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Background: The temporal tendon is a structure often compromised in patients suffering from temporomandibular disorders (TMD), yet its intraoral location makes a standardised assessment difficult., Objectives: To evaluate the variability and accuracy to target force of a newly designed intraoral extension for a palpometer device (Palpeter, Sunstar Suisse) when compared to manual palpation, in addition to clinically assessing the mechanical sensitivity and referred sensations of the temporal tendon in healthy individuals., Methods: Experiment 1: 12 individuals were asked to target on a scale 0.5, 1 and 2 kg, for 2 and 5 s by using five different methods (Palpeter, Palpeter with three different extension shapes and manual palpation). Experiment 2: 10 healthy participants were recruited for a randomised double-blinded assessment by applying pressure of 0.5, 1 and 2 kg to the right temporal tendon with the three extensions and manual palpation. Participants rated the intensity of their sensation/pain on a 0-50-100 numeric rating scale (NRS), unpleasantness on a 0-100 NRS, and if present, they rated and drew the location of referred sensations. Repeated measures analysis of variance (ANOVA) was used in both experiments to compare differences between palpation methods. Tukey's HSD tests were used for the post hoc comparisons, and p values below .05 were considered significant., Results: Experiment 1: The extensions showed no significant differences between them regarding reliability and accuracy for all forces and durations (p > .05). The manual method was significantly less reliable and accurate when compared to the other methods (p < .05). Experiment 2: There were no significant differences between the Palpeter extensions regarding pain intensity or unpleasantness NRS scores (p > .05), but all the extensions had significantly increased pain intensity and unpleasantness when compared to manual palpation (p < .05). Similarly, the frequency of referred sensations was similar between extensions but increased when compared to manual palpation., Conclusions: The new Palpeter extensions proved to be significantly more accurate and have lower test-retest variability than the manual method in a non-clinical setting. Clinically, they showed no significant differences in NRS scores for pain intensity nor unpleasantness, with no major differences in referred sensations, making any of the extensions suitable for clinical testing of the temporal tendon in future studies., (© 2024 The Author(s). Journal of Oral Rehabilitation published by John Wiley & Sons Ltd.)

Older adults have difficulty maintaining balance when faced with postural disturbances, a task that is influenced by the stiffness of the triceps surae and Achilles tendon. Age-related changes in Achilles tendon stiffness have been reported at matched levels of effort, but measures typically have not been made at matched loads, which is important due to age-dependent changes in strength. Moreover, there has been limited investigation into age-dependent changes in muscle stiffness. Here, we investigate how age alters muscle and tendon stiffness and their influence on ankle stiffness. We hypothesized that age-related changes in muscle and tendon contribute to reduced ankle stiffness in older adults and evaluated this hypothesis when either load or effort were matched. We used B-mode ultrasound with joint-level perturbations to quantify ankle, muscle, and tendon stiffness across a range of loads and efforts in seventeen healthy younger and older adults. At matched loads relevant to standing and the stance phase of walking, there was no significant difference in ankle, muscle, or tendon stiffness between groups (all p > 0.13). However, at matched effort, older adults exhibited a significant decrease in ankle (27%; p = 0.008), muscle (37%; p = 0.02), and tendon stiffness (22%; p = 0.03) at 30% of maximum effort. This is consistent with our finding that older adults were 36% weaker than younger adults in plantarflexion (p = 0.004). Together, these results indicate that, at the loads tested in this study, there are no age-dependent changes in the mechanical properties of muscle or tendon, only differences in strength that result in altered ankle, muscle, and tendon stiffness at matched levels of effort., (© 2024. The Author(s) under exclusive licence to Biomedical Engineering Society.)

In this paper, the innovative design of a robotic hand with soft jointed structure is carried out and a tendon-driven mechanism, a master-slave motor coordinated drive mechanism, a thumb coupling transmission mechanism and a thumb steering mechanism are proposed. These innovative designs allow for more effective actuation in each finger, enhancing the load capacity of the robotic hand while maintaining key performance indicators such as dexterity and adaptability. A mechanical model of the robotic finger was made to determine the application limitations and load capacity. The robotic hand was then prototyped for a set of experiments. The experimental results showed that the proposed theoretical model were reliable. Also, the fingertip force of the robotic finger could reach up to 10.3 N, and the load force could reach up to 72.8 N. When grasping target objects of different sizes and shapes, the robotic hand was able to perform the various power grasping and precision grasping in the Cutkosky taxonomy. Moreover, the robotic hand had good flexibility and adaptability by means of adjusting the envelope state autonomously., (© 2024 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.)

Image-based deformation estimation is an important tool used in a variety of engineering problems, including crack propagation, fracture, and fatigue failure. These tools have been important in biomechanics research where measuring in vitro and in vivo tissue deformations are important for evaluating tissue health and disease progression. However, accurately measuring tissue deformation in vivo is particularly challenging due to limited image signal-to-noise ratio. Therefore, we created a novel deep-learning approach for measuring deformation from a sequence of images collected in vivo called StrainNet. Utilizing a training dataset that incorporates image artifacts, StrainNet was designed to maximize performance in challenging, in vivo settings. Artificially generated image sequences of human flexor tendons undergoing known deformations were used to compare benchmark StrainNet against two conventional image-based strain measurement techniques. StrainNet outperformed the traditional techniques by nearly 90%. High-frequency ultrasound imaging was then used to acquire images of the flexor tendons engaged during contraction. Only StrainNet was able to track tissue deformations under the in vivo test conditions. Findings revealed strong correlations between tendon deformation and applied forces, highlighting the potential for StrainNet to be a valuable tool for assessing rehabilitation strategies or disease progression. Additionally, by using real-world data to train our model, StrainNet was able to generalize and reveal important relationships between the effort exerted by the participant and tendon mechanics. Overall, StrainNet demonstrated the effectiveness of using deep learning for image-based strain analysis in vivo., (© 2024. The Author(s).)

Tendons play an important role in the maintenance of motor function by connecting muscles and bones and transmitting forces. Particularly, the role of mechanical stress has primarily focused on the key mechanism of tendon homeostasis, with much research on this topic. With the recent development of molecular biological techniques, the mechanisms of mechanical stress sensing and signal transduction have been gradually elucidated with the identification of mechanosensor in tendon cells and the master regulator in tendon development. This review provides a comprehensive overview of the structure and function of tendon tissue, including the role for physical performance and the detailed mechanism of mechanotransduction in its regulation. An important lesson is that the role of mechanotransduction in tendon tissue is only partially clarified, indicating the complexity of the mechanisms of motor function and fueling increasing interest in uncovering these mechanisms., (© The Author(s) 2024. Published by Oxford University Press on behalf of the American Society for Bone and Mineral Research. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Background: Robot-assisted microsurgery (RAMS) is gradually becoming the preferred method for some delicate surgical procedures. However, the lack of haptic feedback reduces the safety of the surgery. Surgeons are unable to feel the grasping force between surgical instruments and the patient's tissues, which can easily lead to grasping failure or tissue damage., Methods: This paper proposes a tendon-driven grasping force feedback mechanism, consisting of a follower hand and a leader hand, to address the lack of grasping force feedback in flexible surgical robots. Considering the friction in the tendon transmission process, a grasping force estimation model is established for the follower hand. The admittance control model is designed for force/position control of the leader hand., Results: Through experimental validation, it has been confirmed that the grasping force sensing range of the follower hand is 0.5-5 N, with a sensing accuracy of 0.3 N. The leader hand is capable of providing feedback forces in the range of 0-5 N, with a static force accuracy of 0.1 N., Conclusions: The designed mechanism and control strategy can provide the grasping force feedback function. Future work will focus on improving force feedback performance., Trial Registration: This research has no clinical trials., (© 2024 John Wiley & Sons Ltd.)

Abstract: Stone, MH, Hornsby, G, Mizuguchi, S, Sato, K, Gahreman, D, Duca, M, Carroll, K, Ramsey, MW, Stone, ME, and Haff, GG. The use of free weight squats in sports: a narrative review-squatting movements, adaptation, and sports performance: physiological. J Strength Cond Res 38(8): 1494-1508, 2024-The squat and its variants can provide numerous benefits including positively affecting sports performance and injury prevention, injury severity reduction, and rehabilitation. The positive benefits of squat are likely the result of training-induced neural alterations and mechanical and morphological adaptations in tendons, skeletal muscles, and bones, resulting in increased tissue stiffness and cross-sectional area (CSA). Although direct evidence is lacking, structural adaptations can also be expected to occur in ligaments. These adaptations are thought to beneficially increase force transmission and mechanical resistance (e.g., resistance to mechanical strain) and reduce the likelihood and severity of injuries. Adaptations such as these, also likely play an important role in rehabilitation, particularly for injuries that require restricted use or immobilization of body parts and thus lead to a consequential reduction in the CSA and alterations in the mechanical properties of tendons, skeletal muscles, and ligaments. Both volume and particularly intensity (e.g., levels of loading used) of training seem to be important for the mechanical and morphological adaptations for at least skeletal muscles, tendons, and bones. Therefore, the training intensity and volume used for the squat and its variations should progressively become greater while adhering to the concept of periodization and recognized training principles., (Copyright © 2024 National Strength and Conditioning Association.)

Stretch-shortening cycles (SSCs) outperform shortening contractions preceded by isometric contractions in terms of enhanced force/torque, work, and power production during shortening. This so-called SSC effect is presumably related to the active muscle stretch before shortening in SSCs. However, it remains unclear whether the effects of stretch-induced higher preload level or stretch-induced history dependence maximize the SSC effect. Therefore, we analyzed fascicle behavior, muscle-tendon unit (MTU) shortening work, and torque/force ( n = 12 participants) via ultrasound and dynamometry during electrically stimulated submaximal plantar flexion contractions from 10° plantarflexion to 15° dorsiflexion. To elucidate the effects of preload level and preload modality (i.e., contraction type) on shortening performance, muscle-tendon unit shortening was preceded by fixed-end (SHO), active stretch (SSC), and preload-matched fixed-end (MATCHED) contractions. Before shortening, MATCHED and SCC had the same preload level (1% torque difference), similar joint position, and muscle fascicle lengths. Compared with SHO, shortening work was significantly ( P < 0.001, partial η 2 = 0.749) increased by 85% and 55% for SSC and MATCHED, respectively, with SSC shortening work being significantly higher than MATCHED ( P = 0.016). This indicates that preload contributes by 65% to the overall SSC effect so that 35% needs to be referred to stretched-induced history-dependent mechanisms. In addition, SSC showed larger fascicle forces at the end of shortening ( P < 0.001) and 20% less depressed isometric torque following shortening compared with MATCHED ( P < 0.001). As potential decoupling effects by the series elastic element were controlled by matching the preload levels, we conclude that the difference between SSC and MATCHED is related to stretch-induced long-lasting history-dependent effects. NEW & NOTEWORTHY Using a torque-matched preload protocol, we found that 2/3 of the performance enhancement in muscle-tendon unit stretch-shortening cycles (SSCs) is caused by the increased preload level. The remaining 1/3 is owed to the long-lasting history-dependent effects triggered during the stretch in SSCs. This increased performance output is attributed to passive elastic structures within the contractile element that do not require additional muscle activation, therefore contributing to the higher efficiency of the neuromuscular system in SSCs.

Pseudo-haptic techniques are used to modify haptic perception by appropriately changing visual feedback to body movements. Based on the knowledge that tendon vibration can affect our somatosensory perception, this article proposes a method for leveraging tendon vibration to enhance pseudo-haptics during free arm motion. Three experiments were performed to examine the impact of tendon vibration on the range and resolution of pseudo-haptics. The first experiment investigated the effect of tendon vibration on the detection threshold of the discrepancy between visual and physical motion. The results indicated that vibrations applied to the inner tendons of the wrist and elbow increased the threshold, suggesting that tendon vibration can augment the applicable visual motion gain by approximately 13% without users detecting the visual/physical discrepancy. Furthermore, the results demonstrate that tendon vibration acts as noise on haptic motion cues. The second experiment assessed the impact of tendon vibration on the resolution of pseudo-haptics by determining the just noticeable difference in pseudo-weight perception. The results suggested that the tendon vibration does not largely compromise the resolution of pseudo-haptics. The third experiment evaluated the equivalence between the weight perception triggered by tendon vibration and that by visual motion gain, that is, the point of subjective equality. The results revealed that vibration amplifies the weight perception and its effect was equivalent to that obtained using a gain of 0.64 without vibration, implying that the tendon vibration also functions as an additional haptic cue. Our results provide design guidelines and future work for enhancing pseudo-haptics with tendon vibration.

The peripheral nerve injury (PNI) affects the morphology of the whole locomotor apparatus, which can reach the myotendinous junction (MTJ) interface. In the injury condition, the skeletal muscle satellite cells (SC) are triggered, activated, and proliferated to repair their structure, and in the MTJ, the telocytes (TC) are associated to support the interface with the need for remodeling; in that way, these cells can be associated with SC. The study aimed to describe the SC and TC relationship after PNI at the MTJ. Sixteen adult Wistar rats were divided into Control Group (C, n = 8) and PNI Group (PNI, n = 8), PNI was performed by the constriction of the sciatic nerve. The samples were processed for transmission electron microscopy and immunostaining analysis. In the C group was evidenced the arrangement of sarcoplasmic evaginations and invaginations, the support collagen layer with a TC inside it, and an SC through vesicles internally and externally to then. In the PNI group were observed the disarrangement of invaginations and evaginations and sarcomeres degradation at MTJ, as the disposition of telopodes adjacent and in contact to the SC with extracellular vesicles and exosomes in a characterized paracrine activity. These findings can determine a link between the TCs and the SCs at the MTJ remodeling. RESEARCH HIGHLIGHTS: Peripheral nerve injury promotes the myotendinous junction (MTJ) remodeling. The telocytes (TC) and the satellite cells (SC) are present at the myotendinous interface. TC mediated the SC activity at MTJ., (© 2024 Wiley Periodicals LLC.)

The mallard webbed foot represents an exemplary model of biomechanical efficiency in avian locomotion. This study delves into the intricate material assembly and tendon morphology of the mallard webbed foot, employing both macroscopic and microscopic analyses. Through histological slices and scanning electron microscopy (SEM), we scrutinized the coupling assembly of rigid and flexible materials such as skin, tendon, and bone, while elucidating the biomechanical functions of tendons across various segments of the tarsometatarsophalangeal joint (TMTPJ). The histological examination unveiled a complex structural hierarchy extending from the external integument to the skeletal framework. Notably, the bone architecture, characterized by compact bone and honeycombed trabeculae, showcases a harmonious blend of strength and lightweight design. Tendons, traversing the phalangeal periphery, surrounded by elastic fibers, collagen fibers, and fat tissue. Fat chambers beneath the phalanx, filled with adipocytes, provide effective buffering, enabling the phalanx to withstand gravity, provide support, and facilitate locomotion. Furthermore, SEM analysis provided insights into the intricate morphology and arrangement of collagen fiber bundles within tendons. Flexor tendons in proximal and middle TMTPJ segments adopt a wavy-type, facilitating energy storage and release during weight-bearing activities. In contrast, distal TMTPJ flexor tendons assume a linear-type, emphasizing force transmission across phalangeal interfaces. Similarly, extensor tendons demonstrate segment-specific arrangements tailored to their respective biomechanical roles, with wavy-type in proximal and distal segments for energy modulation and linear-type in middle segments for enhanced force transmission and tear resistance. Overall, our findings offer a comprehensive understanding of the mallard webbed foot's biomechanical prowess, underscoring the symbiotic relationship between material composition, tendon morphology, and locomotor functionality. This study not only enriches our knowledge of avian biomechanics but also provides valuable insights for biomimetic design and tissue engineering endeavors., Competing Interests: Declaration of Competing Interest The authors declare no conflict of interest., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Background: The main aim of this study was to ultrasonographically analyse the thickness and the relationship between the Superomedial Bundle of the Spring Ligament and the Posterior Tibial Tendon in healthy subjects and its relationship with different epidemiological variables., Methods: Fifty-five healthy feet with a mean of 47 years old measuring the same ultrasound model and researcher. Demographic variables (age, sex, laterality, BMI, type of sports activity performed, and type of work activity) were collected from all participants. The thickness of the PTT and the Spring Ligament was measured in both longitudinal and transverse diameters. The intraclass correlation coefficient (ICC) was also analysed to assess the agreement of the measurements between a researcher and the ultrasound specialist radiologist., Results: The mean thickness of the Spring ligament was 5.07 mm (95 % CI 4.75-5.38), while that of the PTT in its long axis was 3.58 mm (95 % CI 3.37-3.79). Regarding the interobserver agreement analysis, the intraclass correlation coefficient for measurements between observers was 0.91 (CI95 %: 0.698-0.977) which denotes a high degree of similarity between the clinician and the radiologist., Conclusion: This study describes the relationships between the thickness of the posterior tibial tendon and the superomedial Bundle of the Spring ligament in healthy subjects, as well as their variability according to certain epidemiological variables such as age, gender, occupation, and sport. On the other hand, the measurements taken by a researcher high agreement with those taken by a radiologist specialized in ultrasound., Competing Interests: Declaration of competing interest The authors declare that they have received no funding for this research and have no conflict of interest. Authors declare that manuscripts submitted to the Journal have not been published elsewhere and are not being considered for publication elsewhere, and that the reported research will not be submitted for publication elsewhere until a final decision on its acceptability has been made by the Journal., (Copyright © 2024. Published by Elsevier Ltd.)

Thumb carpometacarpal joint space changes when the surrounding soft tissues including the capsule, ligaments, and tendons are stretched or pulled away. When at rest, joint forces originate from passive contraction of muscles and the involvement of joint capsule and ligaments. Previous biomechanical models of hand and finger joints have mostly focused on the assessment of joint properties when muscles were active. This study aims to present an experimental-numerical biomechanical model of thumb carpometacarpal joint to explore the contribution of tendons, ligaments, and other soft tissues in the passive forces during distraction. Five fresh cadaveric specimens were tested using a distractor device to measure the applied forces upon gradual distraction of the intact joint. The subsequent step involved inserting a minuscule sensor into the joint capsule through a small incision, while preserving the integrity of tendons and ligaments, in order to accurately measure the fundamental intra-articular forces. A numerical model was also used to calculate the passive forces of tendons and ligaments. Before the separation of bones, the forces exerted by tendons and ligaments were relatively small compared to the capsule force, which accounted for approximately 92% of the total applied force. Contribution of tendons and ligaments, however, increased by further distraction. The passive force contribution by tendons at 2-mm distraction was determined less than 11%, whereas it reached up to 74% for the ligaments. The present study demonstrated that the ligament-capsule complex plays significant contribution in passive forces of thumb carpometacarpal joint during distraction., (© 2024. The Author(s) under exclusive licence to Biomedical Engineering Society.)

The present work reports on the multiaxial region and orientation-dependent mechanical properties of two porcine wrap-around tendons under tensile, compressive and combined loads based on an extensive study with n=175 samples. The results provide a detailed dataset of the anisotropic tensile and compressive longitudinal properties and document a pronounced tension-compression asymmetry. Motivated by the physiological loading conditions of these tendons, which include transversal compression at bony abutments in addition to longitudinal tension, we systematically investigated the change in axial tension when the tendon is compressed transversally along one or both perpendicular directions. The results reveal that the transversal compression can increase axial tension (proximal-distal direction) in both cases to orders of 30%, yet by a larger amount in the first case (transversal compression in anterior-posterior direction), which seems to be more relevant for wrap-around tendons in-vivo. These quantitative measurements are in line with earlier findings on auxetic properties of tendon tissue, but show for the first time the influence of this property on the stress response of the tendon, and may thus reveal an important functional principle within these essential elements of force transmission in the body. STATEMENT OF SIGNIFICANCE: The work reports for the first time on multiaxial region and orientation-dependent mechanical properties of wrap-around tendons under various loads. The results indicate that differences in the mechanical properties exist between zones that are predominantly in a uniaxial tensile state and those that experience complex load states. The observed counterintuitive increase of the axial tension upon lateral compression points at auxetic properties of the tendon tissue which may be pivotal for the function of the tendon as an element of the musculoskeletal system. It suggests that the tendon's performance in transmitting forces is not diminished but enhanced when the action line is deflected by a bony pulley around which the tendon wraps, representing an important functional principle of tendon tissue., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Computational models of musculoskeletal systems are essential tools for understanding how muscles, tendons, bones, and actuation signals generate motion. In particular, the OpenSim family of models has facilitated a wide range of studies on diverse human motions, clinical studies of gait, and even non-human locomotion. However, biological structures with many joints, such as fingers, necks, tails, and spines, have been a longstanding challenge to the OpenSim modeling community, especially because these structures comprise numerous bones and are frequently actuated by extrinsic muscles that span multiple joints-often more than three-and act through a complex network of branching tendons. Existing model building software, typically optimized for limb structures, makes it difficult to build OpenSim models that accurately reflect these intricacies. Here, we introduce ArborSim, customized software that efficiently creates musculoskeletal models of highly jointed structures and can build branched muscle-tendon architectures. We used ArborSim to construct toy models of articulated structures to determine which morphological features make a structure most sensitive to branching. By comparing the joint kinematics of models constructed with branched and parallel muscle-tendon units, we found that among various parameters-the number of tendon branches, the number of joints between branches, and the ratio of muscle fiber length to muscle tendon unit length-the number of tendon branches and the number of joints between branches are most sensitive to branching modeling method. Notably, the differences between these models showed no predictable pattern with increased complexity. As the proportion of muscle increased, the kinematic differences between branched and parallel models units also increased. Our findings suggest that stress and strain interactions between distal tendon branches and proximal tendon and muscle greatly affect the overall kinematics of a musculoskeletal system. By incorporating complex muscle-tendon branching into OpenSim models using ArborSim, we can gain deeper insight into the interactions between the axial and appendicular skeleton, model the evolution and function of diverse animal tails, and understand the mechanics of more complex motions and tasks., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Fu et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

The complexity of wrist anatomy and mechanics makes it challenging to develop standardized measurements and establish a normative reference database of wrist biomechanics despite being studied extensively. Moreover, heterogeneity factors in both demographic characteristics (e.g. gender) and physiological properties (e.g. ligament laxity) could lead to differences in biomechanical behaviour even within healthy groups. We investigated the kinematic behaviour of the carpal bones by creating a virtual web-like network between the bones using electromagnetic (EM) sensors. Our objective was to quantify the changes in the carpal bones' biomechanical relative motions and orientations during active wrist motion in the form of orb-web architecture. Models from five cadaveric specimens at different wrist positions: (1) Neutral to 30° Extension, (2) Neutral to 50° Flexion, (3) Neutral to 10° Radial Deviation, (4) Neutral to 20° Ulnar Deviation, and (5) Dart-Throw Motion - Extension (30° Extension/10° RD) to Dart-Throw Motion Flexion (50° Flexion/20° UD), in both neutral and pronated forearm have been analyzed. Quantification analyses were done by measuring the changes in the network thread length, as well as determining the correlation between the threads at different wrist positions. We observed similarities in the kinematic web-network patterns across all specimens, and the interactions between the network threads were aligned to the carpal bones' kinematic behaviour. Furthermore, analyzing the relative changes in the wrist web network has the potential to address the heterogeneity challenges and further facilitate the development of a 3D wrist biomechanics quantitative tool., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

The mechanisms underlying range of motion enhancements via flexibility training discussed in the literature show high heterogeneity in research methodology and study findings. In addition, scientific conclusions are mostly based on functional observations while studies considering the underlying physiology are less common. However, understanding the underlying mechanisms that contribute to an improved range of motion through stretching is crucial for conducting comparable studies with sound designs, optimising training routines and accurately interpreting resulting outcomes. While there seems to be no evidence to attribute acute range of motion increases as well as changes in muscle and tendon stiffness and pain perception specifically to stretching or foam rolling, the role of general warm-up effects is discussed in this paper. Additionally, the role of mechanical tension applied to greater muscle lengths for range of motion improvement will be discussed. Thus, it is suggested that physical training stressors can be seen as external stimuli that control gene expression via the targeted stimulation of transcription factors, leading to structural adaptations due to enhanced protein synthesis. Hence, the possible role of serial sarcomerogenesis in altering pain perception, reducing muscle stiffness and passive torque, or changes in the optimal joint angle for force development is considered as well as alternative interventions with a potential impact on anabolic pathways. As there are limited possibilities to directly measure serial sarcomere number, longitudinal muscle hypertrophy remains without direct evidence. The available literature does not demonstrate the necessity of only using specific flexibility training routines such as stretching to enhance acute or chronic range of motion., (© 2024. The Author(s).)

The muscle tendon junction (MTJ) plays a crucial role in transmitting the force generated by muscles to the tendon and then to the bone. Injuries such as tears and strains frequently happen at the MTJ, where the regenerative process is limited due to poor vascularization and the complex structure of the tissue. Current solutions for a complete tear at the MTJ have not been successful and therefore, the development of a tissue-engineered MTJ may provide a more effective treatment. In this study, decellularised extracellular matrix (DECM) derived from sheep MTJ was used to provide a scaffold for the MTJ with the relevant mechanical properties and differentiation cues such as the relase of growth factors. Human mesenchymal stem cells (MSCs) were seeded on DECM and 10 % cyclic strain was applied using a bioreactor. MSCs cultured on DECM showed significantly higher gene and protein expression of MTJ markers such as collagen 22, paxillin and talin, than MSCs in 2D culture. Although collagen 22 protein expression was higher in the cells with strain than without strain, reduced gene expression of other MTJ markers was observed when the strain was applied. DECM combined with 10 % strain enhanced myogenic differentiation, while tenogenic differentiation was reduced when compared to static cultures of MSCs on DECM. For the first time, these results showed that DECM derived from the MTJ can induce MTJ marker gene and protein expression by MSCs, however, the effect of strain on the MTJ development in DECM culture needs further investigation., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Introduction: Large increases in joint range of motion (ROM) have been reported after eccentric resistance training; however, limited data exist describing the associated mechanisms or potential cross-education effects in the contralateral limb. Therefore, the effects of a 6-wk isokinetic eccentric plantarflexor training program were examined in 26 participants., Methods: Before and after the training program, dorsiflexion ROM, plantarflexor strength, and muscle-tendon unit (MTU) morphology and mechanics were measured in control ( n = 13) and experimental ( n = 13) young adult groups. Training consisted of 5 sets of 12 maximal isokinetic eccentric plantarflexor contractions twice weekly on the right limb., Results: Significant ( P < 0.05) increases in dorsiflexion ROM (4.0-9.5°), stretch tolerance (40.3-95.9%), passive elastic energy storage (47.5-161.3%), and isometric (38.1-40.6%) and eccentric (46.7-67.0%) peak plantarflexor torques were detected in both trained and contralateral limbs in the experimental group. Significant increases in gastrocnemius medialis and soleus thickness (5.4-6.1%), gastrocnemius medialis fascicle length (7.6 ± 8.5%), passive plantarflexor MTU stiffness (30.1 ± 35.5%), and Achilles tendon stiffness (5.3 ± 4.9%) were observed in the trained limb only. Significant correlations were detected between the changes in trained and contralateral limbs for dorsiflexion ROM ( r = 0.59) and both isometric ( r = 0.79) and eccentric ( r = 0.73) peak torques. No significant changes in any metric were detected in the control group., Conclusions: Large ROM increases in the trained limb were associated with neurological, mechanical, and structural adaptations, with evidence of a cross-education effect in the contralateral limb being primarily driven by neurological adaptation (stretch tolerance). The large improvements in ROM, muscle size, and strength confirm that isokinetic eccentric training is a highly effective training tool, with potential for use in athletic and clinical populations where MTU function is impaired and current therapies are ineffective., (Copyright © 2024 by the American College of Sports Medicine.)

Hyaluronan (HA) plays well-recognized mechanical and biological roles in articular cartilage and synovial fluid, where it contributes to tissue structure and lubrication. An understanding of how HA contributes to the structure of other musculoskeletal tissues, including muscle, bone, tendon, and intervertebral discs, is growing. In addition, the use of HA-based therapies to restore damaged tissue is becoming more prevalent. Nevertheless, the relationship between biomechanical stimuli and HA synthesis, degradation, and signaling in musculoskeletal tissues remains understudied, limiting the utility of HA in regenerative medicine. In this review, we discuss the various roles and significance of endogenous HA in musculoskeletal tissues. We use what is known and unknown to motivate new lines of inquiry into HA biology within musculoskeletal tissues and in the mechanobiology governing HA metabolism by suggesting questions that remain regarding the relationship and interaction between biological and mechanical roles of HA in musculoskeletal health and disease.

Background: Methodological heterogeneity hinders data comparisons across isolated studies of tendon and ligament properties, limiting clinical understanding and affecting the development and evaluation of replacement materials., Purpose: To create an open-access data set on the morphological, biomechanical, and biochemical properties of clinically important tendons and ligaments of the lower limb, using consistent methodologies, to enable direct tendon/ligament comparisons., Study Design: Descriptive laboratory study., Methods: Nineteen distinct lower limb tendons and ligaments were retrieved from 8 fresh-frozen human cadavers (5 male, 3 female; aged 49-65 years) including Achilles, tibialis posterior, tibialis anterior, fibularis (peroneus) longus, fibularis (peroneus) brevis, flexor hallucis longus, extensor hallucis longus, plantaris, flexor digitorum longus, quadriceps, patellar, semitendinosus, and gracilis tendons; anterior cruciate, posterior cruciate, medial collateral, and lateral collateral ligaments; and 10 mm-wide grafts from the contralateral quadriceps and patellar tendons. Outcomes included morphology (tissue length, ultrasound-quantified cross-sectional area [CSA US ], and major and minor axes), biomechanics (failure load, ultimate tensile strength [UTS], failure strain, and elastic modulus), and biochemistry (sulfated glycosaminoglycan [sGAG] and hydroxyproline contents). Tissue differences were analyzed using mixed-model regression., Results: There was a range of similarities and differences between tendons and ligaments across outcomes. A key finding relating to potential graft tissue suitability was the comparable failure loads, UTS, CSA US , sGAG, and hydroxyproline present between hamstring tendons (a standard graft source) and 5 tendons not typically used for grafting: fibularis (peroneus) longus and brevis, flexor and extensor hallucis longus, and flexor digitorum longus tendons., Conclusion: This study of lower limb tendons and ligaments has enabled direct comparison of morphological, biomechanical, and biochemical human tissue properties-key factors in the selection of suitable graft tissues. This analysis has identified 6 potential new donor tissues with properties comparable to currently used grafts., Clinical Relevance: This extensive data set reduces the need to utilize data from incompatible sources, which may aid surgical decisions (eg, evidence to expand the range of tendons considered suitable for use as grafts) and may provide congruent design inputs for new biomaterials and computational models. The complete data set has been provided to facilitate further investigations, with the capacity to expand the resource to include additional outcomes and tissues., Competing Interests: One or more of the authors has declared the following potential conflict of interest or source of funding: Funding support was provided by the Australian Orthopaedic Association (AOA), the Lincoln Centre for Bone and Joint Diseases, the Innovative Manufacturing Cooperative Research Centre (IMCRC), and Bone Ligament Tendon (BLT). 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.

Objective: To systematically review the effects of low-load blood flow restriction training (LL-BFR) on healthy adult tendons., Design: A systematic review with meta-analysis., Literature Search: Six electronic databases were searched by two researchers., Study Selection Criteria: Clinical trials comparing the effects of LL-BFR to high-load resistance training (HL-RT) or low-load resistance training (LL-RT) in healthy adult tendons., Data Synthesis: Two reviewers selected the eligible clinical trials, and one reviewer exported the data. Two reviewers evaluated the study quality and risk of bias using the PEDro scale and the ROB2 scale. We performed meta-analysis where appropriate using a random-effects model. We rated the quality of evidence using GRADE., Results: Six studies were eligible. We analyzed tendon cross-sectional area (CSA) and tendon stiffness as the outcomes. Across all comparisons, there was low-to moderate-quality evidence of a difference between LL-BFR and LL-RT immediately after exercise. There was high-quality evidence of no difference between LL-BFR and HL-RT in the long term., Conclusion: The effects of LL-BFR on the tendons depends on the time and dose of the intervention. LL-BFR could be useful to increase the CSA of the tendons in a similar or superior way to HL-RT after 8 weeks of intervention., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Aging is a primary risk factor for degenerative tendon injuries, yet the etiology and progression of this degeneration are poorly understood. While aged tendons have innate cellular differences that support a reduced ability to maintain mechanical tissue homeostasis, the response of aged tendons to altered levels of mechanical loading has not yet been studied. To address this question, we subjected young and aged murine flexor tendon explants to various levels of in vitro tensile strain. We first compared the effect of static and cyclic strain on matrix remodeling in young tendons, finding that cyclic strain is optimal for studying remodeling in vitro. We then investigated the remodeling response of young and aged tendon explants after 7 days of varied mechanical stimulus (stress deprivation, 1%, 3%, 5%, or 7% cyclic strain) via assessment of tissue composition, biosynthetic capacity, and degradation profiles. We hypothesized that aged tendons would show muted adaptive responses to changes in tensile strain and exhibit a shifted mechanical setpoint, at which the remodeling balance is optimal. Interestingly, we found that 1% cyclic strain best maintains native physiology while promoting extracellular matrix (ECM) turnover for both age groups. However, aged tendons display fewer strain-dependent changes, suggesting a reduced ability to adapt to altered levels of mechanical loading. This work has a significant impact on understanding the regulation of tissue homeostasis in aged tendons, which can inform clinical rehabilitation strategies for treating elderly patients., (Copyright © 2024 by ASME.)

Background: Tendon stem/progenitor cell (TSPC) senescence contributes to tendon degeneration and impaired tendon repair, resulting in age-related tendon disorders. Ferroptosis, a unique iron-dependent form of programmed cell death, might participate in the process of senescence. However, whether ferroptosis plays a role in TSPC senescence and tendon regeneration remains unclear. Recent studies reported that Platelet-derived exosomes (PL-Exos) might provide significant advantages in musculoskeletal regeneration and inflammation regulation. The effects and mechanism of PL-Exos on TSPC senescence and tendon regeneration are worthy of further study., Methods: Herein, we examined the role of ferroptosis in the pathogenesis of TSPC senescence. PL-Exos were isolated and determined by TEM, particle size analysis, western blot and mass spectrometry identification. We investigated the function and underlying mechanisms of PL-Exos in TSPC senescence and ferroptosis via western blot, real-time quantitative polymerase chain reaction, and immunofluorescence analysis in vitro. Tendon regeneration was evaluated by HE staining, Safranin-O staining, and biomechanical tests in a rotator cuff tear model in rats., Results: We discovered that ferroptosis was involved in senescent TSPCs. Furthermore, PL-Exos mitigated the aging phenotypes and ferroptosis of TSPCs induced by t-BHP and preserved their proliferation and tenogenic capacity. The in vivo animal results indicated that PL-Exos improved tendon-bone healing properties and mechanical strength. Mechanistically, PL-Exos activated AMPK phosphorylation and the downstream nuclear factor erythroid 2-related factor 2 (Nrf2)/glutathione peroxidase 4 (GPX4) signaling pathway, leading to the suppression of lipid peroxidation. AMPK inhibition or GPX4 inhibition blocked the protective effect of PL-Exos against t-BHP-induced ferroptosis and senescence., Conclusion: In conclusion, ferroptosis might play a crucial role in TSPC aging. AMPK/Nrf2/GPX4 activation by PL-Exos was found to inhibit ferroptosis, consequently leading to the suppression of senescence in TSPCs. Our results provided new theoretical evidence for the potential application of PL-Exos to restrain tendon degeneration and promote tendon regeneration., (© 2024. The Author(s).)

The workflow to simulate motion with recorded data usually starts with selecting a generic musculoskeletal model and scaling it to represent subject-specific characteristics. Simulating muscle dynamics with muscle-tendon parameters computed from existing scaling methods in literature, however, yields some inconsistencies compared to measurable outcomes. For instance, simulating fiber lengths and muscle excitations during walking with linearly scaled parameters does not resemble established patterns in the literature. This study presents a tool that leverages reported in vivo experimental observations to tune muscle-tendon parameters and evaluates their influence in estimating muscle excitations and metabolic costs during walking. From a scaled generic musculoskeletal model, we tuned optimal fiber length, tendon slack length, and tendon stiffness to match reported fiber lengths from ultrasound imaging and muscle passive force-length relationships to match reported in vivo joint moment-angle relationships. With tuned parameters, muscle contracted more isometrically, and soleus's operating range was better estimated than with linearly scaled parameters. Also, with tuned parameters, on/off timing of nearly all muscles' excitations in the model agreed with reported electromyographic signals, and metabolic rate trajectories varied significantly throughout the gait cycle compared to linearly scaled parameters. Our tool, freely available online, can customize muscle-tendon parameters easily and be adapted to incorporate more experimental data., (© 2024. The Author(s).)

Lumbrical muscles originate on the flexor digitorum profundus (FDP) tendons and, during fist making, they move in the same direction when FDP muscle produces maximal proximal tendon gliding. Injuries of the bipennate lumbricals have been described when a shear force acts between the origins on adjacent tendons of the FDP, as they glide in opposite directions in asymmetric hand postures. Other structures of the deep flexors complex can be affected during this injury mechanism, due to the so-called quadriga effect, which can commonly occur during sport climbing practise. Biomechanical studies are needed to better understand the pathomechanism. A cadaveric study was designed to analyse the effects of load during the fourth lumbrical muscle injury mechanism. The amount of FDP tendon gliding and metacarpophalangeal (MCP) joint flexion of the 5th finger were calculated. Ten fresh-frozen cadaveric specimens (ten non-paired forearms and hands) were used. The specimens were placed on a custom-made loading apparatus. The FDP of the 5th finger was loaded, inducing isolated flexion of the 5th finger, until rupture. The rupture occurred in all specimens, under a load of 11 kg (SD 4.94), at 9.23 mm of proximal tendon gliding (SD 3.55) and at 21.4° (SD 28.91) of MCP joint flexion. Lumbrical muscle detachment from the 4th FDP was observed, from distal to proximal, and changes in FDP tendons at the distal forearm level too. The quadriga effect can lead to injury of the bipennate lumbrical muscles and the deep flexors complex in the hand and forearm., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Purpose: Muscle mechanics is paramount in our understanding of motor performance. However, little is known regarding the sensitivity of fascicle dynamics and connective tissues stiffness to exercise duration and ambient temperature during running, both increasing muscle temperature. This study aimed to determine gastrocnemius medialis (GM) fascicle dynamics in vivo during running in temperate and hot conditions, as well as muscle-tendon unit responses., Methods: Using ultrafast ultrasound, 15 participants (8 men, 7 women; 26 ± 3 yr) were tested before, during (2 and 40 min), and after a running task (40 min at 10 km·h -1 ) in temperate (TEMP; ~23°C) and hot (HOT: ~38°C) conditions., Results: Although core, skin temperatures, and heart rate increased from the beginning to the end of the exercise and in a larger extent in HOT than TEMP ( P < 0.001), the physiological stress elicited did not alter running temporal parameters and GM fascicle operating lengths, with similar behavior of the fascicles on their force-length relationship, over time (2 vs 40 min) or across condition (TEMP vs HOT; P ≥ 0.248). Maximal voluntary force production did not reported statistical changes after exercise ( P = 0.060), and the connective tissues stiffness measured (i.e., passive muscle and stiffness of the series-elastic elements) did not show neither time ( P ≥ 0.281), condition ( P ≥ 0.256) nor time-condition interaction ( P ≥ 0.465) effect., Conclusions: This study revealed that prolonged running exercise does not alter muscle-tendon unit properties and interplay, which are not influenced by ambient temperature. These findings may rule out potential detrimental effects of heat on muscle properties and encourage further investigations on longer and more intense running exercise., (Copyright © 2024 by the American College of Sports Medicine.)

Abstract: Sasajima, S and Kubo, K. Effect of static stretching on tendon hysteresis and efficiency during repetitive jumping. J Strength Cond Res 38(6): 1041-1047, 2024-To date, no studies have experimentally shown a relationship between tendon hysteresis and exercise efficiency. However, previous studies showed that tendon hysteresis decreased immediately after static stretching. The purposes of this study were to (a) investigate the change in tendon hysteresis during the recovery period after static stretching and (b) determine whether exercise efficiency is enhanced because of the decline of tendon hysteresis after static stretching. For stretching (1 minute × 4 sets) and control conditions, tendon hysteresis was measured during ramp (i.e., lower strain rate of tendon) and ballistic (i.e., higher strain rate of tendon) contractions before, immediately, 15, 30, 45, and 60 minutes after interventions. In addition, electromyograms of the plantar flexor muscles (medial gastrocnemius [MG], lateral gastrocnemius [LG], and soleus muscles [SOL]) and oxygen consumption (V̇O 2 ) were measured during 10 minutes of submaximal repetitive jumping after both interventions. Tendon hysteresis (during ramp and ballistic contractions) reduced by static stretching persisted for up to 60 minutes (effect of time p < 0.001). During repetitive jumping, no differences in electromyograms of the plantar flexor muscles (effect of condition p = 0.786 for MG, p = 0.124 for LG, p = 0.682 for SOL) or V̇O 2 (effect of condition p = 0.534) were found between stretching and control conditions. These results suggest that the reduction in tendon hysteresis because of static stretching continues until 60 minutes after the end of stretching, and static stretching does not change the efficiency (evaluated by electromyograms of the plantar flexor muscles and V̇O 2 ) during submaximal repetitive jumping., (Copyright © 2024 National Strength and Conditioning Association.)

To compare 2 different graft preparation techniques to determine biomechanical strength and resultant tissue trauma evaluated by histology. Twelve common flexors of the finger's tendons were prepared with either tubulization (SpeedTrap™) or transtendon stiches (Orthocord™). The stiffness, resistance and energy at maximum load were tested for biomechanical assessment in both groups. After load testing, Samples were stained with hematoxylin and eosin (HE) to evaluate histological damage. We observe that the time to prepare tendons with SpeedTrap™ was 8.3 times faster (1:25 min) than traditional ones (15:02 min). In all cases, the mean values for SpeedTrap™ were higher in terms of strength, stiffness and energy at maximum load than for traditional suture but without significant difference (p > 0.05). The Krackow stitch produces greater structural damage to the collagen fibers while SpeedTrap™ maintains better organized arrangement of the fibers after tubulization preparation. With the results obtained, we can conclude that the tubulization technique allows faster graft preparation with less structural damage to the manipulated tissue without altering the biomechanical resistance provided by the transtendon suture technique., (© 2024. The Author(s), under exclusive licence to Springer Nature B.V.)

The bending stiffness modulation mechanism for soft grippers has gained considerable attention to improve grasping versatility, capacity, and stability. However, lateral stability is usually ignored or hard to achieve at the same time with good bending stiffness modulation performance. Therefore, this article presents a bioinspired bidirectional stiffening soft actuator (BISA), enabling compliant and stable performance. BISA combines the air tendon actuation (ATA) and a bone-like structure (BLS). The ATA is the main actuation of the BISA, and the bending stiffness can be modulated with a maximum stiffness of about 0.7 N/mm and a maximum magnification of three times when the bending angle is 45°. Inspired by the morphological structure of the phalanx, the lateral stiffness can be modulated by changing the pulling force of the BLS. The actuator with BLSs can improve the lateral stiffness by about 3.9 times compared to the one without BLSs. The maximum lateral stiffness can reach 0.46 N/mm. And the lateral stiffness can be modulated by decoupling about 1.3 times (e.g., from 0.35 to 0.46 N/mm when the bending angle is 45°). The test results show that the influence of the rigid structures on bending is small with about 1.5 mm maximum position errors of the distal point of the actuator in different pulling forces. The advantages brought by the proposed method enable versatile four-finger grasping. The performance of this gripper is characterized and demonstrated on multiscale, multiweight, and multimodal grasping tasks.

Background: Although experimental research supports that resistance training (RT), especially with greater dietary protein intake, improves muscle mass and strength in older adults, comparable research on tendons is needed., Objectives: We assessed the effects of a protein-rich diet emphasizing lean beef, compared with 2 control diets, on RT-induced changes in skeletal muscle and tendon size and strength in older women., Methods: We randomly assigned women [age: 66 ± 1 y, body mass index (BMI): 28 ± 1] to groups that consumed 1) 0.8 g total protein/kg body weight/day from mixed food sources (normal protein control, n = 16); 2) 1.4 g/kg/d protein from mixed food sources (high protein control, n = 17); or 3) 1.4 g/kg/d protein emphasizing unprocessed lean beef (high protein experimental group, n = 16). Participants were provided with all foods and performed RT 3 times/wk, 70% of 1-repetition maximum for 12 wk. We measured quadriceps muscle volume via magnetic resonance imaging (MRI). We estimated patellar tendon biomechanical properties and cross-sectional area (CSA) using ultrasound and MRI., Results: Dietary intake did not influence RT-induced increases in quadriceps strength (P < 0.0001) or muscle volume (P < 0.05). We noted a trend for an RT effect on mean tendon CSA (P = 0.07), with no differences among diets (P > 0.05). Proximal tendon CSA increased with RT (P < 0.05) with no difference between dietary groups (P > 0.05). Among all participants, midtendon CSA increased with RT (P ≤ 0.05). We found a decrease in distal CSA in the 0.8 g group (P < 0.05) but no change in the 1.4 g group (P > 0.05). Patellar tendon MRI signal or biomechanical properties were unchanged., Conclusions: Our findings indicated that greater daily protein intake, emphasizing beef, did not influence RT-induced changes in quadriceps muscle strength or muscle volume of older women. Although we noted trends in tendon CSA, we did not find a statistically significant impact of greater daily protein intake from beef on tendon outcomes. This trial was registered at clinicaltrials.gov as NCT04347447., Competing Interests: Conflict of interest During the time this research was conducted, WWC received funding for research from NIH, USDA, Beef Checkoff, Foundation for Meat and Poultry Research and Education, Pork Checkoff, North Dakota Beef Commission, Barilla Group, Mushroom Council, National Chicken Council, and the Whey Protein Research Consortium. All other authors report no conflicts of interest., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Tendon allograft and xenograft processing often involves one or more steps of freezing and thawing. As failure strength is an important graft consideration, this study aimed to evaluate effects on failure properties when varying freeze-thaw conditions. Kangaroo tendons, a potential xenograft source, were used to evaluate changes in ultimate tensile strength (UTS), failure strain and elastic modulus after exposure to different freezer-storage temperatures (-20°C vs. -80°C), storage durations (1, 3, 6, 9, or 12 months), number of freeze-thaw cycles (1, 2, 3, 4, 5, or 10), or freeze-thaw temperature ranges (including freezing in liquid nitrogen to thawing at 37°C). Tendons stored for 6 or more months had significantly increased UTS and elastic modulus compared with 1 or 3 months of storage. This increase occurred irrespective of the freezing temperature (-20°C vs. -80°C) or the number of freeze-thaw cycles (1 vs. 10). In contrast, UTS, failure strain and the elastic modulus were no different between storage temperatures, number of freeze-thaw cycles and multiple freeze-thaw cycles across a range of freeze and thaw temperatures. Common freeze-thaw protocols did not negatively affect failure properties, providing flexibility for graft testing, storage, transportation and decellularisation procedures. However, the change in properties with the overall storage duration has implications for assessing the consistent performance of grafts stored for short versus extended periods of time (<6 months vs. >6 months), and the interpretation of data obtained from tissues of varying or unknown storage durations., (© 2024 The Authors. Journal of Orthopaedic Research® published by Wiley Periodicals LLC on behalf of Orthopaedic Research Society.)

Tendons transmit the muscle contraction forces to bones and drive joint movement throughout life. While extensive research have indicated the essentiality of mechanical forces on tendon development, a comprehensive understanding of the fundamental role of mechanical forces still needs to be impaerted. This scoping review aimed to summarize the current knowledge about the role of mechanical forces during the tendon developmental phase. The electronic database search using PubMed, performed in May 2023, yielded 651 articles, of which 16 met the prespecified inclusion criteria. We summarized and divided the methods to reduce the mechanical force into three groups: loss of muscle, muscle dysfunction, and weight-bearing regulation. In contrast, there were few studies to analyze the increased mechanical force model. Most studies suggested that mechanical force has some roles in tendon development in the embryo to postnatal phase. However, we identified species variability and methodological heterogeneity to modulate mechanical force. To establish a comprehensive understanding, methodological commonality to modulate the mechanical force is needed in this field. Additionally, summarizing chronological changes in developmental processes across animal species helps to understand the essence of developmental tendon mechanobiology. We expect that the findings summarized in the current review serve as a groundwork for future study in the fields of tendon developmantal biology and mechanobiology., (© 2023 The Authors. Developmental Dynamics published by Wiley Periodicals LLC on behalf of American Association for Anatomy.)

Sensory feedback for prosthesis control is typically based on encoding sensory information in specific types of sensory stimuli that the users interpret to adjust the control of the prosthesis. However, in physiological conditions, the afferent feedback received from peripheral nerves is not only processed consciously but also modulates spinal reflex loops that contribute to the neural information driving muscles. Spinal pathways are relevant for sensory-motor integration, but they are commonly not leveraged for prosthesis control. We propose an approach to improve sensory-motor integration for prosthesis control based on modulating the excitability of spinal circuits through the vibration of tendons in a closed loop with muscle activity. We measured muscle signals in healthy participants and amputees during different motor tasks, and we closed the loop by applying vibration on tendons connected to the muscles, which modulated the excitability of motor neurons. The control signals to the prosthesis were thus the combination of voluntary control and additional spinal reflex inputs induced by tendon vibration. Results showed that closed-loop tendon vibration was able to modulate the neural drive to the muscles. When closed-loop tendon vibration was used, participants could achieve similar or better control performance in interfaces using muscle activation than without stimulation. Stimulation could even improve prosthetic grasping in amputees. Overall, our results indicate that closed-loop tendon vibration can integrate spinal reflex pathways in the myocontrol system and open the possibility of incorporating natural feedback loops in prosthesis control.

Surgical intervention is a common option for the treatment of wrist joint arthritis and traumatic wrist injury. Whether this surgery is arthrodesis or a motion preserving procedure such as arthroplasty, wrist joint biomechanics are inevitably altered. To evaluate effects of surgery on parameters such as range of motion, efficiency and carpal kinematics, repeatable and controlled motion of cadaveric specimens is required. This study describes the development of a device that enables cadaveric wrist motion to be simulated before and after motion preserving surgery in a highly controlled manner. The simulator achieves joint motion through the application of predetermined displacements to the five major tendons of the wrist, and records tendon forces. A pilot experiment using six wrists aimed to evaluate its accuracy and reproducibility. Biplanar X-ray videoradiography (BPVR) and X-Ray Reconstruction of Moving Morphology (XROMM) were used to measure overall wrist angles before and after total wrist arthroplasty. The simulator was able to produce flexion, extension, radioulnar deviation, dart thrower's motion and circumduction within previously reported functional ranges of motion. Pre- and post-surgical wrist angles did not significantly differ. Intra-specimen motion trials were repeatable; root mean square errors between individual trials and average wrist angle and tendon force profiles were below 1° and 2 N respectively. Inter-specimen variation was higher, likely due to anatomical variation and lack of wrist position feedback. In conclusion, combining repeatable intra-specimen cadaveric motion simulation with BPVR and XROMM can be used to determine potential effects of motion preserving surgeries on wrist range of motion and biomechanics., Competing Interests: The authors declare there are no competing interests., (©2024 Glanville et al.)

Skeletal muscles and the tendons that attach them to bone are structurally complex and deform non-uniformly during contraction. While these tissue deformations dictate force production during movement, our understanding of this behaviour is limited due to challenges in obtaining complete measures of the constituent structures. To address these challenges, we present an approach for simultaneously measuring muscle, fascicle, aponeurosis, and tendon behaviour using sonomicrometry. To evaluate this methodology, we conducted isometric and dynamic contractions in in situ rabbit medial gastrocnemius. We found comparable patterns of strain in the muscle belly, fascicle, aponeurosis, and tendon during the isometric trials to those published in the literature. For the dynamic contractions, we found that our measures using this method were consistent across all animals and aligned well with our theoretical understanding of muscle-tendon unit behaviour. Thus, this method provides a means to fully capture the complex behaviour of muscle-tendon units across contraction types., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2024. Published by The Company of Biologists Ltd.)

Connective tissue attaches to bone across an insertion with spatial gradients in components, microstructure, and biomechanics. Due to regional stress concentrations between two mechanically dissimilar materials, the insertion is vulnerable to mechanical damage during joint movements and difficult to repair completely, which remains a significant clinical challenge. Despite interface stress concentrations, the native insertion physiologically functions as the effective load-transfer device between soft tissue and bone. This review summarizes tendon, ligament, and meniscus insertions cross-sectionally, which is novel in this field. Herein, the similarities and differences between the three kinds of insertions in terms of components, microstructure, and biomechanics are compared in great detail. This review begins with describing the basic components existing in the four zones (original soft tissue, uncalcified fibrocartilage, calcified fibrocartilage, and bone) of each kind of insertion, respectively. It then discusses the microstructure constructed from collagen, glycosaminoglycans (GAGs), minerals and others, which provides key support for the biomechanical properties and affects its physiological functions. Finally, the review continues by describing variations in mechanical properties at the millimeter, micrometer, and nanometer scale, which minimize stress concentrations and control stretch at the insertion. In summary, investigating the contrasts between the three has enlightening significance for future directions of repair strategies of insertion diseases and for bioinspired approaches to effective soft-hard interfaces and other tough and robust materials in medicine and engineering.

Introduction: Magnetic resonance-guided focused ultrasound (MRgFUS) has been demonstrated to be able to thermally ablate tendons with the aim to non-invasively disrupt tendon contractures in the clinical setting. However, the biomechanical changes of tendons permitting this disrupting is poorly understood. We aim to obtain a dose-dependent biomechanical response of tendons following magnetic resonance-guided focused ultrasound (MRgFUS) thermal ablation., Methods: Ex vivo porcine tendons (n = 72) were embedded in an agar phantom and randomly assigned to 12 groups based on MRgFUS treatment. The treatment time was 10, 20, or 30s, and the applied acoustic power was 25, 50, 75, or 100W. Following each MRgFUS treatment, tendons underwent biomechanical tensile testing on an Instron machine, which calculated stress-strain curves during tendon elongation. Rupture rate, maximum treatment temperature, Young's modulus and ultimate strength were analyzed for each treatment energy., Results: The study revealed a dose-dependent response, with tendons rupturing in over 50% of cases when energy delivery exceeded 1000J and 100% disruption at energy levels beyond 2000J. The achieved temperatures during MRgFUS were directly proportional to energy delivery. The highest recorded temperature was 56.8°C ± 9.34 (3000J), while the lowest recorded temperate was 18.6°C ± 0.6 (control). The Young's modulus was highest in the control group (47.3 MPa ± 6.5) and lowest in the 3000J group (13.2 MPa ± 5.9). There was no statistically significant difference in ultimate strength between treatment groups., Conclusion: This study establishes crucial thresholds for reliable and repeatable disruption of tendons, laying the groundwork for future in vivo optimization. The findings prompt further exploration of MRgFUS as a non-invasive modality for tendon disruption, offering hope for improved outcomes in patients with musculotendinous contractures., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Chu Kwan et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

This article presents a study on the neurobiological control of voluntary movements for anthropomorphic robotic systems. A corticospinal neural network model has been developed to control joint trajectories in multi-fingered robotic hands. The proposed neural network simulates cortical and spinal areas, as well as the connectivity between them, during the execution of voluntary movements similar to those performed by humans or monkeys. Furthermore, this neural connection allows for the interpretation of functional roles in the motor areas of the brain. The proposed neural control system is tested on the fingers of a robotic hand, which is driven by agonist-antagonist tendons and actuators designed to accurately emulate complex muscular functionality. The experimental results show that the corticospinal controller produces key properties of biological movement control, such as bell-shaped asymmetric velocity profiles and the ability to compensate for disturbances. Movements are dynamically compensated for through sensory feedback. Based on the experimental results, it is concluded that the proposed biologically inspired adaptive neural control system is robust, reliable, and adaptable to robotic platforms with diverse biomechanics and degrees of freedom. The corticospinal network successfully integrates biological concepts with engineering control theory for the generation of functional movement. This research significantly contributes to improving our understanding of neuromotor control in both animals and humans, thus paving the way towards a new frontier in the field of neurobiological control of anthropomorphic robotic systems.

This study aimed to examine the temporal dynamics of muscle-tendon adaptation and whether differences between their sensitivity to mechano-metabolic stimuli would lead to non-uniform changes within the triceps surae (TS) muscle-tendon unit (MTU). Twelve young adults completed a 12-week training intervention of unilateral isometric cyclic plantarflexion contractions at 80% of maximal voluntary contraction until failure to induce a high TS activity and hence metabolic stress. Each participant trained one limb at a short (plantarflexed position, 115°: PF) and the other at a long (dorsiflexed position, 85°: DF) MTU length to vary the mechanical load. MTU mechanical, morphological, and material properties were assessed biweekly via simultaneous ultrasonography-dynamometry and magnetic resonance imaging. Our hypothesis that tendon would be more sensitive to the operating magnitude of tendon strain but less to metabolic stress exercise was confirmed as tendon stiffness, Young's modulus, and tendon size were only increased in the DF condition following the intervention. The PF leg demonstrated a continuous increment in maximal AT strain (i.e., higher mechanical demand) over time along with lack of adaptation in its biomechanical properties. The premise that skeletal muscle adapts at a higher rate than tendon and does not require high mechanical load to hypertrophy or increase its force potential during exercise was verified as the adaptive changes in morphological and mechanical properties of the muscle did not differ between DF and PF. Such differences in muscle-tendon sensitivity to mechano-metabolic stimuli may temporarily increase MTU imbalances that could have implications for the risk of tendon overuse injury., (© 2024 The Authors. Scandinavian Journal of Medicine & Science In Sports published by John Wiley & Sons Ltd.)

The myotendinous junction (MTJ) is a vulnerable region at the interface of skeletal muscle and tendon that forms an integrated mechanical unit. This study presents a technique for the spatially restrictive co-culture of human embryonic stem cell (hESC)-derived skeletal myocytes and primary tenocytes for two-dimensional modeling of the MTJ. Micropatterned lanes of extracellular matrix and a 2-well culture chamber define the initial regions of occupation. On day 1, both lines occupy less than 20 % of the initially vacant interstitial zone, referred to henceforth as the junction. Myocyte-tenocyte interdigitations are observed by day 7. Immunocytochemistry reveals enhanced organization and alignment of patterned myocyte and tenocyte features, as well as differential expression of multiple MTJ markers. On day 24, electrically stimulated junction myocytes demonstrate negative contractile strains, while positive tensile strains are exhibited by mechanically passive tenocytes at the junction. Unpatterned tenocytes distal to the junction experience significantly decreased strains in comparison to cells at the interface. Unpatterned myocytes have impaired organization and uncoordinated contractile behavior. These findings suggest that this platform is capable of inducing myocyte-tenocyte junction formation and mechanical coupling similar to the native MTJ, showing transduction of force across the cell-cell interface. STATEMENT OF SIGNIFICANCE: The myotendinous junction (MTJ) is an integrated structure that transduces force across the muscle-tendon boundary, making the region vulnerable to strain injury. Despite the clinical relevance, previous in vitro models of the MTJ lack the structure and mechanical accuracy of the native tissue and have difficulty transmitting force across the cell-cell interface. This study demonstrates an in vitro model of the MTJ, using spatially restrictive cues to inform human myocyte-tenocyte interactions and architecture. The model expressed MTJ markers and developed anisotropic myocyte-tenocyte integrations that resemble the native tissue and allow for force transduction from contracting myocytes to passive tenocyte regions. As such, this study presents a system capable of investigating development, injury, and pathology in the human MTJ., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.)

Rotator cuff tendinopathy has a multifactorial etiology, with both aging and external compression found to influence disease progression. However, tendon's response to these factors is still poorly understood and in vivo animal models make it difficult to decouple these effects. Therefore, we developed an explant culture model that allows us to directly apply compression to tendons and then observe their biological responses. Using this model, we applied a single acute compressive injury to C57BL/6J flexor digitorum longus tendon explants and observed changes in viability, metabolic activity, matrix composition, matrix biosynthesis, matrix structure, gene expression, and mechanical properties. We hypothesized that a single acute compressive load would result in an injury response in tendon and that this effect would be amplified in aged tendons. We found that young tendons had increased matrix turnover with a decrease in small leucine-rich proteoglycans, increase in compression-resistant proteoglycan aggrecan, increase in collagen synthesis, and an upregulation of collagen-degrading MMP-9. Aged tendons lacked any of these adaptive responses and instead had decreased metabolic activity and collagen synthesis. This implies that aged tendons lack the adaptation mechanisms required to return to homeostasis, and therefore are at greater risk for compression-induced injury. Overall, we present a novel compressive injury model that demonstrates lasting age-dependent changes and has the potential to examine the long-term response of tendon to a variety of compressive loading conditions., (© 2023 Orthopaedic Research Society.)

The imbalance between endogenous and exogenous healing is the fundamental reason for the poor tendon healing. In this study, a Janus patch was developed to promote endogenous healing and inhibit exogenous healing, leading to improved tendon repair. The upper layer of the patch is a poly(dl-lactide- co -glycolide)/polycaprolactone (PLGA/PCL) nanomembrane (PMCP-NM) modified with poly(2-methylacryloxyethyl phosphocholine) (PMPC), which created a lubricated and antifouling surface, preventing cell invasion and mechanical activation. The lower layer is a PLGA/PCL fiber membrane loaded with fibrin (Fb) (Fb-NM), serving as a temporary chemotactic scaffold to regulate the regenerative microenvironment. In vitro , the Janus patch effectively reduced 92.41% cell adhesion and 79.89% motion friction. In vivo , the patch inhibited tendon adhesion through the TGF-β/Smad signaling pathway and promoted tendon maturation. This Janus patch is expected to provide a practical basis and theoretical guidance for high-quality soft tissue repair.

Tendon properties impact human locomotion, influencing sports performance, and injury prevention. Hamstrings play a crucial role in sprinting, particularly the biceps femoris long head (BFlh), which is prone to frequent injuries. It remains uncertain if BFlh exhibits distinct mechanical properties compared to other hamstring muscles. This study utilized free-hand three-dimensional ultrasound to assess morphological and mechanical properties of distal hamstrings tendons in 15 men. Scans were taken in prone position, with hip and knee extended, at rest and during 20%, 40%, 60%, and 80% of maximal voluntary isometric contraction of the knee flexors. Tendon length, volume, cross-sectional area (CSA), and anteroposterior (AP) and mediolateral (ML) widths were quantified at three locations. Longitudinal and transverse deformations, stiffness, strain, and stress were estimated. The ST had the greatest tendon strain and the lowest stiffness as well as the highest CSA and AP and ML width strain compared to other tendons. Biceps femoris short head (BFsh) exhibited the least strain, AP and ML deformation. Further, BFlh displayed the highest stiffness and stress, and BFsh had the lowest stress. Additionally, deformation varied by region, with the proximal site showing generally the lowest CSA strain. Distal tendon mechanical properties differed among the hamstring muscles during isometric knee flexions. In contrast to other bi-articular hamstrings, the BFlh high stiffness and stress may result in greater energy absorption by its muscle fascicles, rather than the distal tendon, during late swing in sprinting. This could partly account for the increased incidence of hamstring injuries in this muscle., (© 2024 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Whether eccentric exercise involves active fascicle stretch is unclear due to muscle-tendon unit (MTU) series compliance. Therefore, this study investigated the impact of changing the activation timing and level (i.e., preactivation) of the contraction on muscle fascicle kinematics and kinetics of the human tibialis anterior during dynamometer-controlled maximal voluntary MTU-stretch-hold contractions. B-mode ultrasound and surface electromyography were used to assess muscle fascicle kinematics and muscle activity levels, respectively. Although joint kinematics were similar among MTU-stretch-hold contractions (∼40° rotation amplitude), increasing preactivation increased fascicle shortening and stretch amplitudes (9.9-23.2 mm, P ≤ 0.015). This led to increasing positive and negative fascicle work with increasing preactivation. Despite significantly different fascicle kinematics, similar peak fascicle forces during stretch occurred at similar fascicle lengths and joint angles regardless of preactivation. Similarly, residual force enhancement (rFE) following MTU stretch was not significantly affected (6.5-7.6%, P = 0.559) by preactivation, but rFE was strongly correlated with peak fascicle force during stretch ( r rm = 0.62, P = 0.003). These findings highlight that apparent eccentric exercise causes shortening-stretch contractions at the fascicle level rather than isolated eccentric contractions. The constant rFE despite different fascicle kinematics and kinetics suggests that a passive element was engaged at a common muscle length among conditions (e.g., optimal fascicle length). Although it remains unclear whether different fascicle mechanics trigger different adaptations to eccentric exercise, this study emphasizes the need to consider MTU series compliance to better understand the mechanical drivers of adaptation to exercise. NEW & NOTEWORTHY Apparent eccentric exercises do not result in isolated eccentric contractions, but shortening-stretch contractions at the fascicle level. The amount of fascicle shortening and stretch depends on the preactivation during the exercise and cannot be estimated from the muscle-tendon unit (MTU) or joint kinematics. As different fascicle mechanics might trigger different adaptations to eccentric exercise, muscle-tendon unit series compliance and muscle preactivation need to be considered when eccentric exercise protocols are designed.

The effects of a 12-week gait retraining program on the adaptation of the medial gastrocnemius (MG) and muscle-tendon unit (MTU) were investigated. 26 runners with a rearfoot strike pattern (RFS) were randomly assigned to one of two groups: gait retraining (GR) or control group (CON). MG ultrasound images, marker positions, and ground reaction forces (GRF) were collected twice during 9 km/h of treadmill running before and after the intervention. Ankle kinetics and the MG and MTU behavior and dynamics were quantified. Runners in the GR performed gradual 12-week gait retraining transitioning to a forefoot strike pattern. After 12-week, (1) ten participants in each group completed the training; eight participants in GR transitioned to non-RFS with reduced foot strike angles; (2) MG fascicle contraction length and velocity significantly decreased after the intervention for both groups, whereas MG forces increased after intervention for both groups; (3) significant increases in MTU stretching length for GR and peak MTU recoiling velocity for both groups were observed after the intervention, respectively; (4) no significant difference was found for all parameters of the series elastic element. Gait retraining might potentially influence the MG to operate at lower fascicle contraction lengths and velocities and produce greater peak forces. The gait retraining had no effect on SEE behavior and dynamics but did impact MTU, suggesting that the training was insufficient to induce mechanical loading changes on SEE behavior and dynamics., (© 2024 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

The ability of the median nerve (MN) to adapt in response to altered carpal tunnel conditions is important to mitigate compressive stress on the nerve. We assessed changes in MN deformation and position throughout the entire time course of hand force exertions. Fourteen right-handed participants ramped up force from 0% to 50% of maximal voluntary force (MVF) before ramping force back down in three different hand force exertion tasks (pulp pinch, chuck pinch, power grip). Pinch and grip forces were measured with a digital dynamometer, which were time synchronized with transverse carpal tunnel images obtained via ultrasound. Ultrasound images were extracted in 10% increments between 0% and 50% MVF while ramping force up (loading phase) and down (unloading phase). MN deformation and position relative to the flexor digitorum superficialis tendon of the long finger were assessed in concert. During loading, the nerve became more circular while displacing dorsally and ulnarly. These changes primarily occurred at the beginning of the hand force exertions while ramping force up from 0% to 20%, with very little change between 20% and 50% MVF. Interestingly, deformation and position changes during loading were not completely reversed during unloading while ramping force down. These findings indicate an initial reorganization of carpal tunnel structures. Mirrored changes in nerve deformation and position may also reflect strain-related characteristics of adjoining subsynovial connective tissue. Regardless, time-varying changes in nerve deformation and position appear to be an important accommodative mechanism in the healthy carpal tunnel in response to gripping and pinching tasks., (© 2023 The Authors. Journal of Orthopaedic Research® published by Wiley Periodicals LLC on behalf of Orthopaedic Research Society.)

Previous in vitro and in situ studies have reported a shift in optimal muscle fibre length for force generation (L 0 ) towards longer length at decreasing activation levels (also referred to as length-dependent activation), yet the relevance for in vivo human muscle contractions with a variable activation pattern remains largely unclear. By a combination of dynamometry, ultrasound and electromyography (EMG), we experimentally obtained muscle force-fascicle length curves of the human soleus at 100%, 60% and 30% EMG max levels from 15 participants aiming to investigate activation-dependent shifts in L 0 in vivo. The results showed a significant increase in L 0 of 6.5 ± 6.0% from 100% to 60% EMG max and of 9.1 ± 7.2% from 100% to 30% EMG max (both P < 0.001), respectively, providing evidence of a moderate in vivo activation dependence of the soleus force-length relationship. Based on the experimental results, an approximation model of an activation-dependent force-length relationship was defined for each individual separately and for the collective data of all participants, both with sufficiently high accuracy (R 2 of 0.899 ± 0.056 and R 2  = 0.858). This individual approximation approach and the general approximation model outcome are freely accessible and may be used to integrate activation-dependent shifts in L 0 in experimental and musculoskeletal modelling studies to improve muscle force predictions. KEY POINTS: The phenomenon of the activation-dependent shift in optimal muscle fibre length for force generation (length-dependent activation) is poorly understood for human muscle in vivo dynamic contractions. We experimentally observed a moderate shift in optimal fascicle length towards longer length at decreasing electromyographic activity levels for the human soleus muscle in vivo. Based on the experimental results, we developed a freely accessible approximation model that allows the consideration of activation-dependent shifts in optimal length in future experimental and musculoskeletal modelling studies to improve muscle force predictions., (© 2024 The Authors. The Journal of Physiology published by John Wiley & Sons Ltd on behalf of The Physiological Society.)

Abstract: Takeuchi, K, Nakamura, M, Matsuo, S, Samukawa, M, Yamaguchi, T, and Mizuno, T. Combined effects of static and dynamic stretching on the muscle-tendon unit stiffness and strength of the hamstrings. J Strength Cond Res 38(4): 681-686, 2024-Combined static and dynamic stretching for 30 seconds is frequently used as a part of a warm-up program. However, a stretching method that can both decrease muscle-tendon unit (MTU) stiffness and increase muscle strength has not been developed. The purpose of this study was to examine the combined effects of 30 seconds of static stretching at different intensities (normal-intensity static stretching [NS] and high-intensity static [HS]) and dynamic stretching at different speeds (low-speed dynamic [LD] and high-speed dynamic stretching [HD]) on the MTU stiffness and muscle strength of the hamstrings. Thirteen healthy subjects (9 men and 4 women, 20.9 ± 0.8 years, 169.3 ± 7.2 cm, 61.1 ± 8.2 kg) performed 4 types of interventions (HS-HD, HS-LD, NS-HD, and NS-LD). Range of motion (ROM), passive torque, MTU stiffness, and muscle strength were measured before and immediately after interventions by using an isokinetic dynamometer machine. In all interventions, the ROM and passive torque significantly increased (p < 0.01). Muscle-tendon unit stiffness significantly decreased in HS-HD and HS-LD (both p < 0.01), but there was no significant change in NS-HD (p = 0.30) or NS-LD (p = 0.42). Muscle strength significantly increased after HS-HD (p = 0.02) and NS-LD (p = 0.03), but there was no significant change in HS-LD (p = 0.23) or NS-LD (p = 0.26). The results indicated that using a combination of 30 seconds of high-intensity static stretching and high-speed dynamic stretching can be beneficial for the MTU stiffness and muscle strength of the hamstrings., (Copyright © 2023 National Strength and Conditioning Association.)