRS (standard, 60 points; delayed, 72 points; P = .032) at 52 weeks., Conclusion: Delayed loading was not superior to standard loading in reducing the heel-rise height difference at 1 year. The data indirectly suggested reduced inflammation in the initial months and a better patient-reported outcome at 1 year in the delayed group. The soleus part of the tendon was already markedly elongated (35%) 1 week after surgery, while the length of the gastrocnemius tendon was unchanged at 1 week but was 6% elongated at 1 year. Together, these data indirectly suggest that the delayed group fared better, although this finding needs to be confirmed in future investigations., Registration: NCT04263493 (ClinicalTrials.gov identifier)., Competing Interests: One or more of the authors has declared the following potential conflict of interest or source of funding: This work was supported by the Novo Nordisk Foundation (NNF180C0052371), Team Denmark, and Bispebjerg-Frederiksberg Hospital Research Foundation. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

Background: Age-related loss of strength is disproportionally greater than the loss of mass, suggesting maladaptations in the neuro-myo-tendinous system. Myofibers are often misshaped in aged and diseased muscle, but systematic analyses of large sample sets are lacking. Our aim was to investigate myofiber shape in relation to age, exercise, myofiber type, species and sex., Methods: Vastus lateralis muscle biopsies (n = 265) from 197 males and females, covering an age span of 20-97 years, were examined. The gastrocnemius and soleus muscles of 11 + 22-month-old male C57BL/6 mice were also examined. Immunofluorescence and ATPase stainings of muscle cross-sections were used to measure myofiber cross-sectional area (CSA) and perimeter. From these, a shape factor index (SFI) was calculated in a fibre-type-specific manner (type I/II in humans; type I/IIa/IIx/IIb in mice), with higher values indicating increased deformity. Heavy resistance training (RT) was performed three times per week for 3-4 months by a subgroup (n = 59). Correlation analyses were performed comparing SFI and CSA with age, muscle mass, maximal voluntary contraction (MVC), rate of force development and specific force (MVC/muscle mass)., Results: In human muscle, SFI was positively correlated with age for both type I (R 2  = 0.20) and II (R 2  = 0.38) myofibers. When subjects were separated into age cohorts, SFI was lower for type I (4%, P < 0.001) and II (6%, P < 0.001) myofibers in young (20-36) compared with old (60-80) and higher for type I (5%, P < 0.05) and II (14%, P < 0.001) myofibers in the oldest old (>80) compared with old. The increased SFI in old muscle was observed in myofibers of all sizes. Within all three age cohorts, type II myofiber SFI was higher than that for type I myofiber (4-13%, P < 0.001), which was also the case in mice muscles (8-9%, P < 0.001). Across age cohorts, there was no difference between males and females in SFI for either type I (P = 0.496/0.734) or II (P = 0.176/0.585) myofibers. Multiple linear regression revealed that SFI, after adjusting for age and myofiber CSA, has independent explanatory power for 8/10 indices of muscle mass and function. RT reduced SFI of type II myofibers in both young and old (3-4%, P < 0.001)., Conclusions: Here, we identify type I and II myofiber shape in humans as a hallmark of muscle ageing that independently predicts volumetric and functional assessments of muscle health. RT reverts the shape of type II myofibers, suggesting that a lack of myofiber recruitment might lead to myofiber deformity., (© 2023 The Authors. Journal of Cachexia, Sarcopenia and Muscle published by Wiley Periodicals LLC.)

Permanent loss of muscle function seen after an Achilles tendon rupture may partly be explained by tendon elongation and accompanying shortening of the muscle. Muscle fascicle length shortens, serial sarcomere number is reduced, and the sarcomere length is unchanged after Achilles tendon transection (ATT), and these changes are mitigated with suturing. The method involved in this study was a controlled laboratory study. Two groups of rats underwent ATT on one side with a contralateral control (CTRL): A ) ATT with 3 mm removal of the Achilles tendon and no suturing (substantial tendon elongation), and B ) ATT with suture repair (minimal tendon elongation). The operated limb was immobilized for 2 wk to reduce load. Four weeks after surgery the rats were euthanized, and hindlimbs were analyzed for tendon length, gastrocnemius medialis (GM) muscle mass, length, fascicle length, sarcomere number and length. No differences were observed between the groups, and in both groups the Achilles tendon length was longer (15.2%, P < 0.001), GM muscle mass was smaller (17.5%, P < 0.001), and muscle length was shorter (8.2%, P < 0.001) on the ATT compared with CTRL side. GM fascicle length was shorter (11.2%, P < 0.001), and sarcomere number was lower (13.8%, P < 0.001) on the ATT side in all regions. Sarcomere length was greater in the proximal (5.8%, P < 0.001) and mid (4.2%, P = 0.003), but not distal region on the ATT side. In this animal model, regardless of suturing, ATT resulted in tendon elongation, loss of muscle mass and length, and reduced serial sarcomere number, which resulted in an "overshoot" lengthening of the sarcomeres. NEW & NOTEWORTHY Following acute Achilles tendon rupture, patients are often left with functional deficits. The specific reason remains largely unknown. The shortened muscle leads to reduced fascicle length, in turn leading to adaptation by reduced serial sarcomere numbers. Surprisingly, this adaptation appears to "overshoot" and lead to increased sarcomere length. The present animal model advances understanding of how muscle sarcomeres, which are difficult to measure in humans, are affected when undue elongation takes place after tendon rupture.

Both aging and physical activity can influence the amount of intramuscular connective tissue in skeletal muscle, but the impact of these upon specific extracellular matrix (ECM) proteins in skeletal muscle is unknown. We investigated the proteome profile of intramuscular connective tissue by label-free proteomic analysis of cellular protein-depleted extracts from lateral gastrocnemius muscle of old (22-23 mo old) and middle-aged (11 mo old) male mice subjected to three different levels of regular physical activity for 10 wk (high-resistance wheel running, low-resistance wheel running, or sedentary controls). We hypothesized that aging is correlated with an increased amount of connective tissue proteins in skeletal muscle and that regular physical activity can counteract these age-related changes. We found that dominating cellular proteins were diminished in the urea/thiourea extract, which was therefore used for proteomics. Proteomic analysis identified 482 proteins and showed enrichment for ECM proteins. Statistical analysis revealed that the abundances of 86 proteins changed with age. Twenty-three of these differentially abundant proteins were identified as structural ECM proteins (e.g., collagens and laminins) and all of these were significantly more abundant with aging. No significant effect of training or interaction between training and advance in age was found for any proteins. Finally, we found a lower protein concentration in the urea/thiourea extracts from the old mice compared with the middle-aged mice. Our findings indicate that the intramuscular ECM solubility is affected by increased age but is not altered by physical training. NEW & NOTEWORTHY We investigated the impact of aging and exercise on extracellular matrix components of intramuscular connective tissue using proteomics. Middle-aged and old mice were subjected to three different levels of regular physical activity for 10 wk (high-resistance wheel running, low-resistance wheel running, or sedentary controls). We prepared extracts of extracellular matrix proteins depleted of cellular proteins. Our findings indicate that intramuscular connective tissue alters its soluble protein content with age but is unaffected by training.

Skeletal muscle possesses adaptability to mechanical loading and regenerative potential following muscle injury due to muscle stem cell activity. So far, it is known that muscle stem cell activity is supported by the roles of several interstitial cells within skeletal muscle in response to muscle damage. The adjacent tendon is also exposed to repetitive mechanical loading and possesses plasticity like skeletal muscle. However, the interplay between the skeletal muscle and adjacent tendon tissue has not been fully investigated. In this study, we tested whether factors released by three-dimensional engineered human tendon constructs in response to uniaxial tensile loading can stimulate the proliferation and differentiation of human-derived myogenic cells (myoblasts). Tendon constructs were subjected to repetitive mechanical loading (4% strain at 0.5 Hz for 4 h) and nonrepetitive loading (0% strain at 0 Hz for 4 h), and the conditioned media from mechanically loaded and nonmechanically loaded control constructs were applied to myoblasts. Immunofluorescence analysis revealed both an increase of myotube fusion index (≥5 nuclei within one desmin+ myotube) and the myotube diameter when conditioned medium from mechanically loaded tendon constructs was applied. Myostatin, myosin heavy chain 7, and AXIN2 gene expressions were downregulated in myotubes treated with conditioned medium from mechanically loaded tendon constructs. However, proliferative potential (number of Ki67+ and bromodeoxyuridine+ myoblasts) did not differ between the two groups. These results indicate that tendon fibroblasts enhance myotube formation by mechanical loading-induced factors. Our finding suggests that mechanical loading affects the signaling interplay between skeletal muscle and tendon tissue and is thus important for musculoskeletal tissue development and regeneration in humans. Impact statement The interplay between satellite cells and various types of resident cells within the skeletal muscle for muscle regeneration has been extensively studied. However, even though tendon tissue is located adjacent to skeletal muscle tissue and cells in these tissues are exposed to repetitive mechanical loading together, the interaction between muscle and tendon tissues for muscle regeneration remains to be elucidated. In this study, we report that the conditioned media from engineered human tendon tissues undergoing repetitive tensile mechanical loading enhanced myotube formation. Our in vitro findings extend the fundamental understanding of the crosstalk between adjacent tissues of the muscle-tendon unit.

Purpose/aim of the Study: Osteogenesis imperfecta is a heritable bone disorder that is usually caused by mutations in collagen type I encoding genes. The impact of such mutations on tendons, a structure with high collagen type I content, remains largely unexplored. We hypothesized that tendon properties are abnormal in the context of a mutation affecting collagen type I. The main purpose of the study was to assess the anatomical, mechanical, and material tendon properties of Col1a1 Jrt/+ mice, a model of severe dominant OI., Materials and Methods: The Flexor Digitorum Longus (FDL) tendon of Col1a1 Jrt/+ mice and wild-type littermates (WT) was assessed with in vitro mechanical testing., Results: The results showed that width and thickness of FDL tendons were about 40% larger in WT (p < 0.01) than in Col1a1 Jrt/+ mice, whereas the cross-sectional area was 138% larger (p < 0.001) . The stiffness, peak- and yield-force were between 160% and 194% higher in WT vs. Col1a1 Jrt/+ mice. The material properties did not show significant differences between mouse strains with differences <15% between WT and Col1a1 Jrt/+ (p > 0.05) . Analysis of the Achilles tendon collagen showed no difference between mice strains for the content but collagen solubility in acetic acid was 66% higher in WT than in Col1a1 Jrt/+ (p < 0.001)., Conclusions: This study shows that the FDL tendon of Col1a1 Jrt/+ mice has reduced mechanical properties but apparently normal material properties. It remains unclear whether the tendon phenotype of Col1a1 Jrt/+ mice is secondary to muscle weakness or a direct effect of the Col1a1 mutation or a combination of both.

Overuse injury in tendon tissue (tendinopathy) is a frequent and costly musculoskeletal disorder and represents a major clinical problem with unsolved pathogenesis. Studies in mice have demonstrated that circadian clock-controlled genes are vital for protein homeostasis and important in the development of tendinopathy. We performed RNA sequencing, collagen content and ultrastructural analyses on human tendon biopsies obtained 12 h apart in healthy individuals to establish whether human tendon is a peripheral clock tissue and we performed RNA sequencing on patients with chronic tendinopathy to examine the expression of circadian clock genes in tendinopathic tissues. We found time-dependent expression of 280 RNAs including 11 conserved circadian clock genes in healthy tendons and markedly fewer (23) differential RNAs with chronic tendinopathy. Further, the expression of COL1A1 and COL1A2 was reduced at night but was not circadian rhythmic in synchronised human tenocyte cultures. In conclusion, day-to-night changes in gene expression in healthy human patellar tendons indicate a conserved circadian clock as well as the existence of a night reduction in collagen I expression. KEY POINTS: Tendinopathy is a major clinical problem with unsolved pathogenesis. Previous work in mice has shown that a robust circadian rhythm is required for collagen homeostasis in tendons. The use of circadian medicine in the diagnosis and treatment of tendinopathy has been stifled by the lack of studies on human tissue. Here, we establish that the expression of circadian clock genes in human tendons is time dependent, and now we have data to corroborate that circadian output is reduced in diseased tendon tissues. We consider our findings to be of significance in advancing the use of the tendon circadian clock as a therapeutic target or preclinical biomarker for tendinopathy., (© 2023 The Authors. The Journal of Physiology published by John Wiley & Sons Ltd on behalf of The Physiological Society.)

Aging is accompanied by morphological and mechanical changes to the intramuscular connective tissue (IMCT) of skeletal muscles, but whether physical exercise can influence these changes is debated. We investigated the effects of aging and exercise with high or low resistance on composition and mechanical properties of the IMCT, including direct measurements on isolated IMCT which has rarely been reported. Middle-aged (11 months, n = 24) and old (22 months, n = 18) C57BL/6 mice completed either high (HR) or low (LR) resistance voluntary wheel running or were sedentary (SED) for 10 weeks. Passive mechanical properties of the intact soleus and plantaris muscles and the isolated IMCT of the plantaris muscle were measured in vitro. IMCT thickness was measured on picrosirius red stained cross sections of the gastrocnemius and soleus muscle and for the gastrocnemius hydroxyproline content was quantified biochemically and advanced glycation end-products (AGEs) estimated by fluorometry. Mechanical stiffness, IMCT content and total AGEs were all elevated with aging in agreement with previous findings but were largely unaffected by training. Conclusion: IMCT accumulated with aging with a proportional increase in mechanical stiffness, but even the relatively high exercise volume achieved with voluntary wheel-running with or without resistance did not significantly influence these changes., (Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.)

Persistent muscle weakness, tendon elongation, and incomplete return to preinjury level are frequent sequelae after acute Achilles tendon rupture, and evidence-based knowledge of how to best rehabilitate the injury is largely absent in the literature. The objective of this review is to illuminate and discuss to what extent an Achilles tendon rupture affects muscle, tendon, and function when assessed with the Achilles tendon total rupture score (ATRS), muscle strength, muscle cross-sectional area, tendon length, and the heel-rise test. The patient-reported outcome measures (PROM) data in the literature suggest that the recovery takes longer than 6 months (ATRS, 70 out of 100), that one-year postinjury, the ATRS only reaches 82, and that this does not appear to noticeably improve thereafter. Loss of muscle mass, strength, and function can in some cases be permanent. Over the first 6 months postinjury, the tendon undergoes elongation, which appears to be negatively correlated to heel-rise function. More recently, there has been some interest in how muscle length and excursion is related to the reduced function. The available literature indicates that further research is highly warranted and that efforts to restore normal tendon length may improve the likelihood of returning to preinjury level after an Achilles tendon rupture., Competing Interests: The authors declare that they have no conflicts of interest., (Copyright © 2022 Rikke Hoeffner et al.)

Tendons are vital collagen-dense specialized connective tissues transducing the force from skeletal muscle to the bone, thus enabling movement of the human body. Tendon cells adjust matrix turnover in response to physiological tissue loading and pathological overloading (tendinopathy). Nevertheless, the regulation of tendon matrix quality control is still poorly understood and the pathogenesis of tendinopathy is presently unsolved. Autophagy, the major mechanism of degradation and recycling of cellular components, plays a fundamental role in the homeostasis of several tissues. Here, we investigate the contribution of autophagy to human tendons' physiology, and we provide in vivo evidence that it is an active process in human tendon tissue. We show that selective autophagy of the endoplasmic reticulum (ER-phagy), regulates the secretion of type I procollagen (PC1), the major component of tendon extracellular matrix. Pharmacological activation of autophagy by inhibition of mTOR pathway alters the ultrastructural morphology of three-dimensional tissue-engineered tendons, shifting collagen fibrils size distribution. Moreover, autophagy induction negatively affects the biomechanical properties of the tissue-engineered tendons, causing a reduction in mechanical strength under tensile force. Overall, our results provide the first evidence that autophagy regulates tendon homeostasis by controlling PC1 quality control, thus potentially playing a role in the development of injured tendons., (© 2022. The Author(s).)

Muscle fibre denervation and declining numbers of muscle stem (satellite) cells are defining characteristics of ageing skeletal muscle. The aim of this study was to investigate the potential for lifelong recreational exercise to offset muscle fibre denervation and compromised satellite cell content and function, both at rest and under challenged conditions. Sixteen elderly lifelong recreational exercisers (LLEX) were studied alongside groups of age-matched sedentary (SED) and young subjects. Lean body mass and maximal voluntary contraction were assessed, and a strength training bout was performed. From muscle biopsies, tissue and primary myogenic cell cultures were analysed by immunofluorescence and RT-qPCR to assess myofibre denervation and satellite cell quantity and function. LLEX demonstrated superior muscle function under challenged conditions. When compared with SED, the muscle of LLEX was found to contain a greater content of satellite cells associated with type II myofibres specifically, along with higher mRNA levels of the beta and gamma acetylcholine receptors (AChR). No difference was observed between LLEX and SED for the proportion of denervated fibres or satellite cell function, as assessed in vitro by myogenic cell differentiation and fusion index assays. When compared with inactive counterparts, the skeletal muscle of lifelong exercisers is characterised by greater fatigue resistance under challenged conditions in vivo, together with a more youthful tissue satellite cell and AChR profile. Our data suggest a little recreational level exercise goes a long way in protecting against the emergence of classic phenotypic traits associated with the aged muscle. KEY POINTS: The detrimental effects of ageing can be partially offset by lifelong self-organized recreational exercise, as evidence by preserved type II myofibre-associated satellite cells, a beneficial muscle innervation status and greater fatigue resistance under challenged conditions. Satellite cell function (in vitro), muscle fibre size and muscle fibre denervation determined by immunofluorescence were not affected by recreational exercise. Individuals that are recreationally active are far more abundant than master athletes, which sharply increases the translational perspective of the present study. Future studies should further investigate recreational activity in relation to muscle health, while also including female participants., (© 2022 The Authors. The Journal of Physiology published by John Wiley & Sons Ltd on behalf of The Physiological Society.)

Tendons are connective tissues that transmit mechanical forces from muscle to bone and consist mainly of nano-scale fibrils of type I collagen. Aging has been associated with reduced mechanical function of tendons at the whole-tendon level and also with increased glycation of tendon collagen fibrils. Yet, the mechanical effects of aging at the fibril level remain unknown. In vitro glycation has previously been reported to substantially increase fibril strength and stiffness in young rats, suggesting a potentially large effect of aging through the glycation mechanism. We therefore expected that aging would have a similar major impact on fibril mechanical properties. In addition, differences in fibril mechanical properties between men and women have never been studied. This study investigated human patellar tendon biopsies from young (26 ± 4 years) and elderly (66 ± 1 years), men and women by measuring the mechanical properties of individual collagen fibrils using a custom nano-mechanical device. There were no major mechanical differences with either age or sex, but there were modestly greater failure stress (22%) and tensile modulus at both low and high strain (16% and 26% respectively) in the elderly group. No significant differences in mechanical properties were observed between men and women. The slightly greater strength and stiffness in the elderly group are in contrasts to the age-related deficits observed for whole-tendons in vivo, although the study was not designed to investigate these minor differences., (Copyright © 2021 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Loading intervention is currently the preferred management of tendinopathy, but to what extent different loading regimes influence the mechanical response in tendons is scarcely investigated. Therefore, the purposes of the investigation were to examine the effect of exercise interventions with either high or low load magnitude applied to the tendinopathic patellar tendon and the influence on its mechanical, material, and morphological properties. Forty-four men with chronic patellar tendinopathy were randomized to 12 weeks of exercising with either; 55% of 1RM throughout the period (MSR group) or 90% of 1RM (HSR group), and with equal total exercise volume in both groups. Mechanical (stiffness), material (T2* relaxation time), and morphological (cross-sectional area (CSA)) properties were assessed at baseline and after 12 weeks of intervention. MRI with ultra-short echo times (UTE) and T2*-mapping was applied to explore if T2* relaxation time could be used as a noninvasive marker for internal material alteration and early change thereof in response to intervention. There was no effect of HSR or MSR on the mechanical (stiffness), material (T2* relaxation time) or morphological (CSA) properties, but both regimes resulted in significant strength gain. In conclusion, there were no statistically superior effect of exercising with high (90%) compared to moderate (55%) load magnitude on the mechanical, material or morphological properties., (© 2021 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Effects of lifelong physical activity on tendon function have been investigated in cross-sectional studies, but these are at risk of "survivorship" bias. Here, we investigate whether lifelong side-specific loading is associated with greater cross-sectional area (CSA), mechanical properties, cell density (DNA content), and collagen cross-link composition of the male human patellar tendon (PT), in vivo. Nine seniors and six young male lifelong elite badminton players and fencers were included. CSA of the PT obtained by 3-tesla MRI and ultrasonography-based bilateral PT mechanics were assessed. Collagen fibril characteristics, enzymatic cross links, nonenzymatic glycation (autofluorescence), collagen, and DNA content were measured biochemically in PT biopsies. The elite athletes had a ≥15% side-to-side difference in maximal knee extensor strength, reflecting chronic unilateral sport-specific loading patterns. The PT CSA was greater on the lead extremity compared with the nonlead extremity (17%, P = 0.0001). Furthermore, greater tendon stiffness (18%, P = 0.0404) together with lower tendon stress (22%, P = 0.0005) and tendon strain (18%, P = 0.0433) were observed on the lead extremity. No effects were demonstrated from side-to-side for glycation, enzymatic cross link, collagen, and DNA content (50%, P = 0.1160). Moreover, tendon fibril density was 87 ± 28 fibrils/μm 2 on the lead extremity and 68 ± 26 fibrils/μm 2 on the nonlead extremity (28%, P = 0.0544). Tendon fibril diameter was 86 ± 14 nm on the lead extremity and 94 ± 14 nm on the nonlead extremity (-9%, P = 0.1076). These novel data suggest that lifelong side-specific loading in males yields greater patellar tendon size and stiffness possibly with concomitant greater fibril density but without changes of collagen cross-link composition. NEW & NOTEWORTHY The present data demonstrate that lifelong side-specific loading yields greater patellar tendon structure on the lead extremity without affecting glycation that is associated with aging. These novel data suggest that lifelong side-specific habitual loading induce structural alterations that may serve to improve the mechanical properties of the tendon.

Background: T 2 * mapping has proven useful in tendon research and may have the ability to detect subtle changes at an early stage of tendinopathy., Purpose: To investigate the difference in T 2 * between patients with early tendinopathy and healthy controls, and to investigate the relationship between T 2 * and clinical outcomes, tendon size, and mechanical properties., Study Type: Prospective cross-sectional., Subjects: Sixty-five patients with early tendinopathy and 25 healthy controls., Field Strength/sequence: Three Tesla, ultrashort time to echo magnetic resonance imaging., Assessment: Tendon T 2 * was quantified using a monoexponential fitting algorithm. Clinical symptoms were evaluated using the Victorian Institute of Sports Assessment-Achilles/Patella (VISA-A/VISA-P). In vivo mechanical properties were measured using an ultrasound-based method that determined force and deformation simultaneously in tendons of patellar tendinopathy patients., Statistical Tests: A generalized linear model adjusted for age was applied to investigate the difference between patients and controls. In the two patient groups, linear regressions were applied to investigate the association between T 2 * and tendon size, clinical outcomes, and biomechanical properties., Results: There was a significant difference in T 2 * between patients and healthy controls (204.8 [95% CI: 44.5-365.0] μsec, P < 0.05). There was a positive correlation between tendon size and T 2 * for both Achilles (r = 0.72; P < 0.05) and patellar tendons (r = 0.53; P < 0.05). There was no significant correlation between VISA-A and T 2 * (r = -0.2; P = 0.17) or VISA-P and T 2 * (r = -0.5; P = 0.0504). Lastly, there was a negative correlation between modulus and T 2 * (r = -0.51; P < 0.05)., Data Conclusions: T 2 * mapping can detect subtle structural changes that translate to altered mechanical properties in early-phase tendinopathy. However, T 2 * did not correlate with clinical scores in patients with early-phase Achilles and patellar tendinopathy. Thus, T 2 * mapping may serve as a tool for early detection of structural changes in tendinopathy but does not necessarily describe the clinical severity of disease., Level of Evidence: 1 TECHNICAL EFFICACY STAGE: 2., (© 2021 International Society for Magnetic Resonance in Medicine.)

The myotendinous junction (MTJ) is a specialized interface for transmitting high forces between the muscle and tendon and yet the MTJ is a common site of strain injury with a high recurrence rate. The aim of this study was to identify previously unknown MTJ components in mature animals and humans. Samples were obtained from the superficial digital flexor (SDF) muscle-tendon interface of 20 horses, and the tissue was separated through a sequential cryosectioning approach into muscle, MTJ (muscle tissue enriched in myofiber tips attached to the tendon), and tendon fractions. RT-PCR was performed for genes known to be expressed in the three tissue fractions and t-distributed stochastic neighbor embedding (t-SNE) plots were used to select the muscle, MTJ, and tendon samples from five horses for RNA sequencing. The expression of previously known and unknown genes identified through RNA sequencing was studied by immunofluorescence on human hamstring MTJ tissue. The main finding was that RNA sequencing identified the expression of a panel of 61 genes enriched at the MTJ. Of these, 48 genes were novel for the MTJ and 13 genes had been reported to be associated with the MTJ in earlier studies. The expression of known [COL22A1 (collagen XXII), NCAM (neural cell adhesion molecule), POSTN (periostin), NES (nestin), OSTN (musclin/osteocrin)] and previously undescribed [MNS1 (meiosis-specific nuclear structural protein 1), and LCT (lactase)] MTJ genes was confirmed at the protein level by immunofluorescence on tissue sections of human MTJ. In conclusion, in muscle-tendon interface tissue enriched with myofiber tips, we identified the expression of previously unknown MTJ genes representing diverse biological processes, which may be important in the maintenance of the specialized MTJ.

Background: Muscle strain injury leads to a high risk of recurrent injury in sports and can cause long-term symptoms such as weakness and pain. Scar tissue formation after strain injuries has been described, yet what ultrastructural changes might occur in the chronic phase of this injury have not. It is also unknown if persistent symptoms and morphological abnormalities of the tissue can be mitigated by strength training., Purpose: To investigate if heavy resistance training improves symptoms and structural abnormalities after strain injuries., Study Design: Randomized controlled trial; Level of evidence, 1., Methods: A total of 30 participants with long-term weakness and/or pain after a strain injury of the thigh or calf muscles were randomized to eccentric heavy resistance training of the injured region or control exercises of the back and abdominal muscle. Isokinetic (hamstring) or isometric (calf) muscle strength was determined, muscle cross-sectional area measured, and pain and function evaluated. Scar tissue ultrastructure was determined from biopsy specimens taken from the injured area before and after the training intervention., Results: Heavy resistance training over 3 months improved pain and function, normalized muscle strength deficits, and increased muscle cross-sectional area in the previously injured region. No systematic effect of training was found upon pathologic infiltration of fat and blood vessels into the previously injured area. Control exercises had no effect on strength, cross-sectional area, or scar tissue but a positive effect on patient-related outcome measures, such as pain and functional scores., Conclusion: Short-term strength training can improve sequelae symptoms and optimize muscle function even many years after a strain injury, but it does not seem to influence the overall structural abnormalities of the area with scar tissue., Registration: NCT02152098 (ClinicalTrials.gov identifier).

It is well known that cells can generate endogenous forces onto the extracellular matrix, but to what extent the mechanical properties of the matrix influences these endogenous cellular forces remains unclear. We therefore sought to quantify the influence of matrix rigidity on cell-matrix interactions by inducing cross-links using increasing concentrations of genipin (0.01-1 mM) or by blocking cross-link formation using beta-aminopropionitrile (BAPN) in engineered human tendon tissue constructs. The cell-matrix mechanics of the tendon constructs were evaluated as cell-generated tissue re-tensioning and stress-relaxation responses using a novel custom-made force monitor, which can apply and detect tensional forces in real-time in addition to mechanical failure testing. Genipin treatment had no influence on the biochemical profile (hydroxyproline, glycosaminoglycan and DNA content) of the constructs and cell viability was comparable between genipin-treated and control constructs, except at the highest genipin concentration. Endogenous re-tension after unloading was significantly decreased with increasing genipin concentrations compared to controls. Mechanical failure testing of tendon constructs showed increased (56%) peak stress at the highest genipin concentration but decreased (72%) with BAPN treatment when compared to controls. Tendon construct stiffness increased with high genipin concentrations (0.1 and 1 mM) and decreased by 70% in BAPN-treated constructs, relative to the controls. These data demonstrate that human tendon fibroblasts regulate their force exertion inversely proportional to increased cross-link capacity but did so independently of matrix stiffness. Overall, these findings support the notion of an interaction between cell force generation and cross-linking, and thus a role for this interplay in mechanical homeostasis of the tissue., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Background: Nonsteroidal anti-inflammatory drugs (NSAIDs) are commonly used in the treatment of Achilles tendinopathy, but whether they have any additive clinical effect on physical rehabilitation in the early phase of tendinopathy remains unknown., Purpose/hypothesis: To investigate whether an initial short-term NSAID treatment added to a physical rehabilitation program in the early phase of Achilles tendinopathy would have an additive effect. We hypothesized that the combination of NSAID and rehabilitation would be superior to rehabilitation alone., Study Design: Randomized controlled trial; Level of evidence, 1., Methods: A total of 69 patients with early phase Achilles tendinopathy (lasting <3 months) were randomly assigned to either a naproxen group (7 days of treatment; 500 mg twice daily; n = 34) or a placebo group (7 days of placebo treatment; n = 35). Both groups received an identical 12-week physical rehabilitation program. The clinical outcome of the study was evaluated using the Victorian Institute of Sports Assessment-Achilles (VISA-A) questionnaire and a numerical rating scale (NRS), and the physiological outcome was evaluated using ultrasonography, magnetic resonance imaging (MRI), and ultra-short time to echo T2* mapping MRI (UTE T2* MRI). Follow-up was performed at 1 week, 3 months, and 1 year. Time effects are presented as mean difference ± SEM., Results: No significant differences were found between the 2 treatment groups for any of the outcome measures at any time point ( P > .05). For the VISA-A score, a significant time effect was observed between baseline and 3-month follow-up (14.9 ± 2.3; P < .0001), and at 1-year follow-up, additional improvements were observed (6.1 ± 2.3; P < .01). Furthermore, the change in VISA-A score between baseline and 3-month follow-up was greater in patients with very short symptom duration (<1 month) at baseline compared with patients who had longer symptom duration (>2 months) (interaction between groups, 11.7 ± 4.2; P < .01). Despite clinical improvements, total weekly physical activity remained lower compared with preinjury levels at 3 months (-2.7 ± 0.5 h/wk; P < .0001) and 1 year (-3.0 ± 0.5 h/wk; P < .0001). At baseline, ultrasonography showed increased thickness (0.12 ± 0.03 cm; P < .0001) and vascularity (0.3 ± 0.1 cm 2 ; P < .005) on the tendinopathic side compared with the contralateral side, but no changes over time were observed for ultrasonography, MRI, or UTE T2* MRI results., Conclusion: Clinical symptoms in early tendinopathy improved with physical rehabilitation, but this improvement was not augmented with the addition of NSAID treatment. Furthermore, this clinical recovery occurred in the absence of any measurable structural alterations. Finally, clinical improvements after a physical rehabilitation program were greater in patients with very short symptom duration compared with patients who had longer symptom duration., Registration: NCT03401177 (ClinicalTrials.gov identifier) and BFH-2016-019 (Danish Data Protection Agency).

A recent COSMIN review found that the Victorian Institute of Sports Assessment-Achilles tendinopathy questionnaire (VISA-A) has flawed construct validity. The objective of the current study was to assess specifically the process of how VISA-A was constructed and validated, and whether the Danish version of VISA-A is a valid patient-reported outcome measure (PROM) for measuring the perceived impact of Achilles tendinopathy. The original item generation strategy for content validity and the process for confirming the scaling properties (construct validity) were examined. In addition, construct validity was evaluated directly using several psychometric methods (Rasch analysis, confirmatory factor analysis (CFA), and multivariable linear regression) in a cohort of 318 persons with Achilles tendinopathy with symptom duration groups ranging from less than 3 months to more than 1 year of chronicity, and a group of 120 healthy persons. We found that the item generation and item reduction in the original construction of VISA-A was based on literature review and clinician consensus with little or no patient involvement. We determined that 1) VISA-A consists of ambiguous conceptual item themes and thus lacks content validity, 2) there was no thorough investigation of the psychometric properties of the original version of VISA-A, which thus lacks construct validity, and 3) rigorous direct assessment of the psychometric properties of the Danish VISA-A revealed inadequate psychometric properties. In agreement with the COSMIN study, we conclude that when used as a single score, VISA-A is not an adequate scale for measuring self-reported impact of Achilles tendinopathy., Competing Interests: The authors have declared that no competing interests exist.

Background: Loading interventions have become a predominant treatment strategy for tendinopathy, and positive clinical outcomes and tendon tissue responses may depend on the exercise dose and load magnitude., Purpose/hypothesis: The purpose was to investigate if the load magnitude influenced the effect of a 12-week loading intervention for patellar tendinopathy in the short term (12 weeks) and long term (52 weeks). We hypothesized that a greater load magnitude of 90% of 1 repetition maximum (RM) would yield a more positive clinical outcome, tendon structure, and tendon function compared with a lower load magnitude of 55% of 1 RM when the total exercise volume was kept equal in both groups., Study Design: Randomized clinical trial; Level of evidence, 1., Methods: A total of 44 adult participants with chronic patellar tendinopathy were included and randomized to undergo moderate slow resistance (MSR group; 55% of 1 RM) or heavy slow resistance (HSR group; 90% of 1 RM). Function and symptoms (Victorian Institute of Sport Assessment-Patella questionnaire [VISA-P]), tendon pain during activity (numeric rating scale [NRS]), and ultrasound findings (tendon vascularization and swelling) were assessed before the intervention, at 6 and 12 weeks during the intervention, and at 52 weeks from baseline. Tendon function (functional tests) and tendon structure (ultrasound and magnetic resonance imaging) were investigated before and after the intervention period., Results: The HSR and MSR interventions both yielded significant clinical improvements in the VISA-P score (mean ± SEM) (HSR: 0 weeks, 58.8 ± 4.3; 12 weeks, 70.5 ± 4.4; 52 weeks, 79.7 ± 4.6) (MSR: 0 weeks, 59.9 ± 2.5; 12 weeks, 72.5 ± 2.9; 52 weeks, 82.6 ± 2.5), NRS score for running, NRS score for squats, NRS score for preferred sport, single-leg decline squat, and patient satisfaction after 12 weeks, and these were maintained after 52 weeks. HSR loading was not superior to MSR loading for any of the measured clinical outcomes. Similarly, there were no differences in functional (strength and jumping ability) or structural (tendon thickness, power Doppler area, and cross-sectional area) improvements between the groups undergoing HSR and MSR loading., Conclusion: There was no superior effect of exercising with a high load magnitude (HSR) compared with a moderate load magnitude (MSR) for the clinical outcome, tendon structure, or tendon function in the treatment of patellar tendinopathy in the short term. Both HSR and MSR showed equally good, continued improvements in outcomes in the long term but did not reach normal values for healthy tendons., Registration: NCT03096067 (ClinicalTrials.gov identifier).

Background: In the treatment of an Achilles tendon rupture the patients are commonly equipped with an orthopaedic walker boot with wedges. To what extent this influences the tensile force placed on the Achilles tendon is unclear., Purpose: To assess the forefoot force and describe changes in muscle activity of the medial gastrocnemius, soleus and tibialis anterior when using one or three wedges during ambulation in a weightbearing orthopaedic walker boot., Methods: The force on the forefoot was measured with a force sensor insole and muscle activity of the medial gastrocnemius, soleus and tibialis anterior were measured using surface electromyography in 10 healthy participants. Three different types of ambulation were performed (walking without crutches (unass.), walking with crutches (+crutch) and walking with crutches and verbal instructions to place body weight on heel (heel+crutch) with one and three heel wedges respectively., Findings: The total peak force displayed an interaction where forefoot force decreased when wearing three wedges only for the +crutch ambulation type (80N, p=0.001) although there was a trend to decrease with three wedges also for the heel+crutch ambulation type (48N, p=0.05). The relative peak force on the forefoot showed a main effect with a significant decrease when using three wedges compared to one wedge across all three ambulation types (19.1%, p=0.009)., Interpretation: The force on the forefoot and hereby the Achilles tendon significantly decreased when using three wedges compared to one wedge. These findings have important implications for the rehabilitation post Achilles tendon rupture., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Ultrasound has risen to the forefront as one of the primary tools in tendon research, with benefits including its relatively low cost, ease of use, and high safety. Moreover, it has been shown that cine ultrasound can be used to evaluate tendon deformation by tracking the motion of anatomical landmarks during physical movement. Estimates from landmark tracking, however, are typically limited to global tissue properties, such that clinically relevant regional nonuniformities may be missed. Fortunately, advancements in ultrasound scanning have led to the development of speckle-tracking algorithms, which enable the noninvasive measurement of in vivo local deformation patterns. Despite the successes in other fields, the adaptation of speckle-tracking to tendon research has presented some unique challenges as a result of tissue anisotropy and microstructural changes under load. With no generally accepted standards for its use, current methodological approaches vary substantially between studies and research groups. Therefore, the goal of this paper is to provide a summative review of the technical complexities and variations of speckle-tracking approaches being used and the impact these decisions may have on measured results and their interpretation. Variations in these approaches currently being used with relevant technical aspects are discussed first (for the technician), followed by a discussion of the more clinical considerations (for the clinician). Finally, a summary table of common challenges encountered when implementing speckle-tracking is provided, with suggested recommendations for minimizing the impact of such potential sources of error.

Background: There is currently a lack of imaging modalities that can be used as a sensitive measure in tendinopathy. Recent findings suggest the applicability of ultra-short echo time (UTE) magnetic resonance imaging (MRI) T2* mapping in tendons, but the reproducibility remains unknown., Purpose: To evaluate test-retest reproducibility of UTE MRI T2* mapping of tendinopathic patellar tendons and to evaluate the intra- and inter-observer reproducibility of the measurement., Material and Methods: Fifteen patients with chronic patellar tendinopathy were evaluated with UTE MRI twice in a 3.0-T scanner on the same day. Manual segmentation of the patellar tendon was performed by two blinded investigators and automated T2*map reconstruction was performed in custom-made software., Results: There was a significant and numerically small difference in test-retest T2* values (T2*mean diff  = 0.06 ± 0.07 ms ≈ 3.7%; P  = 0.006) with an ICC = 0.91 (95% confidence interval [CI] 0.58-0.98; typical error of 3.0%). The intra- and inter-observer reproducibility showed no significant bias ( P  = 0.493 and P  = 0.052), and generally substantial reproducibility was demonstrated for T2* (intra-observer ICC = 0.99; 95% CI 0.98-1.00 and inter-observer ICC = 0.99; 95% CI 0.96-1.00, and typical error 1.3% and 1.3%, respectively)., Conclusion: These data demonstrate a small bias between repeated measurements for UTE T2*, but with a very low associated mean difference (3.7%) between the two tests. The high ICC values and low typical error % demonstrate reproducibility of repeated T2*-mapping sessions. Further, the method showed substantial intra- and inter-observer reproducibility for T2* values proving feasibility for use of UTE T2* mapping in research and clinical practice.

Background: To investigate how anatomical cross-sectional area and volume of quadriceps and triceps surae muscles were affected by ageing, and by resistance training in older and younger men, in vivo., Methods: The old participants were randomly assigned to moderate (O55, n = 13) or high-load (O80, n = 14) resistance training intervention (12 weeks; 3 times/week) corresponding to 55% or 80% of one repetition maximum, respectively. Young men (Y55, n = 11) were assigned to the moderate-intensity strengthening exercise program. Each group received the exact same training volume on triceps surae and quadriceps group (Reps x Sets x Intensity). The fitting polynomial regression equations for each of anatomical cross-sectional area-muscle length curves were used to calculate muscle volume (contractile content) before and after 12 weeks using magnetic resonance imaging scans., Results: Only Rectus femoris and medial gastrocnemius muscle showed a higher relative anatomical cross-sectional area in the young than the elderly on the proximal end. The old group displayed a higher absolute volume of non-contractile material than young men in triceps surae (+ 96%). After training, Y55, O55 and O80 showed an increase in total quadriceps (+ 4.3%; + 6.7%; 4.2% respectively) and triceps surae (+ 2.8%; + 7.5%; 4.3% respectively) volume. O55 demonstrated a greater increase on average gains compared to Y55, while no difference between O55 and O80 was observed., Conclusions: Muscle loss with aging is region-specific for some muscles and uniform for others. Equivalent strength training volume at moderate or high intensities increased muscle volume with no differences in muscle volume gains for old men. These data suggest that physical exercise at moderate intensity (55 to 60% of one repetition maximum) can reverse the aging related loss of muscle mass., Trial Registration: NCT03079180 in ClinicalTrials.gov . Registration date: March 14, 2017.

Objective: Age-related loss of muscle mass and function can be attenuated in rodents with life-long voluntary wheel running with moderate resistance. The present study assessed if sarcopenia could be counteracted with ten weeks high intensity training., Method: Old (22-23 months) and middle-aged (11 months) mice were divided into three physical activity groups: Ten weeks of voluntary running in wheels with high (HR) or low resistance (LR), or no running wheel (SED). The wheel resistance was 0.5-1.5 g in the LR group and progressed from 5 g to 10 g in the HR group. Six, 8 and 5 old and 8, 9 and 9 middle-aged mice of the SED, LR and HR groups, respectively, were included in the analysis. Wheel activity was monitored throughout the intervention. Muscle mass of the tibialis anterior, gastrocnemius, soleus and plantaris muscles were measured post-mortem. Fiber type distribution and myofiber cross sectional areal (CSA) were quantified in the gastrocnemius and soleus muscles as well as total number of fibers in the soleus muscle., Results: In the SED, the mass of all individual muscles was reduced in the old vs middle-aged (P < 0.001). In the training groups, the old mice ran significantly less, slower and for shorter bouts than the middle-aged throughout the intervention (P < 0.05). HR running increased the gastrocnemius and soleus muscle mass by 6% and 18% respectively in the old compared to SED. Fiber CSA was significantly reduced in the old SED mice, whereas fiber CSA in the old HR gastrocnemius and soleus muscles was comparable to the SED middle-aged. Fiber type shifted from 2b towards 2a in the gastrocnemius muscle of the trained old mice. HR running was more efficient than LR in maintaining muscle mass and myofiber size, and in shifting fiber types. In the middle-aged mice, similar effects were found, but less pronounced. Interestingly, fiber CSA was unaffected by running in the middle-aged., Conclusion: Ten weeks of HR running had a positive effect on muscle mass and morphology in both middle-aged and old mice. The old HR fiber CSA was greater than in old SED and comparable to the middle-aged, and the fibers shifted to a more oxidative composition (2b → 2a). Albeit less pronounced, similar training effects were observed in the middle-aged mice despite running faster and longer than the old., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Tendon is a highly organized, dense connective tissue that has been demonstrated to have very little turnover. In spite of the low turnover, tendon can grow in response to loading, which may take place primarily at the periphery. Tendon injuries and recurrence of injuries are common in both humans and animals in sports. It is unclear why some areas of the tendon are more susceptible to such injuries and whether this is due to intrinsic regional differences in extracellular matrix (ECM) production or tissue turnover. This study aimed to compare populations of tenocytes derived from the tendon core and periphery. Tenocytes were isolated from equine superficial digital flexor tendons (SDFTs), and the proliferation capacity was determined. ECM production was characterized by immuno- and histological staining and by liquid chromatography-mass spectrometry-based proteomics. Core and periphery SDFT cultures exhibited comparable proliferation rates and had very similar proteome profiles, but showed biological variation in collagen type I deposition. In conclusion, the intrinsic properties of tenocytes from different regions of the tendon are very similar, and other factors in the tissue may contribute to how specific areas respond to loading or injury.

Purpose: Recent data suggest that there is a lack of turnover in the core of human tendon, but it remains unknown whether there are regional differences between core and periphery of the cross section. The purpose of this project was to investigate regional differences in turnover as estimated by the accumulation of fluorescent Advanced Glycation End-products (AGEs) and regional differences in mechanical properties., Materials and Methods: Tendons were obtained from lean control ( n = 4) and diabetic Göttingen minipigs (streptozotocin-induced, n = 6). The deep digital flexor tendon of one hind limb was separated into a proximal, central and distal part. Autofluorescence was measured in the core and periphery of the proximal and distal parts of the tendon, and mechanical properties were tested on fascicles taken from the core and periphery of the central tendon (only diabetic animals)., Results: Autofluorescence was greater in the proximal than the distal part. In the distal part of the lean control animals, autofluorescent AGE accumulation was also greater in the core than the periphery. Peak modulus in the core region (704 ± 79 MPa) was higher than the periphery (466 ± 53 MPa, p < 0.05) in diabetic tendons., Conclusion: Taken together, autofluorescence varied both along the length and across the tendon cross section, indicating higher turnover in the distal and peripheral regions. In addition, mechanical properties differed across the tendon cross-section.

Integrins are important for mechanosensation in tissue and play, together with nutrition, a role in regulating extracellular matrix (ECM) in skeletal muscle and tendon. Integrin receptors are dimers that consist of an α and β subunit and bridge extracellular and intracellular signals. The present study investigates whether the deletion of the integrin receptor α 1 subunit influences collagen and other matrix proteins in the musculotendinous tissue and whether it causes any compensatory changes in other integrin subunits in C57BL/6J mice. In addition, we study whether a high-fat diet (HFD) influences these responses in muscle or tendon. Mice on a HFD had a higher number of non-enzymatic cross-links in skeletal muscle ECM and increased gene expression of collagen and other extracellular matrix proteins. In contrast to gene expression, total collagen protein content was decreased by HFD in the muscle with no change in tendon. Integrin α 1 subunit knockout resulted in a decrease of collagen type I and III, TGF-β1 and IGF-1 gene expression in muscle of HFD mice but did not affect total collagen protein compared with wild-type (WT) littermates in either muscle or tendon. There was no compensatory increase in the genes that express other integrin subunits. In conclusion, HFD induced a significant increase in expression of ECM genes in muscle. On the protein level, HFD resulted in a lower collagen content in muscle. Tendons were unaffected by the diet. Deletion of the integrin α 1 subunit did not affect collagen protein or gene expression in muscle or tendon.

Background: Tendon loading might play a role in the development of heterotopic ossification after Achilles tendon ruptures. Early heavy loading on a healing tendon in animals has been shown to prolong the proinflammatory response, and inflammatory cells are thought to drive heterotopic ossification formation. Taken together, this suggests that early rehabilitation might influence heterotopic ossification development., Questions/purposes: The purposes of this study were to investigate (1) whether the presence of heterotopic ossification after Achilles tendon ruptures influences clinical outcome and (2) whether early mobilization or weightbearing prevents the development of heterotopic ossification., Methods: This was a retrospective analysis of 69 patients from a previous clinical trial. All patients were treated surgically, but with three different early rehabilitation protocols after surgery: late weightbearing and ankle immobilization, late weightbearing and ankle mobilization, and early weightbearing and ankle mobilization. Plain radiographs taken 2, 6, 12, 26, and 52 weeks postoperatively were analyzed for heterotopic ossification, which was detected in 19% of patients (13 of 69) at 52 weeks. Heterotopic ossification was measured, scored, and correlated to clinical outcomes; heel-raise index (HRI), ankle joint ROM, tendon strain, Achilles tendon rupture score (ATRS), and Victorian Institute of Sport Assessment-Achilles (VISA-A) questionnaire scores at 26 and 52 weeks postoperatively., Results: Heterotopic ossification had no adverse effects on patient-reported outcomes (ATRS or VISA-A), tendon strain, or ROM. In fact, patients with heterotopic ossification tended to have a better HRI at 52 weeks compared with patients without (mean difference 14% [95% CI -0.2 to 27]; p = 0.053). Neither the occurrence (heterotopic ossification/no heterotopic ossification) nor the heterotopic ossification severity (ossification score) differed between the three rehabilitation groups. Seventeen percent of the patients (four of 24) with early functional rehabilitation (early weightbearing and ankle joint mobilization exercise) had heterotopic ossification (score, 2-3) while late weightbearing and immobilization resulted in heterotopic ossification in 13% of the patients (score, 3-4)., Conclusions: Heterotopic ossification occurs relatively frequently after Achilles tendon ruptures but appears to have no adverse effects on functional outcomes. Furthermore, heterotopic ossification develops during the first 6 weeks after rupture, and weightbearing or ankle-joint mobilization does not prevent this from occurring., Level of Evidence: Level III, prognostic study.

Tendon pathology (tendinopathy) typically occurs in specific regions of a tendon, and growth in response to exercise also appears to be more pronounced in specific regions. In a previous study in animals we found evidence of regional differences in tendon turnover, but whether the turnover of human patellar tendon differs in different regions still remains unknown. Patellar tendons were obtained from cadavers of healthy men and women (body donation program, n = 5 donors, >60 yr of age). Samples were taken from 10 different regions along the length, width, and thickness of the tendon. Turnover was measured by 14 C bomb pulse dating and also estimated from the accumulation of advanced glycation end products (AGEs) by fluorescence (340/460 nm) in addition to measurement of specific AGEs by mass spectrometry. Composition in terms of collagen, glycosaminoglycans (GAGs), and DNA was also assessed in each region. 14 C results showed that all tendon regions had a similar 14 C concentration, which was equal to the average atmospheric 14 C concentration during the first 15 yr of the person's life. Fluorescence normalized to dry weight did not differ between regions, nor did specific AGEs. Higher GAG content was observed in the proximal and near the distal insertion of the tendon. In conclusion, healthy human patellar tendon displays no regional differences in collagen turnover throughout life. NEW & NOTEWORTHY Tendon injuries and tendinopathies typically occur in specific regions of the tendon, but the reason for this specificity is not well understood. A potential factor in injury susceptibility is tissue turnover, and previous work suggests that the tendon core has practically no turnover during adult life; however, it is not known whether this is true for other regions of the tendon. Our present results on healthy human patellar tendon clearly demonstrate that turnover does not differ between regions and thereby cannot explain differences in injury susceptibility. The findings also indicate that all regions of the tendon are formed simultaneously during skeletal maturation and do not turn over appreciably during adulthood. This is an important finding because little is known about tendon growth during maturation in humans.

Purpose: Differential displacement between tendon layers has been shown to occur within the healthy Achilles tendon, and changes of this mechanism have been proposed to result in shear forces, which potentially could lead to tendinopathy. The magnitude of displacement between the tendon layers in tendinopathy is unknown. The purpose of this study was to investigate Achilles tendon layer displacement in individuals suffering from unilateral tendinopathy compared with the asymptomatic contralateral side., Methods: Ten participants (9 men and 1 woman 45 ± 10 years, BMI: 28 ± 5) with unilateral Achilles tendinopathy were included. Intra-tendinous motion was assessed using ultrasonography during dynamic unilateral heel rises in standing and seated position. Speckle displacement was determined using a cross-correlation algorithm, in four independent rows, representing superficial and deep tendon layers., Results: The most superficial layer displaced less than the deepest in all condition, except standing for the tendinopathic leg. There was a strong tendency (p = 0.054) for the displacement difference being reduced in the tendinopathic tendon (Tendinopathic side: 0.52 ± 0.16 mm vs. asymptomatic contralateral side: 1.02 ± 0.18 mm)., Conclusion: These novel data suggest that the presence of tendinopathy diminishes intra-tendinous sliding in the Achilles tendon.

Overloading of tendon tissue with resulting chronic pain (tendinopathy) is a common disorder in occupational-, leisure- and sports-activity, but its pathogenesis remains poorly understood. To investigate the very early phase of tendinopathy, Achilles and patellar tendons were investigated in 200 physically active patients and 50 healthy control persons. Patients were divided into three groups: symptoms for 0-1 months (T1), 1-2 months (T2) or 2-3 months (T3). Tendinopathic Achilles tendon cross-sectional area determined by ultrasonography (US) was ~25% larger than in healthy control persons. Both Achilles and patellar anterior-posterior diameter were elevated in tendinopathy, and only later in Achilles was the width increased. Increased tendon size was accompanied by an increase in hypervascularization (US Doppler flow) without any change in mRNA for angiogenic factors. From patellar biopsies taken bilaterally, mRNA for most growth factors and tendon components remained unchanged (except for TGF-beta1 and substance-P) in early tendinopathy. Tendon stiffness remained unaltered over the first three months of tendinopathy and was similar to the asymptomatic contra-lateral tendon. In conclusion, this suggests that tendinopathy pathogenesis represents a disturbed tissue homeostasis with fluid accumulation. The disturbance is likely induced by repeated mechanical overloading rather than a partial rupture of the tendon., (© 2019 Federation of American Societies for Experimental Biology.)

Fibrillin-1 mutations cause pathological changes in connective tissue that constitute the complex phenotype of Marfan syndrome. In this study, we used fibrillin-1 hypomorphic and haploinsufficient mice (Fbn1 mgr/mgR and Fbn1 +/- mice, respectively) to investigate the impact of fibrillin-1 deficiency alone or in combination with regular physical activity on tendon tissue morphology and mechanical properties. Morphological and biomechanical analyses revealed that Fbn1 mgr/mgR but not Fbn1 +/- mice displayed smaller tendons with physical properties that were unremarkable when normalized to tendon size. Fbn1 mgR/mgR mice (n = 43) Fbn1 +/- mice (n = 27) and wild-type mice (WT, n = 25) were randomly assigned to either control cage conditions (n = 54) or to a running on a running wheel for 4 weeks (n = 41). Both fibrillin-1-deficient mice ran voluntarily on the running wheel in a manner similar to WT mice (3-4 km/24 h). Regular exercise did not mitigate aneurysm progression in Fbn1 mgR/mgR mice (P < 0.05) as evidenced by unmodified median survival. In spite of the smaller size, tendons of fibrillin-1-deficient mice subjected to regular exercise showed no evidence of overt histopathological changes or tissue overload. We therefore concluded that lack of optimal fibrillin-1 synthesis leads to a down regulation of integrated tendon formation, rather than to a loss of tendon quality, which also implies that fibrillin-1 deficiency in combination with exercise is not a suitable animal model for studying the development of tendon overuse (tendinopathy)., (© 2019 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of The Physiological Society and the American Physiological Society.)

Traumatic strain injury in skeletal muscle is often associated with fluid accumulation at the site of rupture, but the role of this injury exudate (EX) in cellular responses and healing is unknown. We aimed to characterize the EX sampled from human hamstring or calf muscles following a strain injury ( n = 12). The cytokine and growth-factor profile, gene expression, and transcriptome analysis of EX-derived cells were compared with blood taken simultaneously from the same individuals. Cellular responses to the EX were tested in 3-dimensional (3D) culture based on primary human fibroblasts and myoblasts isolated from hamstring muscles. The EX contained a highly proinflammatory profile with a substantial expression of angiogenic factors. The proinflammatory profile was present in samples taken early postinjury and in samples aspirated several weeks postinjury, suggesting persistent inflammation. Cells derived from the EX demonstrated an increased expression of fibrogenic, adipogenic, and angiogenesis-related genes in comparison with blood cells. The injury EX stimulated fibroblast proliferation 2-fold compared with plasma, whereas such an effect was not seen for myoblasts. Finally, in 3D cell culture, the EX induced an up-regulation of connective tissue-related genes. In summary, EX formation following a muscle-strain injury stimulates fibroblast proliferation and the synthesis of connective tissue in fibroblasts. This suggests that the EX promotes an acute tissue-healing response but potentially also contributes to the formation of fibrotic tissue in the later phases of tissue repair.-Bayer, M. L., Bang, L., Hoegberget-Kalisz, M., Svensson, R. B., Olesen, J. L., Karlsson, M. M., Schjerling, P., Hellsten, Y., Hoier, B., Magnusson, S. P., Kjaer, M. Muscle-strain injury exudate favors acute tissue healing and prolonged connective tissue formation in humans.

Advanced glycation end-products (AGEs) accumulate with aging and have been associated with tissue modifications and metabolic disease. Regular exercise has several health benefits, and the purpose of this study was to investigate the effect of regular long-term exercise and diet on skin autofluorescence (SAF) as a measure of glycation and on Achilles tendon structure. In connection with the 2017 European Masters Athletics Championships Stadia, high-level male athletes ( n = 194) that had regularly trained for more than 10 years were recruited, in addition to untrained controls ( n = 34). SAF was non-invasively determined using an AGE Reader. Achilles tendon thickness and vascular Doppler activity were measured by ultrasonography, and diet was assessed by a questionnaire. There was no significant difference in SAF between the athletes and controls. However, greater duration of exercise was independently associated with lower SAF. Diet also had an effect, with a more "Western" diet in youth being associated with increased SAF. Furthermore, our data demonstrated that greater Achilles tendon thickness was associated with aging and training. Together, our data indicate that long-term exercise may yield a modest reduction in glycation and substantially increase Achilles tendon size, which may protect against injury.

The decline in skeletal muscle regenerative capacity with age is partly attributed to muscle stem cell (satellite cell) dysfunction. Recent evidence has pointed to a strong interaction between myoblasts and fibroblasts, but the influence of age on this interaction is unknown. Additionally, while the native tissue environment is known to determine the properties of myogenic cells in vitro, how the aging process alters this cell memory has not been established at the molecular level. We recruited 12 young and 12 elderly women, who performed a single bout of heavy resistance exercise with the knee extensor muscles of one leg. Five days later, muscle biopsies were collected from both legs, and myogenic cells and nonmyogenic cells were isolated for in vitro experiments with mixed or separated cells and analyzed by immunostaining and RT-PCR. A lower myogenic fusion index was detected in the cells from the old versus young women, in association with differences in gene expression levels of key myogenic regulatory factors and senescence, which were further altered by performing exercise before tissue sampling. Coculture with nonmyogenic cells from the elderly led to a higher myogenic differentiation index compared with nonmyogenic cells from the young. These findings show that the in vitro phenotype and molecular profile of human skeletal muscle myoblasts and fibroblasts is determined by the age and exercise state of the original in vivo environment and help explain how exercise can enhance muscle stem cell function in old age.

Background: Regular loading of tendons may counteract the negative effects of aging. However, the influence of strength training loading magnitude on tendon mechanical properties and its relation to matrix collagen content and collagen cross-linking is sparsely described in older adults. The purpose of the present study was to compare the effects of moderate or high load resistance training on tendon matrix and its mechanical properties., Methods: Seventeen women and 19 men, age 62-70 years, were recruited and randomly allocated to 12 months of heavy load resistance training (HRT), moderate load resistance training (MRT) or control (CON). Pre- and post-intervention testing comprised isometric quadriceps strength test (IsoMVC), ultrasound based testing of in vivo patellar tendon (PT) mechanical properties, MRI-based measurement of PT cross-sectional area (CSA), PT biopsies for assessment of fibril morphology, collagen content, enzymatic cross-links, and tendon fluorescence as a measure of advanced glycation end-products (AGEs)., Results: Thirty three participants completed the intervention and were included in the data analysis. IsoMVC increased more after HRT (+ 21%) than MRT (+ 8%) and CON (+ 7%) (p < 0.05). Tendon stiffness (p < 0.05) and Young's modulus (p = 0.05) were also differently affected by training load with a reduction in CON and MRT but not in HRT. PT-CSA increased equally after both MRT and HRT. Collagen content, fibril morphology, enzymatic cross-links, and tendon fluorescence were unaffected by training., Conclusion: Despite equal improvements in tendon size after moderate and heavy load resistance training, only heavy. load training seemed to maintain tendon mechanical properties in old age. The effect of load magnitude on tendon biomechanics was unrelated to changes of major load bearing matrix components in the tendon core. The study is a sub-study of the LISA study, which was registered at http://clinicaltrials.gov (NCT02123641) April 25th 2014.

Muscle strain injuries disrupt the muscle-tendon unit, early rehabilitation is associated with a faster return to sports (RTS), but the time course of tissue healing remains sparsely described. The purpose was to examine tissue regeneration and the effectiveness of early versus delayed rehabilitation onset on functional and structural recovery after strain injuries. A total of 50 recreational athletes with a severe acute strain injury in their thigh or calf muscles were randomized to early or delayed rehabilitation onset. Magnetic resonance imaging (MRI) was obtained initially, 3 and 6 months postinjury, and dynamic contrast-enhanced MRI (DCE-MRI) estimated tissue inflammation initially and after 6 months. Muscle strength was determined 5 weeks, 3 months, and 6 months postinjury, and a questionnaire determined soreness, pain, and confidence. DCE-MRI microvascular perfusion was higher in the injured compared to an uninjured muscle acutely (P < 0.01) and after 6 months (P < 0.01), for both groups (P > 0.05) and unrelated to RTS (P > 0.05). Total volume of the injured muscle decreased from the acute to the 3-month scan, and to the 6-month scan (P < 0.01) in both groups. Muscle strength was similar in both groups at any time. There was a nonsignificant trend (P ≤ 0.1) toward less pain and higher confidence with early rehabilitation. One reinjury was recorded. In conclusion, our data showed prolonged tissue repair with the initial response linked to muscle atrophy but did not explain why early rehabilitation onset accelerated recovery considering that structural and functional recovery was similar with early and delayed rehabilitation., (© 2018 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Tendons transmit contractile muscular force to bone to produce movement, and it is believed cells can generate endogenous forces on the extracellular matrix to maintain tissue homeostasis. However, little is known about the direct mechanical measurement of cell-matrix interaction in cell-generated human tendon constructs. In this study we examined if cell-generated force could be detected and quantified in engineered human tendon constructs, and if glycosaminoglycans (GAGs) contribute to tendon force transmission. Following de-tensioning of the tendon constructs it was possible to quantify an endogenous re-tensioning. Further, it was demonstrated that the endogenous re-tensioning response was markedly blunted after interference with the cytoskeleton (inhibiting non-muscle myosin-dependent cell contraction by blebbistatin), which confirmed that re-tensioning was cell generated. When the constructs were elongated and held at a constant length a stress relaxation response was quantified, and removing 27% of the GAG content of tendon did not alter the relaxation behavior, which indicates that GAGs do not play a meaningful role in force transmission within this system., (Copyright © 2018 The Author(s). Published by Elsevier Ltd.. All rights reserved.)