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In soccer, knee and hip muscle strength assessments have been recommended for injury prevention. The aims of this study were threefold: (1) to compare knee and hip muscle strength between professional players competing at different levels; (2) to compare strength performance according to the preferred leg (PL) and the non-preferred leg (NPL); and (3) to compare knee and hip muscle strength performance at two moments of the season. This study included 33 professional soccer players: 13 were in the elite group (EG), and 20 were in the sub-elite group (SEG). Body composition, isokinetic knee strength at 60°/s, and hip adduction strength were assessed at two different moments (M1 and M2). Values of peak torque (PT), peak torque/bodyweight (PT/BW), and the hamstring-to-quadriceps strength ratio (H:Q) for knee extensors (KEs) and knee flexors (KFs) for both legs were used for analysis. The statistical analysis included the Mann-Whitney U and the Wilcoxon Signed Rank tests. At M1, the EG presented a significantly better performance in KF PT/BW and in the squeeze strength test for the PL and the NPL (p ≤ 0.01). At M2, the EG performed substantially better in KE PT/BW and KF PT/BW (p ≤ 0.01). No substantial strength differences were observed in knee and hip muscle performance between the PL and the NPL. From M1 to M2, significant increases were found in knee strength in both groups (p ≤ 0.01). Overall, the EG players outperformed significantly their lower-division peers in strength assessments. The results indicate significant knee and hip muscle strength increases during the season, probably as a response to the exposure to training and competition. [ABSTRACT FROM AUTHOR]

Background: Muscle strength and balance abilities have been related to game performance injury prevention. This study aimed to (1) assess knee muscle strength performance based on intra- and inter-limb asymmetries, (2) investigate the relationships between knee muscle strength, body composition, and balance, and (3) analyze the variance in the overall stability index (OSI) explained by knee muscle strength indicators after controlling for age and body composition. Methods: The sample included 104 young male soccer players (16.0 ± 1.6 years). Body composition, knee muscle strength, and balance ability were assessed. A paired samples t-test was used to examine differences between the preferred leg (PL) and the non-preferred leg (NPL). Pearson correlations and hierarchical multiple regression were conducted to explore the relationships between strength, body composition, and balance. Results: No significant inter-limb asymmetries were detected for the knee flexors (KF) and knee extensors (KE) performance. The hamstrings-to-quadriceps ratio does not suggest the existence of intra-limb asymmetries (p = 0.06). Significant and negative correlations were found between KF (r = -0.38, p ≤ 0.01) and KE (r = -0.58, p ≤ 0.01) for the PL. Concerning the NPL, KE also revealed a significant and negative relationship with balance (r = -0.30, p ≤ 0.01). Significant and positive relationships were found between bodyweight, fat-free mass, and balance. After controlling for age and body composition, knee muscle strength could explain between 13% and 30% of the variance observed in the OSI (NPL and PL, respectively). Conclusions: The results underline knee muscle strength as an important predictor of balance. Thus, training programs targeting knee muscle strength development should be considered during the season periodization by coaches and their staff throughout the players' developmental stages. [ABSTRACT FROM AUTHOR]

Abstract Background Muscle strength and balance abilities have been related to game performance injury prevention. This study aimed to (1) assess knee muscle strength performance based on intra- and inter-limb asymmetries, (2) investigate the relationships between knee muscle strength, body composition, and balance, and (3) analyze the variance in the overall stability index (OSI) explained by knee muscle strength indicators after controlling for age and body composition. Methods The sample included 104 young male soccer players (16.0 ± 1.6 years). Body composition, knee muscle strength, and balance ability were assessed. A paired samples t-test was used to examine differences between the preferred leg (PL) and the non-preferred leg (NPL). Pearson correlations and hierarchical multiple regression were conducted to explore the relationships between strength, body composition, and balance. Results No significant inter-limb asymmetries were detected for the knee flexors (KF) and knee extensors (KE) performance. The hamstrings-to-quadriceps ratio does not suggest the existence of intra-limb asymmetries (p = 0.06). Significant and negative correlations were found between KF (r = -0.38, p ≤ 0.01) and KE (r = -0.58, p ≤ 0.01) for the PL. Concerning the NPL, KE also revealed a significant and negative relationship with balance (r = -0.30, p ≤ 0.01). Significant and positive relationships were found between bodyweight, fat-free mass, and balance. After controlling for age and body composition, knee muscle strength could explain between 13% and 30% of the variance observed in the OSI (NPL and PL, respectively). Conclusions The results underline knee muscle strength as an important predictor of balance. Thus, training programs targeting knee muscle strength development should be considered during the season periodization by coaches and their staff throughout the players’ developmental stages.

Abstract Post‐activation potentiation (PAP) is defined as an enhanced contractile response of a muscle following its own contractile activity and is influenced by the intensity and duration of the conditioning contraction. The aim of this study was to determine if the combination of intensity and duration, that is, torque–time integral (TTI) is a determinant of PAP amplitude. We compared PAP amplitude following low‐to‐maximal voluntary conditioning contraction intensities with and without similar TTI in the knee extensors. Twelve healthy males completed two experimental sessions. Femoral nerve stimulation was applied to evoke single twitches on the relaxed quadriceps before and after isometric conditioning contractions of knee extensors. In one session, participants performed conditioning contractions without similar TTI (6 s at 100, 80, 60, 40 and 20% maximal voluntary contraction (MVC)), while they performed conditioning contractions with similar TTI in the other session (6 s at 100%, 7.5 s at 80%, 10 s at 60%, 15 s at 40%, and 30 s at 20% MVC). In both sessions, PAP amplitude was related to conditioning contraction intensity. The higher the conditioning contraction intensity with or without similar TTI, the higher PAP. Significant correlations were found (i) between PAP and conditioning contraction intensity with (r2 = 0.70; P

The aim of our study was to compare the effects of two different plyometric training programs (targeting knee extensors or plantar flexors) on jump height and strength of leg muscles. Twenty‐nine male basketball players were assigned to the knee‐flexed (KF), knee‐extended (KE), or control groups. In addition to regular training, the KF group performed plyometric jumps (10 sets of 10 jumps, 3 sessions/week, 4 weeks) from 50 cm boxes with the knee flexed (90°–120°), whereas the KE group performed the jumps from 30 cm boxes with the knee much more extended (130°–170°). Jumping ability was evaluated with squat jumps (SJs), countermovement jumps (CMJs), and drop jumps from 20 cm (DJ20) and 40 cm (DJ40). Knee and ankle muscles were assessed during maximal isokinetic and isometric tests, and EMG activity was recorded from vastus lateralis and medial gastrocnemius. The KF group increased SJ (+10%, d = 0.86) and CMJ (+11%, d = 0.70) but decreased DJ40 height (−7%, d = −0.40). Conversely, the KE group increased DJ20 (+10%, d = 0.74) and DJ40 (+12%, d = 0.77) but decreased SJ height (−4%, d = −0.23). The reactivity index during DJs increased (+10% for DJ20, d = 0.47; +20% for DJ40, d = 0.91) for the KE group but decreased (−10%, d = −0.48) for the KF group during DJ40. Plantar flexor strength increased for the KE group (d = 0.72–1.00) but not for the KF group. Negative transfer across jumps is consistent with the principle of training specificity. Basketball players interested to perform fast rebounds in their training should avoid plyometric jumps with large knee flexions and long contact times. Highlights: Plyometric training specificity: different jump techniques (knee flexed vs. knee extended) elicited specific adaptations in jumping performance. Training with the knees flexed improved squat jump and countermovement jump height, whereas training with the knees extended augmented drop jump height.Negative transfer: the different jump techniques can have a negative influence on jump height and reactivity index. Drop jump height declined after knee‐flexed training, whereas squat jump height decreased after knee‐extended training.Plyometric training exercises should be aligned with sport‐specific movements to optimize performance. Basketball players who perform fast, powerful movements should avoid plyometric jumps with large knee flexions and long contact times. [ABSTRACT FROM AUTHOR]

Post‐activation potentiation (PAP) is defined as an enhanced contractile response of a muscle following its own contractile activity and is influenced by the intensity and duration of the conditioning contraction. The aim of this study was to determine if the combination of intensity and duration, that is, torque–time integral (TTI) is a determinant of PAP amplitude. We compared PAP amplitude following low‐to‐maximal voluntary conditioning contraction intensities with and without similar TTI in the knee extensors. Twelve healthy males completed two experimental sessions. Femoral nerve stimulation was applied to evoke single twitches on the relaxed quadriceps before and after isometric conditioning contractions of knee extensors. In one session, participants performed conditioning contractions without similar TTI (6 s at 100, 80, 60, 40 and 20% maximal voluntary contraction (MVC)), while they performed conditioning contractions with similar TTI in the other session (6 s at 100%, 7.5 s at 80%, 10 s at 60%, 15 s at 40%, and 30 s at 20% MVC). In both sessions, PAP amplitude was related to conditioning contraction intensity. The higher the conditioning contraction intensity with or without similar TTI, the higher PAP. Significant correlations were found (i) between PAP and conditioning contraction intensity with (r2 = 0.70; P < 0.001) or without similar TTI (r2 = 0.64; P < 0.001), and (ii) between PAP with and without similar TTI (r2 = 0.82; P < 0.001). The results provide evidence that TTI has a minor influence on PAP in the knee extensors. This suggests that to optimize the effect of PAP, it is more relevant to control the intensity of the contraction rather than the TTI. What is the central question of this study?Post‐activation potentiation is defined as an enhanced contractile response of a muscle following its own contractile activity and is greatly influenced by the intensity and duration of the conditioning contraction: is the combination of intensity and duration, that is, the torque–time integral, a determinant of post‐activation potentiation amplitude?What is the main finding and its importance?The main findings of this study provide evidence that the torque–time integral has a minor influence on post‐activation potentiation in the knee extensors. This suggests that to optimize the effect of post‐activation potentiation, it is more relevant to control the intensity of the contraction rather than the torque–time integral. [ABSTRACT FROM AUTHOR]

IntroducationThis study aimed to clarify whether or not the prolonged effect of a 180-s static stretching (SS) exercise could be maintained for a longer period by performing an additional short-term (15-s) SS exercise following a 15-min rest.MethodsThe participants were 15 healthy untrained adult males in three conditions: (1) SS condition (180-s SS exercise only); (2) SS + 15 s condition (180-s SS and 15-s SS exercise during the rest period); and (3) noSS + 15-s condition (15-s SS exercise only). The knee flexion range of motion (ROM), pain pressure threshold (PPT), and tissue hardness were measured before (PRE), immediately after (POST), and 15 min and 30 min after the SS exercise.Results and discussionSignificant interaction effects were observed in all variables. Post hoc tests showed that knee flexion ROM showed an immediate significant change (P < 0.05) and also at 15 min and 30 min after the SS exercise in the SS + 15 s condition. Tissue hardness showed an immediate significant decrease (P < 0.05) and also at 15 min, but not at 30 min after the SS exercise in the SS + 15 s condition. The additional short-term (15 s) SS exercise in the rest period may have a potential long-lasting effect on ROM increase and tissue hardness decrease.

Good preoperative knee function (KF) and quadriceps strength (QS) relate to good prognoses after anterior cruciate ligament reconstruction (ACLR). This study aimed to investigate the associations between patient characteristics, rate of torque development (RTD), voluntary activation (VA) of the quadriceps against preoperative KF and QS. A cross-sectional study was conducted. Forty patients with a primary, unilateral ACL injury who had finished the preoperative rehabilitation and scheduled for an ACLR were included. KF was evaluated using the International Knee Documentation Committee score. QS was measured by maximal voluntary isometric contractions. RTD was divided into the early (RTD0-50) and the late (RTD100-200) phases. Quadriceps VA was measured using the superimposed burst technique. Our results showed that patients with poor preoperative KF had more deficits in RTD0-50 (P = 0.025), higher BMI(P = 0.043), and more meniscus injuries (MI) (P = 0.001) than those with good KF. Patients with asymmetrical QS showed lower BMI (P = 0.020) and shorter time from injury (TFI) (P = 0.027) than those with symmetrical QS. Additionally, a higher pre-injury Tegner score combined with greater RTD100-200 contributed to greater QS on the injured limb (R2 = 0.357, P = 0.001). To conclude, BMI, TFI, concomitant MI, pre-injury physical activity level, and quadriceps RTD are related to QS and KF before ACLR. [ABSTRACT FROM AUTHOR]

The force drop after transcranial magnetic stimulation (TMS) delivered to the motor cortex during voluntary muscle contractions could inform about muscle relaxation properties. Because of the physiological relation between skeletal muscle fibertype distribution and size and muscle relaxation, TMS could be a noninvasive index of muscle relaxation in humans. By combining a noninvasive technique to record muscle relaxation in vivo (TMS) with the gold standard technique for muscle tissue sampling (muscle biopsy), we investigated the relation between TMS-induced muscle relaxation in unfatigued and fatigued states, and muscle fiber-type distribution and size. Sixteen participants (7F/9M) volunteered to participate. Maximal knee-extensor voluntary isometric contractions were performed with TMS before and after a 2-min sustained maximal voluntary isometric contraction. Vastus lateralis muscle tissue was obtained separately from the participants’ dominant limb. Fiber type I distribution and relative cross-sectional area of fiber type I correlated with TMS-induced muscle relaxation at baseline (r = 0.67, adjusted P = 0.01; r = 0.74, adjusted P = 0.004, respectively) and normalized TMS-induced muscle relaxation as a percentage of baseline (r = 0.50, adjusted P = 0.049; r = 0.56, adjusted P = 0.031, respectively). The variance in the normalized peak relaxation rate at baseline (59.8%, P < 0.001) and in the fatigue resistance (23.0%, P = 0.035) were explained by the relative cross-sectional area of fiber type I to total fiber area. Fiber type I proportional area influences TMS-induced muscle relaxation, suggesting TMS as an alternative method to noninvasively inform about skeletal muscle relaxation properties. NEW & NOTEWORTHY Transcranial magnetic stimulation (TMS)-induced muscle relaxation reflects intrinsic muscle contractile properties by interrupting the drive from the central nervous system during voluntary muscle contractions. We showed that fiber type I proportional area influences the TMS-induced muscle relaxation, suggesting that TMS could be used for the noninvasive estimation of muscle relaxation in unfatigued and fatigued human muscles when the feasibility of more direct method to study relaxation properties (i.e., muscle biopsy) is restricted. [ABSTRACT FROM AUTHOR]

Background: This study aimed to investigate the correlations of serum vitamin D insufficiency with quadriceps neuromuscular function in patients with anterior cruciate ligament (ACL) injury. Methods: A cross-sectional study was conducted. Eighteen patients with a primary, unilateral ACL injury who had insufficient serum vitamin D concentrations (

The study aimed to investigate the impairments in quadriceps neuromuscular function, including strength, rate of torque development (RTD) and activation failure (QAF) early after an ACL injury. A cross-sectional study was conducted. Thirty physically active patients with a primary ACL injury within three months, aged 18 to 40 years old, and who were scheduled for ACL reconstruction were included. Thirty matched healthy controls were also recruited. All the outcomes were measured on an isokinetic dynamometer with knee flexion at 45°. Quadriceps strength was measured by maximal voluntary isometric contractions (MVIC). Early (RTD0-50) and late (RTD100-200) phases of RTD were retrieved from the MVIC test from 0 to 50 ms and 100–200 ms, respectively. QAF was quantified by the central activation ratio (CAR) measured by superimposed burst technique. The results of Mann–Whitney U test showed that compared with the healthy limbs, the injured limbs of the ACL group showed lower quadriceps strength (P < 0.001), RTD0-50 (P < 0.001) and RTD100-200 (P < 0.001); the uninjured limbs showed lower quadriceps strength (P = 0.009), RTD0-50 (P = 0.006) as well as greater QAF (P = 0.010). To conclude, bilateral quadriceps suffered from neuromuscular impairments early after an ACL injury. [ABSTRACT FROM AUTHOR]

High-intensity interval training (HIIT) is an effective alternative to moderate intensity continuous training for improvements in body composition and aerobic capacity; however, there is little work comparing different modalities of HIIT. The purpose of this study was to compare the effects of plyometric- (PLYO) and cycle-oriented (CYC) HIIT on body composition, aerobic capacity, and skeletal muscle size, quality, and function in recreationally trained females. Young (21.7 ± 3.1 yrs), recreationally active females were quasi-randomized (1:1 ratio) to 8 weeks of twice weekly PLYO (n = 15) or CYC (n = 15) HIIT. Body composition (four-compartment model), VO2peak, countermovement jump performance, muscle size, and echo intensity (muscle quality), as well as strength and power of the knee extensors and plantar flexors were measured before and after training. Both groups showed a similar decrease in body fat percentage (p < 0.001; η p 2 = 0.409) and echo intensity (p < 0.001; η p 2 = 0.558), and an increase in fat-free mass (p < 0.001; η p 2 = 0.367) and VO2peak (p = 0.001; η p 2 = 0.318). Muscle size was unaffected (p > 0.05), whereas peak torque was reduced similarly in both groups (p = 0.017; η p 2 = 0.188) and rapid torque capacity was diminished only for the knee extensors after CYC (p = 0.022; d = −0.67). These results suggest that PLYO and CYC HIIT are similarly effective for improving body composition, aerobic capacity, and muscle quality, whereas muscle function may express moderate decrements in recreationally active females. ClinicalTrials.gov (NCT05821504) [ABSTRACT FROM AUTHOR]

The present study compared the ipsilateral repeated bout effect (IL‐RBE) and contralateral repeated bout effect (CL‐RBE) of the elbow flexors (EF) and knee flexors (KF) for the same interval between bouts to shed light on their mechanisms. Fifty‐two healthy sedentary young (20–28 years) men were randomly assigned to the IL‐EF, IL‐KF, CL‐EF, and CL‐KF groups (n = 13/group). Thirty maximal eccentric contractions of the EF were performed in IL‐EF and CL‐EF, and 60 maximal eccentric contractions of the KF were performed in IL‐KF and CL‐KF, with a 2‐week interval between bouts. Changes in muscle damage markers such as maximal voluntary contraction (MVC) torque, muscle soreness, and plasma creatine kinase activity, and proprioception measures before to 5 days post‐exercise were compared between groups. Changes in all variables were greater (p < 0.05) after the first than second bout for all groups, and the changes were greater (p < 0.05) for the EF than KF. The changes in all variables after the second bout were greater (p < 0.05) for the CL than IL condition for both EF and KF. The magnitude of the average protective effect was similar between CL‐EF (33%) and CL‐KF (32%), but slightly greater (p < 0.05) for IL‐EF (67%) than IL‐KF (61%). These demonstrate that the magnitude of CL‐RBE relative to IL‐RBE was similar between the EF and KF (approximately 50%), regardless of the greater muscle damage for the EF than KF. It appears that the CL‐RBE is more associated with neural adaptations at cerebrum, cerebellum, interhemispheric inhibition, and coricospinal tract, but the IL‐RBE is induced by additional adaptations at muscles. [ABSTRACT FROM AUTHOR]

In football, knee and hip muscle strength assessment have been recommended for injury prevention (Mosler et al., 2017; Namazi et al., 2019). Additionally, bilateral strength differences in the knee and hip joints have been related to an increased risk of injury. The aims of this study were threefold: (1) to compare knee and hip muscle strength performance between professional players competing at different levels; (2) to assess bilateral strength differences according to the preferred leg (PL) and the non-preferred leg (NPL); and (3) to compare knee and hip muscle strength performance in two-time moments of the season. This study considered 33 professional football players: 13 were in the elite group (EG) competing in the First Portuguese Football League, and 20 were in the sub-elite group (SEG) competing in the Fourth Portuguese Football League. Participants were assessed for body composition, isokinetic knee strength at 60º/s, and hip adduction strength at two-time moments (TM) separated by five months. Peak torque normalised by bodyweight (PT/BW) of knee extensors (KE) and knee flexors (KF) were used for analysis. The differences between groups in strength were analysed using the Mann-Whitney U test. The Wilcoxon Signed Rank Test was conducted to verify differences between the performance of the PL and NPL in each group and to compare the performance between TM1 and TM2. At TM1, the EG presented a significantly better performance in KF PT/BW and hip adduction strength for the PL and NPL (p £ 0.01). In TM2, the EG performed substantially better in KE PT/BW and KF PT/BW (p £ 0.01). In both groups, no substantial bilateral strength differences were observed in knee and hip adduction performance. From TM1 to TM2, significant increases were found in knee strength in both groups (p £ 0.01). Overall, the EG players outperformed their lower-division peers in strength assessments significantly. The results also indicate significant knee and hip muscle strength increases during the season (except for the EG in hip adduction strength), probably as a response to the exposure to training and competition.

We assessed lower limb muscle activity during the execution of first and second tennis serves, exploring whether the extent of these differences is influenced by the chosen method for normalising surface electromyography (EMG) data. Ten male competitive tennis players first completed three rounds of maximal isometric voluntary contractions (MVC) of knee extensors and plantar flexors for the left (front) and right (back) leg separately, and three squat jumps. Afterward, they executed ten first and ten-second serves. Surface EMG activity of four lower limb muscles (vastus lateralis, rectus femoris, gastrocnemius lateralis, and soleus muscles) on each leg was recorded and normalised in three different ways: to MVC; to peak/maximal activity measured during squat jump; and to the actual serve. For the rectus femoris and soleus muscles of the left leg, and the gastrocnemius lateralis and soleus muscles of the right leg, EMG amplitude differed significantly between normalisation techniques (P ≤ 0.012). All muscles showed greater activity during the first serve, although this difference was only statistically significant for the right vastus lateralis muscle (P = 0.014). In conclusion, the EMG normalisation method selected may offer similar information when comparing first and second serve, at least for leg muscles studied here. [ABSTRACT FROM AUTHOR]

Problem statement: Bilateral assessment of knee strength is crucial for identifying and understanding lower limb strength asymmetry. However, there is a scarcity of data in the literature regarding the bilateral strength asymmetry of knee extensors in healthy, untrained individuals, particularly in relation to a limited number of tested positions. Aims: 1) To investigate and quantify the bilateral knee extensor strength asymmetry between the dominant (DL) and non-dominant leg (NDL) in nonathletes. 2) To utilize dynamometric measurements to assess isometric peak torque (PT) across the full range of motion. 3) To apply an optimal mathematical function for modelling the torque-angle relationship, allowing a detailed examination of the respective curves for both legs using the bilateral asymmetry index (BAI). Methods: Isometric peak torque (PT) was measured using isokinetic dynamometry (Biodex 4Pro) in a cohort of 12 male nonathletes (age of 26 ± 7 years, height of 1.8 ± 0.1 m, and body mass of 78 ± 10 kg). Measurements were taken at various positions (°): 20, 40, 60, 70, 80, 90, 100, and 110. The data were modelled with rational function, established to be the optimal and the obtained torque-angle curves were grouped in three asymmetry patterns, based on the BAI values and the behaviour of the DL and NDL curves. The physical activity of nonathletes was evaluated as a sum of points for current weekly exercise and for sport training in the childhood-adolescent age. Results: The first pattern presents low levels of bilateral asymmetry (with BAI<15%) in the whole range of motion. The second one demonstrate high asymmetry in the angular range 80°-110° (BAI up to 27%), while in the third pattern, asymmetry was observed in both ranges: 20°-40° (BAI between 28 -31%) and 90°-110° (BAI up to 26.4% and 39.7%in the range of optimal or close to optimal muscle length). Particular for the third pattern was: - a rightward shift of the maximal PT in the knee joint torque-angle curve only in the DL; - a stronger NDL in the range where extensors are shorter; and - a highest score for the level of physical activity. Conclusions: These results pointed out that leg dominance appears to form different motor skills of knee extensors, which can result in strength asymmetry variations in nonathletes. Therefore, we recommend paying attention to knee extensors strength diagnostics in nonathletes. Thus, based on the results from this research, we suggest a minimum of two knee positions for bilateral asymmetry evaluation in healthy men, one of which at shortened extensors in the range 30-50° and one at lengthened in the range 80-90°. [ABSTRACT FROM AUTHOR]

Single-pulse Transcranial Magnetic Stimulation (TMS) and, very recently, lumbar stimulation (LS) have been used to measure cortico-spinal excitability from various interventions using maximal or submaximal contractions in the lower limbs. However, reliability studies have overlooked a wide range of contraction intensities for MEPs, and no reliability data is available for LEPs. This study investigated the reliability of motor evoked potentials and lumbar evoked potentials at different stimulation intensities and contraction levels in m.rectus femoris. Twenty-two participants performed non-fatiguing isometric knee extensions at 20 and 60% of maximum voluntary contraction (MVC). LS induced a lumbar-evoked potential (LEP) of 25 and 50% resting maximal compound action potential (M-max). TMS stimulator output was adjusted to 120, 140, and 160% of active motor threshold (aMT). In each contraction, a single MEP or LEP was delivered. Ten contractions were performed at each stimulator intensity and contraction level in random order. Moderate-to-good reliability was found when LEP was normalized to M-max/ Root Mean Square in all conditions (ICC:0.74-0.85). Excellent reliability was found when MEP was normalized to Mmax for all conditions (ICC >0.90) at 60% of MVC. Good reliability was found for the rest of the TMS conditions. Moderateto-good reliability was found for silent period (SP) elicited by LS (ICC: 0.7-0.83). Good-to-excellent reliability was found for SP elicited by TMS (ICC>0.82). MEPs and LEPs elicited in m.rectus femoris appear to be reliable to assess changes at different segments of the cortico-spinal tract during different contraction levels and stimulator output intensities. Furthermore, the TMS- and LS- elicited SP was a reliable tool considered to reflect inhibitory processes at spinal and cortical levels. [ABSTRACT FROM AUTHOR]

Since the 1970s, researchers have studied a potential difference in muscle fatigue (acute strength loss) between maximal eccentric (ECCmax) and concentric (CONmax) resistance exercise. However, a clear answer to whether such a difference exists has not been established. Therefore, the aim of our paper was to overview methods and results of studies that compared acute changes in muscle strength after bouts of ECCmax and CONmax resistance exercise. We identified 30 relevant studies. Participants were typically healthy men aged 20–40 years. Exercise usually consisted of 40–100 isokinetic ECCmax and CONmax repetitions of the knee extensors or elbow flexors. Both ECCmax and CONmax exercise caused significant strength loss, which plateaued and rarely exceeded 60% of baseline, suggesting strength preservation. In upper‐body muscles, strength loss at the end of ECCmax (31.4 ± 20.4%) and CONmax (33.6 ± 17.5%) exercise was similar, whereas in lower‐body muscles, strength loss was less after ECCmax (13.3 ± 12.2%) than CONmax (39.7 ± 13.3%) exercise. Muscle architecture and daily use of lower‐body muscles likely protects lower‐body muscles from strength loss during ECCmax exercise. We also reviewed seven studies on muscle fatigue during coupled ECCmax‐CONmax exercise and found similar strength loss in the ECC and CON phases. We also found evidence from three studies that more ECC than CON repetitions can be completed at equal relative loads. These results indicate that muscle fatigue may manifest differently between ECCmax and CONmax resistance exercise. An implication of the results is that prescriptions of ECC resistance exercise for lower‐body muscles should account for greater fatigue resilience of these muscles compared to upper‐body muscles. [ABSTRACT FROM AUTHOR]

Muscle strength and balance ability have been related to injury prevention and game actions performance in football. The aims of this study are twofold: (1) to examine bilateral asymmetries in isokinetic strength and balance assessments, and (2) to evaluate the relationship between muscle strength and balance measures. Eighty-eight male adolescent football players were assessed for body composition (InBody 770), isokinetic strength (Biodex System 4 Pro Dynamometer), and balance performance (Biodex Balance System). Paired samples t-tests were conducted to determine bilateral differences in strength and balance. Pearson correlations and multiple linear regression analyses evaluated the relationship between strength and balance. No significant bilateral strength differences were observed in knee flexors (KF) and knee extensors (KE) peak torque (PT) scores. No bilateral differences were found for balance measures, except in the lateromedial stability index, which was better while performing with the preferred leg (p ≤ 0.01; d = 0.29). Significant correlations were found between KE PT and balance tasks (p ≤ 0.01). KF and KE PT shared between 18 and 22% of the common variance in the overall stability index in the non-preferred and preferred leg, respectively. Monitoring strategies of bilateral asymmetries may be crucial to enhance performance in tasks underpinned by unilateral movements, such as changes of direction and sprints, and to identify players at risk of injury. Adopting these strategies during the early stages of football training might be crucial for players' long-term development. [ABSTRACT FROM AUTHOR]

Transcranial magnetic stimulation (TMS) is a research tool that has potential to provide new insights into strength training-induced adaptations. However, using TMS to study the lower limbs is challenging, and sex-specific reliability has yet to be reported. We examined the reliability of corticospinal excitability and silent periods for the rectus femoris, vastus lateralis, and biceps femoris in both sexes. Thirteen males and 14 females reported to the laboratory twice. During both trials, a double cone coil was used to deliver 20 pulses to the rectus femoris hotspot with a stimulator output of 130% of active motor threshold. Motor-evoked potential peak-to-peak amplitude, which reflects corticospinal excitability, and silent period duration were quantified. Our results offer 4 novel findings. First, corticospinal excitability and silent period demonstrated higher reliability for the females. Second, regardless of sex and muscle, the silent period was more reliable than corticospinal excitability. Third, reliability was highest for our target muscle (rectus femoris), with lower reliability for the vastus lateralis and biceps femoris, suggesting that these methods cannot be used to study coactivation. Fourth, active motor threshold showed less variability than corticospinal excitability and silent period but increased at trial 2 in females. Many of the intraclass correlation coefficients were excellent ($0.90), although we attribute this finding to variability between subjects. Reliability of lower-limb TMS measures may be sex, muscle, and variable dependent. Our findings suggest that both males and females should be included in lower-limb TMS research, although combining data between sexes should be approached cautiously. [ABSTRACT FROM AUTHOR]

In this study, we compared the knee strength parameters tested in professional football players at the preparatory and competitive seasons, separated by 12 weeks. Knee extensors and flexors peak torque, hamstring to quadriceps conventional ratio (H/Qcon), and bilateral asymmetry index (AI) on the dominant and non-dominant limbs of 10 football players were evaluated in an isokinetic dynamometer. Paired t-test was used to compare all variables between the preparatory and competitive seasons. A one-sample t-test was used to compare the H/Qcon and AI with theoretical means defined as 0.6 and 10%, respectively. The results showed a significant peak torque improvement in the dominant side for both knee extensors (p=0.013) and flexors (p=0.003), whereas on the non-dominant limb there was no significant difference. Regarding the H/Qcon, dominant and non-dominant sides were below 0.6 during the preparatory and competitive seasons (p

Studies have reported that a stiff triceps surae muscle and tendonaponeurosis and also a more compliant quadriceps muscle and tendon-aponeurosis, are related to lower oxygen cost during running. However, to date, no study has investigated in a single experiment how oxygen cost during running is related to the stiffness of the free tendons (Achilles tendon, patellar tendon) and all the superficial muscles of two major muscle groups for running (i.e., quadriceps, triceps surae). Thus, 17 male trained runners/triathletes participated in this study and visited the laboratory on three occasions. On the first day, the participants were familiarized with the tests. On the second day, the passive compression stiffness of the triceps surae muscle (i.e., gastrocnemii), Achilles tendon, quadriceps muscle (i.e., vastii, rectus femoris), and patellar tendon was non-invasively measured using a digital palpation device (MyotonPRO). In addition, an incremental test was applied to test the VO2max of the participants. Thereafter, in the third visit, after at least 48-h of rest, participants performed a 15-min run on the treadmill with a speed reflecting a velocity of 70% VO2max, to assess oxygen costs during running. The Spearman correlation showed a significant negative correlation between passive Achilles tendon compression stiffness and running oxygen consumption, with a large effect size (rρ = -0.52; CI (95%) -0.81 to -0.33; P = 0.03). Moreover, no further significant relationship between oxygen cost during running and the passive compression stiffness of the quadriceps muscle and patellar tendon, as well as the triceps surae muscle, was detected. The significant correlation indicates that a stiffer passive Achilles tendon can lead to a lower oxygen cost during running. Future studies will have to test the causality of this relationship with training methods such as strength training that are able to increase the Achilles tendon stiffness. [ABSTRACT FROM AUTHOR]

This study examined the effects of unilateral resistance training (RT) on maximal velocity parameters in the ipsilateral and contralateral legs in young and older males. Young (n = 22; age = 21.55 ± 2.23 years) and older (n = 20; age = 65.10 ± 9.65 years) males were assigned to training or control groups. Unilateral isokinetic RT of the knee extensors was performed for 4 weeks. Peak velocity and acceleration were identified during a dynamic maximal voluntary contraction before (PRE), at Week 2 (MID), and after Week 4 (POST) of RT. Age-independent increases in peak velocity (1.5%) and acceleration (4.5%) were demonstrated at POST for the trained leg. For the untrained leg, acceleration increased (4.3%) at POST similarly between training groups. These findings provide evidence for the high degree of neuromuscular plasticity, regardless of age, during the early phase of RT, and the potential for cross education of acceleration. [ABSTRACT FROM AUTHOR]

This study investigated the etiology of neuromuscular fatigue induced by a 5-m shuttle run test (5MSRT) in soccer players. Nineteen adult male amateur soccer players (age: 20.0 ± 2.9 years) participated in the present study. Before and after the 5MSRT, they were instructed to complete a maximal voluntary isometric contraction (MVIC) of the knee extensors (KE) during and after which two electrical stimulations were applied at the femoral nerve. Voluntary activation level (VAL), surface electromyography recordings (sEMG), electrophysiological (Mmax) and potentiated resting twitch (Ptw) responses of the KE were compared between pre- and post-5MSRT. Rating of perception exertion (RPE) was also assessed before, during the test immediately following each sprint repetition and after the test. The distance covered during each sprint significantly decreased as the number of trials performed increased (p<.05). The RPE reported following each sprint significantly increased throughout the test. In addition, MVIC (-9%), sEMG (-23%), VAL (-15%), Ptw (-26%) and Mmax (~22%) of the KE were lowered from pre-to-post 5MSRT (.001 < p < .01). The 5MSRT induced a decrease of repeated-sprint running performance and MVIC of the KE. These decrements were accompanied by lowered VAL, sEMG, Ptw and Mmax values of the KE reflecting the involvement of both the central and peripheral origins in the 5MSRT-induced fatigue. Given the important muscle stress induced by 5MSRT, this strenuous test must be applied with caution, after an inevitable familiarization phase, and not during the competition period to avoid the risk of serious injury. [ABSTRACT FROM AUTHOR]

Purpose: The present study aimed to directly compare the effects of 30 min muscle (VIBmuscle) vs. tendon (VIBtendon) local vibration (LV) to the quadriceps on maximal voluntary isometric contraction (MVIC) and rate of torque development (RTD) as well as on central nervous system excitability (i.e. motoneuron and cortical excitability).Methods: Before (PRE) and immediately after (POST) LV applied to the quadriceps muscle or its tendon, we investigated MVIC and RTD (STUDY #1; n = 20) or vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) electromyography responses to thoracic electrical stimulation (TMEPs; motoneuron excitability) and transcranial magnetic stimulation (MEPs; corticospinal excitability) (STUDY #2; n = 17). MEP/TMEP ratios were further calculated to quantify changes in cortical excitability.Results: MVIC decreased at POST (P = 0.017) without any difference between VIBtendon and VIBmuscle, while RTD decreased for VIBtendon (P = 0.013) but not VIBmuscle. TMEP amplitudes were significantly decreased for all muscles (P = 0.014, P < 0.001 and P = 0.004 for VL, VM and RF, respectively) for both LV sites. While no changes were observed for MEP amplitude, MEP/TMEP ratios increased at POST for VM and RF muscles (P = 0.009 and P = 0.013, respectively) for both VIBtendon and VIBmuscle.Conclusion: The present results suggest that prolonged muscle and tendon LV are similarly effective in modulating central nervous system excitability and decreasing maximal force. Yet, altered explosive performance after tendon but not muscle LV suggests greater neural alterations when tendons are vibrated. [ABSTRACT FROM AUTHOR]

Purpose: Although quadriceps weakness after ACL reconstruction (ACLR) is well documented, the magnitude of reported weakness varies considerably. Such variation raises the possibility that certain patients may be more susceptible to quadriceps weakness after ACLR. This meta-analysis identified factors explaining between-study variability in quadriceps weakness post-ACLR.Methods: Studies between 2010 and 2020 were screened for the following criteria: human subjects, unilateral ACLR, and strength reported both for the ACLR leg and the uninjured or healthy-control leg. 122 studies met the criteria, resulting in 303 and 152 Cohen's d effect sizes (ESs) comparing ACLR legs to uninjured legs (a total of 4135 ACLR subjects) and to healthy controls (a total of 1,507 ACLR subjects vs. 1-193 healthy controls), respectively. Factors (time, graft, sex, activity, mass/height, geographic area, concomitant injury, and type of strength testing) that may affect study ES were examined.Results: Meta-regressions indicated an association between time post-ACLR and study ESs (P < 0.001) and predicted full recovery (ES = 0) to occur at 54-59 months post-ACLR. When compared to uninjured legs, patients with patellar tendon autografts had greater deficits than studies using hamstring tendon autografts (P = 0.023). When compared to uninjured legs, studies including only males reported greater deficits than studies combining males and females (P = 0.045); whereas when compared to healthy controls, studies combining males and females reported greater deficits than studies with males (P = 0.013). When compared to controls, studies from USA reported greater deficits than studies from Europe (P = 0.003). Increased isokinetic-testing speed was associated with smaller deficits (P ≤ 0.025). Less than 25% of patients achieved a between-limb symmetry in quadriceps strength > 90% between 6 and 12 months post-ACLR.Conclusion: Time post-surgery, graft, sex, geographic location, and isokinetic speed influenced the magnitude of post-ACLR quadriceps weakness. Patients with patellar tendon autografts demonstrated greater between-limb asymmetry in quadriceps strength, while female strength deficits were underestimated to a greater extent. A slower isokinetic speed provided a more sensitive assessment of quadriceps strength post-ACLR. The overwhelming majority of patients were returning to sport with significantly impaired quadriceps strength.Level Of Evidence: III. [ABSTRACT FROM AUTHOR]

This study investigated the effect of 1-week oral administration of propolis on muscle fatigue and recovery after performing a fatigue task (total 100 maximal voluntary concentric knee extension repetitions). In this placebo-controlled, double-blind study, 18 young men consumed a formulation with high Brazilian green propolis dose (H-BGP), a formulation with low Brazilian green propolis dose, or a placebo, for 1 week before performing the fatigue task (an interval between each intervention: 1–2 weeks). Maximal voluntary contraction torque, central fatigue (voluntary activation and root mean square values of the surface electromyography amplitude), and peripheral fatigue (potentiated triplet torque) were assessed before, immediately after, and 2 minutes after the fatigue task. Maximal voluntary contraction torque decreased immediately after the fatigue task in all conditions (P<0.001); however, it recovered from immediately after to 2 minutes after the fatigue task only in the H-BGP condition (P<0.001). Furthermore, there was a significant decrease in voluntary activation (P<0.001) and root mean square values of the surface electromyography amplitude (P≤0.035) only in the placebo condition. No significant difference was observed in the time-course change in potentiated triplet torque between the conditions. These results suggest that oral administration of propolis promotes muscle fatigue recovery by reducing central fatigue. [ABSTRACT FROM AUTHOR]

A novel exoskeleton robotic system was developed to assist stair climbing. This active demonstrator consists of a motor with a cable system, various sensors, and a control system with a power supply. The objective of this preliminary study is a biomechanical evaluation of the novel system to determine its effectiveness in use. For this purpose, three test persons were biomechanically investigated, who performed stair ascents and descents with and without the exoskeleton. Kinematics, kinetics, and muscle activity of the knee extensors were measured. The measured data were biomechanically simulated in order to evaluate the characteristics of joint angles, moments, and reaction forces. The results show that the new exoskeleton assists both the ascent and the descent according to the measured surface electromyography (sEMG) signals, as the knee extensors are relieved by an average of 19.3%. In addition, differences in the interaction between the test persons and the system were found. This could be due to a slightly different operation of the assisting force or to the different influence of the system on the kinematics of the users. [ABSTRACT FROM AUTHOR]

Mechanomyography (MMG) may be used to quantify very small motor responses resulting from muscle activation, voluntary or involuntary. The purpose of this study was to investigate the MMG mean peak amplitude (MPA) and area under the curve (AUC) and the corresponding mechanical responses following delivery of transcranial magnetic stimulation (TMS) to the knee extensors. Fourteen adults (23 ± 1 years) received single TMS pulses at intensities from 30-80 % maximum stimulator output to elicit muscle responses in the relaxed knee extensors while seated. An accelerometer-based sensor was placed on the rectus femoris (RF) and vastus lateralis (VL) muscle bellies to measure the MMG signal. Pearson correlation revealed a positive linear relationship between MMG MPA and TMS intensity for RF (r=0.569; p<0.001) and VL (r=0.618; p<0.001). TMS intensity of ≥60 % maximum stimulator output produced significantly higher MPA than at 30 % TMS intensity and evoked measurable movement at the knee joint. MMG MPA was positively correlated to AUC (r=0.957 for RF and r=0.603 for VL; both p<0.001) and knee extension angle (r=0.596 for RF and r=0.675 for VL; both p<0.001). In conclusion, MMG captured knee extensor mechanical responses at all TMS intensities with the response increasing with increasing TMS intensity. These findings suggest that MMG can be an additional tool for assessing muscle activation., (© 2024 Walter de Gruyter GmbH, Berlin/Boston.)

Low fractions of inspired oxygen (FI O2 ; i.e., hypoxia) affect aspects of skeletal muscle contractility in humans, but it remains unclear if post-activation potentiation (PAP) and the torque-frequency (T-F) relationship are altered. We investigated the effects of two (H2) and four hours (H4) of normobaric hypoxia (FI O2 =0.11±0.47) on the magnitude of PAP of the knee extensors (KE) and the T-F relationship of the dorsiflexors (DF) in 13 and 12 healthy participants, respectively. To assess PAP, a resting twitch was evoked via femoral nerve stimulation before, and 2-300 s following a 10-s maximal voluntary contraction (MVC). A T-F relationship was obtained by stimulating the common fibular nerve with a single pulse and 1-s trains between 5-100 Hz. During hypoxia, peripheral oxygen saturation decreased by ~18% from 98.0±0.8% at baseline (P<0.001). MVC force and voluntary activation (VA) of the KE were lower than baseline throughout hypoxia (e.g., ~8 and ~5%, respectively at H2; P≤0.027); however, the magnitude of PAP was not altered by hypoxia (P≥0.711). Surprisingly, PAP did increase with time across the control day (P≤0.012). MVC torque and VA of the DF were unaffected by hypoxia (P≥0.127), but the estimated frequency required to evoke 50 % of 100-Hz torque increased by ~1.2 Hz at H2 (P≤0.021). These results imply that two hours of normobaric hypoxia were sufficient to: i) impair neural drive to the KE but not the mechanism(s) responsible for PAP, and ii) lead to a rightward shift of the T-F relationship for the DF.

Purpose: Drawing on correlations between the mechanomyographic (MMG) and the force signal, we devised a novel approach based on MMG signal analysis to detect voluntary activation (VA) of the synergistic superficial heads of the quadriceps muscle. We hypothesized that, after a fatiguing exercise, the changes in the evoked MMG signal of each quadriceps head would correlate with the changes in the level of VA in the whole quadriceps. Methods: Twenty-five men underwent a unilateral single-leg quadriceps exercise to failure. Before and after exercise, VA was assessed by interpolated-twitch-technique via nerve stimulation during and after maximum voluntary contraction (MVC). The force and MMG signal were recorded from vastus lateralis, vastus medialis, and rectus femoris. The MMG peak-to-peak was calculated and the voluntary activation index (VAMMG), defined as the superimposed/potentiated MMG peak-to-peak ratio, was determined from the MMG signal for each head. Results: VAMMG presented a very high intraclass correlation coefficient (0.981–0.998) and sensitivity (MDC95%: 0.42–6.97%). MVC and VA were decreased after exercise in both the exercising [MVC:−17(5)%, ES −0.92; VA: −7(3)%, ES −1.90] and the contralateral limb [MVC: −9(4)%, ES −0.48; VA: −4(1)%, ES −1.51]. VAMMG was decreased in both the exercising [~ −9(6)%, ES −1.77] and contralateral limb [~ −3(2)%, ES −0.57], with a greater decrease in VAMMG noted only in the vastus medialis of the exercising limb. Moderate-to-very high correlations were found between VAMMG and VA (R-range: 0.503–0.886) before and after exercise. Conclusion: VAMMG may be implemented to assess VA and provide further information when multiple synergistic muscle heads are involved in fatiguing exercises. [ABSTRACT FROM AUTHOR]

To give people more specific information on the quality of their daily motion, it is necessary to continuously measure muscular activity during everyday occupations in an easy way. The traditional methods to measure muscle activity using a combination of surface electromyography (sEMG) sensors and optical motion capture system are expensive and not suitable for non-technical users and unstructured environment. For this reason, in our group we are researching methods to estimate leg muscle activity using non-contact wearable sensors, improving ease of movement and system usability. In a previous study, we developed a method to estimate muscle activity via only a single inertial measurement unit (IMU) on the shank. In this study, we describe a method to estimate muscle activity during walking via two IMU sensors, using an original sensing system and specifically developed estimation algorithms based on ANN techniques. The muscle activity estimation results, estimated by the proposed algorithm after optimization, showed a relatively high estimation accuracy with a correlation efficient of R2 = 0.48 and a standard deviation STD = 0.10, with a total system average delay of 192 ms. As the average interval between different gait phases in human gait is 250–1000 ms, a 192 ms delay is still acceptable for daily walking requirements. For this reason, compared with the previous study, the newly proposed system presents a higher accuracy and is better suitable for real-time leg muscle activity estimation during walking. [ABSTRACT FROM AUTHOR]

The purpose of this study was to assess whether dynamic torque ratios (DCR) from isokinetic strength assessments of eccentric knee flexors (eccKF) and concentric knee extensors (conKE) display differences when stratified into specific angle-specific DCR (DCRAST) groups. Fifty-two professional female soccer players (age 21.30 ± 4.44 years; height 166.56 ± 5.17 cm; mass 61.55 ± 5.73 kg) from the English Women's Super League completed strength assessments of both lower limbs on an isokinetic dynamometer at 60°∙s−1. Angle-specific torque (AST) were used to calculate DCRAST to create sub-groups using clustering algorithms. The results identified for the dominant side that the Medium DCRAST group elicited significantly higher conKE AST when compared to Low and High DCRAST groups at increased knee extension (P ≤ 0.05). For the non-dominant side, the High DCRAST group had significantly higher and lower eccKF and conKE AST compared to the Low DCRAST group at increased knee extension (P ≤ 0.05). This study highlights that the inclusion of AST data may subsequently help practitioners to prescribe exercise that promotes strength increases at targeted joint angles. In turn, these approaches can be used to help reduce injury risk, identify rehabilitation responses and help inform return to play. [ABSTRACT FROM AUTHOR]

Purpose: To describe the effects of uphill walking versus cycling exercises on knee-extensor (KE) neuromuscular properties and subsequent running exercise. Methods: Nine athletes performed 4 different sessions (1 familiarization and 3 experimental sessions, visit 2-4). Visit 2 (cycling + 10-km condition) consisted of the completion of 1-h cycling followed by a 10-km running time trial. Visit 3 consisted of the completion of 1-h uphill walking followed by a 10-km running exercise (RE). During the fourth visit, athletes only ran 10 km. Visits 3 and 4 were randomized. The uphill walking and cycling exercises were performed at the same intensity, and pacing of the RE was similar between conditions. Neuromuscular function of the KE was assessed before warm-up, after first exercise, and after RE. Heart rate and rating of perceived exertion (RPE) were recorded during all exercises. Results: RPE during RE was greater following the 1-h cycling and uphill walking exercises than during RE alone. KE force (-21%), twitch torque (-20%), doublet torque (-16%), and twitch rate of force development (-13%) significantly decreased following cycling exercise and not after uphill walking exercise. Postactivation potentiation was observed after uphill walking and RE. KE force-production capacity partially recovered after running in the cycling + 10-km condition. Conclusion: Uphill walking and running induced postactivation potentiation, limiting the decrease in KE force postexercise. Despite different alterations in force-production capacity induced by cycling and uphill walking, both exercises increased perception of effort during the subsequent RE. [ABSTRACT FROM AUTHOR]

Participants exposed to a simulated slip with forward loss of balance (FLB) develop large lower limb joint moments which may be a limiting factor for those whose muscle-tendon units' (MTUs) properties are deteriorated. Whether the age-related decline in these properties limits participants' capacity to recover their balance following a slip with FLB remains unclear. We combined isokinetic dynamometry, ultrasound and EMG to understand how knee extensor and ankle plantarflexor muscle strength and power, rate of moment development, electromechanical delay, and tendon stiffness affected the balance of young (25.3 ± 3.9 years) and older adults (62.8 ± 7.1 years) when recovering from a single slip with FLB triggered whilst walking on a split-belt instrumented treadmill. Except for the patellar tendon's stiffness, knee extensor and ankle plantarflexor electromechanical delays, older adults' MTUs properties were deteriorated compared to those of young participants (p < 0.05). We found no significant relationship between age or the MTUs properties of participants and balance recovery. These findings provide additional support that neither maximal nor explosive strength training are likely to be successful in preventing a fall for healthy older adults, and that other type of interventions, such as task-specific training that has already proved efficacious in reducing the risk of falling, should be developed. [ABSTRACT FROM AUTHOR]

Little is known about how muscle length affects residual force enhancement (rFE) in humans. We therefore investigated rFE at short, long, and very long muscle lengths within the human quadriceps and patellar tendon (PT) using conventional dynamometry with motion capture (rFETQ) and a new, non-invasive shear-wave tensiometry technique (rFEWS). Eleven healthy male participants performed submaximal (50% max.) EMG-matched fixed-end reference and stretch-hold contractions across these muscle lengths while muscle fascicle length changes of the vastus lateralis (VL) were captured using B-mode ultrasound. We found significant rFETQ at long (7±5%) and very long (12±8%), but not short (2±5%) muscle lengths, whereas rFEWS was only significant at the very long (38±27%), but not short (8±12%) or long (6±10%) muscle lengths. We also found significant relationships between VL fascicle length and rFETQ (r=0.63, p=0.001) and rFEWS (r=0.52, p=0.017), but relationships were not significant between VL fascicle stretch amplitude and rFETQ (r=0.33, p=0.126) or rFEWS (r=0.29, p=0.201). Squared PT shear-wave-speed-angle relationships did not agree with estimated PT force-angle relationships, which indicates that estimating PT loads from shear-wave tensiometry might be inaccurate. We conclude that increasing muscle length rather than stretch amplitude contributes more to rFE during submaximal voluntary contractions of the human quadriceps.

This study examined the influence of knee extensors' hip and knee angle on force production capacity and their neuromuscular and architectural consequences. Sixteen healthy men performed sub-maximal and maximal voluntary isometric contractions (MVIC) of knee extensors with four different combinations of the knee and hip angles. Muscle architecture, excitation-contraction coupling process, muscular activity, and corticospinal excitability were evaluated on the vastus lateralis (VL) and rectus femoris (RF) muscles. MVIC and evoked peak twitch (Pt) torques of knee extensors increased significantly (p < 0.05) by 42 ± 12% and 47 ± 16% on average, respectively, under knee flexed positions (110° flexion, 0° = full extension) compared to knee extended positions (20° flexion) but were not different between hip positions (i.e., 0° or 60° flexion). Knee flexion also affected VL and RF muscle and fascicle lengths toward greater length than under knee extended position, while pennation angle decreased for both muscles with knee flexion. Pennation angles of the VL muscle were also lower under extended hip positions. Alternatively, no change in maximal muscle activation or corticospinal activity occurred for the VL and RF muscles across the different positions. Altogether these findings evidenced that MVIC torque of knee extensors depended particularly upon peripheral contractile elements, such as VL and RF muscle and fascicle lengths, but was unaffected by central factors (i.e., muscle activation). Furthermore, the hip position can affect the pennation angle of the VL, while VL muscle length can affect the pennation angle of the RF muscle. These elements suggest that the VL and RF muscles exert a mutual influence on their architecture, probably related to the rectus-vastus aponeurosis. [ABSTRACT FROM AUTHOR]

The purpose of this study was to investigate muscle activity and intermuscular coherence of the rectus femoris (RF) and biceps femoris (BF) during forward (FW) and backward (BW) pedaling. Sixteen healthy volunteers performed FW and BW pedaling in 30, 45, and 60 revolutions per minute (RPM), while electromyographic (EMG) signals of the RF and BF were recorded bilaterally to determine integral EMG and intermuscular coherence. BW pedaling showed a statistically significant larger EMG activity on the left BF (P = 0.023) in 30 RPM; on the left BF (P = 0.01), right BF (P = 0.05), and right RF (P = 0.006) in 45 RPM, and on the left BF (P = 0.014) and right RF (P = 0.011) in 60 RPM than FW pedaling. In 45 RPM, higher coherence was demonstrated on the left leg (P = 0.011) during the left flexor and right extensor phases and on the right leg (P = 0.043) during the right flexor and left extensor phases in BW compared with FW pedaling. In 60 RPM, higher coherence was observed on both legs (left, P = 0.037; right, P < 0.001) during the left flexor and right extensor phases in BW compared with FW pedaling. Our results suggest that BW pedaling increased the muscle activity of both biarticular muscles and intermuscular coherence.

This study examined the influence of knee extensors’ hip and knee angle on force production capacity and their neuromuscular and architectural consequences. Sixteen healthy men performed sub-maximal and maximal voluntary isometric contractions (MVIC) of knee extensors with four different combinations of the knee and hip angles. Muscle architecture, excitation-contraction coupling process, muscular activity, and corticospinal excitability were evaluated on the vastus lateralis (VL) and rectus femoris (RF) muscles. MVIC and evoked peak twitch (Pt) torques of knee extensors increased significantly (p

Abstract Background Low level laser therapy (LLLT) is among novel methods for preventing and treating muscle damage and soreness induced by volitional exercise, but little is known about using LLLT before neuromuscular electrical stimulation. The aim of this first randomised, double blind, crossover trial addressing this issue was to evaluate effects of LLLT on muscle damage and oxidative stress, as well as recovery of muscle function after a single session of isometric neuromuscular electrical stimulation(NMES). Methods Twenty four moderately active, healthy men aged 21–22 years received 45 electrically evoked tetanic, isometric contractions of the quadriceps femoris, preceded by LLLT or sham-LLLT. Maximal isometric voluntary muscle torques, perceived soreness, and blood samples were analysed from baseline to 96 h post intervention. We measured plasma markers of muscle damage (the activity of creatine kinase), and inflammation (C-reactive protein), and evaluated redox state parameters. Results NMES-evoked contractions induced oxidative stress, demonstrated by an increase in lipid peroxidation and impairments in enzymatic antioxidant system. LLLT irradiations had a protective effect on NMES-induced decrease in enzymatic antioxidant defence and shortened the duration of inflammation. This effect of irradiations on redox state and inflammation did not affect lipid peroxidation, muscle damage, and muscle torque. Conclusions LLLT may protect from impairments in enzymatic antioxidant system and may shorten inflammation induced by a single NMES session in moderately active, healthy men. However, the effects of LLLT on redox state and inflammatory processes do not seem to affect muscle damage and recovery of muscle function after NMES. Trial registration The study was retrospectively registered in the Australian New Zealand Clinical Trials Registry (ANZCTR); The trial registration number: ACTRN12619000678190; date of registration: 6 May 2019.

The etiology of changes in lower‐limb neuromuscular function, especially to the central nervous system, may be affected by exercise duration. Direct evidence is lacking as few studies have directly compared different race distances. This study aimed to investigate the etiology of deficits in neuromuscular function following short versus long trail‐running races. Thirty‐two male trail runners completed one of five trail‐running races as LONG (>100 km) or SHORT (<60 km). Pre‐ and post‐race, maximal voluntary contraction (MVC) torque and evoked responses to electrical nerve stimulation during MVCs and at rest were used to assess voluntary activation and muscle contractile properties of knee‐extensor (KE) and plantar‐flexor (PF) muscles. Transcranial magnetic stimulation (TMS) was used to assess evoked responses and corticospinal excitability in maximal and submaximal KE contractions. Race distance correlated with KE MVC (ρ = −0.556) and twitch (ρ = −0.521) torque decreases (p ≤.003). KE twitch torque decreased more in LONG (−28 ± 14%) than SHORT (−14 ± 10%, p =.005); however, KE MVC time × distance interaction was not significant (p =.073). No differences between LONG and SHORT for PF MVC or twitch torque were observed. Maximal voluntary activation decreased similarly in LONG and SHORT in both muscle groups (p ≥.637). TMS‐elicited silent period decreased in LONG (p =.021) but not SHORT (p =.912). Greater muscle contractile property impairment in longer races, not central perturbations, contributed to the correlation between KE MVC loss and race distance. Conversely, PF fatigability was unaffected by race distance. [ABSTRACT FROM AUTHOR]

A novel exoskeleton robotic system was developed to assist stair climbing. This active demonstrator consists of a motor with a cable system, various sensors, and a control system with a power supply. The objective of this preliminary study is a biomechanical evaluation of the novel system to determine its effectiveness in use. For this purpose, three test persons were biomechanically investigated, who performed stair ascents and descents with and without the exoskeleton. Kinematics, kinetics, and muscle activity of the knee extensors were measured. The measured data were biomechanically simulated in order to evaluate the characteristics of joint angles, moments, and reaction forces. The results show that the new exoskeleton assists both the ascent and the descent according to the measured surface electromyography (sEMG) signals, as the knee extensors are relieved by an average of 19.3%. In addition, differences in the interaction between the test persons and the system were found. This could be due to a slightly different operation of the assisting force or to the different influence of the system on the kinematics of the users.

The spectral properties of surface electromyographic (EMG) signal in the rectus femoris (RF) and the coactivation in the medial hamstrings (MH) were investigated in 45 stroke subjects (22 ± 12 days post-onset) and 30 age-matched healthy controls who performed unilateral knee extensions at maximum effort (100% MVC) and during 5-s force-matching tasks (10, 30, 50% MVC). The spectral properties were obtained through a power spectrum analysis based on Fast Fourier Transform. The coactivation was measured as the MH amplitude (%max) and MH/RF amplitude ratio. Force variability was expressed as the coefficient of variation. Both knee extensors and flexors were weaker in the paretic leg than the non-paretic and control legs (p < 0.001). A significantly higher relative power in the 5–13 and 13–30 Hz bands was found in the paretic than the non-paretic leg across all force levels (p ≤ 0.001) without changes in the 30–60 and 60–100 Hz bands or the mean and median frequencies. Regarding the antagonist coactivation, MH amplitude in the paretic leg was higher than in the non-paretic leg (submaximal levels, p < 0.0001) and the control leg (all force levels, p = 0.0005) with no differences between legs in the MH/RF ratio. The steadiness of the knee extension force was not related to the spectral properties of the agonist EMG or antagonistic coactivation. Greater coactivation was associated with weaker paretic knee flexors (p ≤ 0.0002). The overall results suggest variably altered agonist activation and antagonistic coactivation over the range of isometric knee extension contractions in subacute stroke. [ABSTRACT FROM AUTHOR]

Purpose: The magnitude of muscle damage induced by maximal eccentric exercise is attenuated when the same exercise is repeated by homologous muscle of the ipsilateral or contralateral limb. It is not known if the muscle damage--protective effect is also transferred to nonhomologous muscles. The present study investigated the effects of unilateral knee extensor (KE) or flexor (KF) eccentric exercise on muscle damage induced by elbow flexor (EF) eccentric exercise of the ipsilateral or contralateral side. Methods: Young healthy sedentary men were assigned to four experimental groups (n = 13 per group) that performed five sets of six maximal eccentric contractions (MaxEC) of the KE or KF of the same or opposite side of the arm that performed MaxEC of the EF 1 wk later, and a control group that performed two bouts of MaxEC of the EF using a different arm for each bout separated by 1 wk. Changes in several indirect muscle damage markers were compared among the groups by mixed-design, two-way ANOVA. Results: Changes in maximal voluntary concentric contraction torque, range of motion, muscle soreness, and plasma creatine kinase activity after KE or KFMaxEC were not different (P > 0.05) between legs, but greater (P < 0.05) after KF than KEMaxEC. The changes in the variables after EFMaxEC in the experimental groups were not different (P > 0.05) from the first bout of the control group but larger (P < 0.05) than the second bout of the control group, and no differences between the ipsilateral and contralateral sides were evident. Conclusions: These results showed that no protective effect on EFMaxEC was conferred by the leg exercises, suggesting that muscle damage protection was not transferred from KE or KF to EF. [ABSTRACT FROM AUTHOR]

The effects of high-load resistance training on muscle strength and muscle mass depend on rest periods between sets. However, whether differences in rest period length during low-load resistance exercise (RE) has an influence on improving muscle characteristics remains unclear. Better understanding such effects would enable us to prescribe low-load resistance exercise more safely and effectivity. The purpose of this study was to investigate the acute effects of low-load RE on muscle swelling with different length rest periods between sets. A total of 42 young men (age, 22.9 ± 2.4 years; height, 172.1 ± 5.4 cm; body mass, 65.6 ± 6.5 kg) were recruited to participate in this study. They were assigned to one of three groups with different rest periods between sets (20 s [seconds], 60 s, or 180 s). A total of 12 sets of 10 repetitions of RE with 30% of one repetition maximum on knee extensor muscles were performed. Muscle thickness of the vastus lateralis was measured using ultrasonography as an indicator of muscle swelling every 3 sets. Muscle thickness significantly increased after 3 sets of RE in the 20-s (3.9 ± 3.3%) and 60-s groups (5.9 ± 3.8%), but only after 12 sets in the 180-s group (4.3 ± 3.1%). RE with rest periods shorter than 60 s could result in exercise-induced muscle swelling after fewer sets of RE.

To give people more specific information on the quality of their daily motion, it is necessary to continuously measure muscular activity during everyday occupations in an easy way. The traditional methods to measure muscle activity using a combination of surface electromyography (sEMG) sensors and optical motion capture system are expensive and not suitable for non-technical users and unstructured environment. For this reason, in our group we are researching methods to estimate leg muscle activity using non-contact wearable sensors, improving ease of movement and system usability. In a previous study, we developed a method to estimate muscle activity via only a single inertial measurement unit (IMU) on the shank. In this study, we describe a method to estimate muscle activity during walking via two IMU sensors, using an original sensing system and specifically developed estimation algorithms based on ANN techniques. The muscle activity estimation results, estimated by the proposed algorithm after optimization, showed a relatively high estimation accuracy with a correlation efficient of R2 = 0.48 and a standard deviation STD = 0.10, with a total system average delay of 192 ms. As the average interval between different gait phases in human gait is 250–1000 ms, a 192 ms delay is still acceptable for daily walking requirements. For this reason, compared with the previous study, the newly proposed system presents a higher accuracy and is better suitable for real-time leg muscle activity estimation during walking.

Participants exposed to a simulated slip with forward loss of balance (FLB) develop large lower limb joint moments which may be a limiting factor for those whose muscle-tendon units’ (MTUs) properties are deteriorated. Whether the age-related decline in these properties limits participants’ capacity to recover their balance following a slip with FLB remains unclear. We combined isokinetic dynamometry, ultrasound and EMG to understand how knee extensor and ankle plantarflexor muscle strength and power, rate of moment development, electromechanical delay, and tendon stiffness affected the balance of young (25.3 ± 3.9 years) and older adults (62.8 ± 7.1 years) when recovering from a single slip with FLB triggered whilst walking on a split-belt instrumented treadmill. Except for the patellar tendon’s stiffness, knee extensor and ankle plantarflexor electromechanical delays, older adults’ MTUs properties were deteriorated compared to those of young participants (p < 0.05). We found no significant relationship between age or the MTUs properties of participants and balance recovery. These findings provide additional support that neither maximal nor explosive strength training are likely to be successful in preventing a fall for healthy older adults, and that other type of interventions, such as task-specific training that has already proved efficacious in reducing the risk of falling, should be developed.

The purpose of this study was to determine the effects of training load (25% vs. 75% of one repetition maximum [1RM]) and fatigue (failure vs. non‐failure) during four weeks of unilateral knee extension resistance training (RT) on maximal voluntary force in the trained and the untrained knee extensors. Healthy young adults (n = 42) were randomly assigned to control (CON, n = 9, 24 ± 4.3 years), low‐load RT to failure (LLF, n = 11, 21 ± 1.3 years, three sets to failure at 25% of 1RM), high‐load RT to failure (HLF, n = 11, 21 ± 1.4 years, three sets to failure at 75% of 1RM), and high‐load RT without failure (HLNF, n = 11, 22 ± 1.5 years, six sets of five repetitions at 75% of 1RM) groups. Before and after the four weeks of training, 1RM, maximal voluntary isometric force, and corticospinal excitability (CSE) were measured. 1RM in the trained (20%, d = 0.70, 15%, d = 0.61) and the untrained knee extensors (5%, d = 0.27, 6%, d = 0.26) increased only in the HLF and HLNF groups, respectively. MVIC force increased only in the trained leg of the HLF (5%, d = 0.35) and HLNF groups (12%, d = 0.67). CSE decreased in the VL of both legs in the HLNF group (−19%, d = 0.44) and no changes occurred in the RF. In conclusion, high‐ but not low‐load RT improves maximal voluntary force in the trained and the untrained knee extensors and fatigue did not further enhance these adaptations. Voluntary force improvements were unrelated to CSE changes in both legs. [ABSTRACT FROM AUTHOR]