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There is a marked difference between males and females in sprint running performance, yet a comprehensive investigation of sex differences in the muscle morphology of sprinters, which could explain the performance differences, remains to be completed. This study compared muscle volumes of 23 individual leg muscles and 5 functional muscle groups, assessed with 3 T magnetic resonance imaging, between male (n = 31) and female (n = 22) sprinters, as well as subgroups of elite males (EM, n = 5), elite females (EF, n = 5), and performance-matched (to elite females) males (PMMEF, n = 6). Differences in muscle volume distribution between EM, EF, and unathletic male (UM) controls were also assessed. For the full cohorts, male sprinters were more muscular than their female counterparts, but the differences were nonuniform and anatomically variable, with the largest differences in the hip extensors and flexors. However, among elite sprinters the sex differences in the volume of the functional muscle groups were almost uniform (absolute volume +47–53%), and the muscle volume distribution of EM was more similar to EF than to UM (P < 0.039). For PMMEF, relative hip extensor volume, but not stature or percent body fat, differentiated for performance (PMMEF and EF < EM) rather than sex. In conclusion, although the full cohorts of sprinters showed a marked sex difference in the amount and distribution of muscle mass, elite sprinters appeared to be selected for a common muscle distribution phenotype that for these elite subgroups was a stronger effect than that of sex. Relative hip extensor muscle volume, rather than stature, percent body fat, or total relative muscle volume, appeared to be the primary determinant of the sex difference in performance. NEW & NOTEWORTHY: We present novel evidence suggesting muscle volume, specifically relative hip extensor volume, may be a primary deterministic variable for the sex difference in sprint performance, such that with matched sprint times, male and female sprinters may be expected to have equivalent muscle morphology. We highlight striking similarities in distribution of leg muscle mass between elite male and female sprinters and provide evidence for the existence of a muscular distribution phenotype specific to elite sprinters, irrespective of sex. [ABSTRACT FROM AUTHOR]

Purpose: Collagen peptide supplementation has been reported to enhance synthesis rates or growth in a range of musculoskeletal tissues and could enhance tendinous tissue adaptations to resistance training (RT). This double-blind placebo-controlled study aimed to determine if tendinous tissue adaptations, size (patellar tendon cross-sectional area (CSA) and vastus lateralis (VL) aponeurosis area), and mechanical properties (patellar tendon), after 15 wk of RT, could be augmented with collagen peptide (CP) versus placebo (PLA) supplementation. Methods: Young healthy recreationally active men were randomized to consume either 15 g of CP (n = 19) or PLA (n = 20) once every day during a standardized program of lower-body RT (3 times aweek). Measurements pre- and post-RT included patellar tendon CSA and VL aponeurosis area (via magnetic resonance imaging), and patellar tendon mechanical properties during isometric knee extension ramp contractions. Results: No between-group differences were detected for any of the tendinous tissue adaptations to RT (ANOVA group-time, 0.365 = P = 0.877). There were within-group increases in VL aponeurosis area (CP, +10.0%; PLA, +9.4%), patellar tendon stiffness (CP, +17.3%; PLA, +20.9%) and Young's modulus (CP, +17.8%; PLA, +20.6%) in both groups (paired t-tests (all), P = 0.007). There were also within-group decreases in patellar tendon elongation (CP, -10.8%; PLA, -9.6%) and strain (CP, -10.6%; PLA, -8.9%) in both groups (paired t-tests (all), P = 0.006). Although no within-group changes in patellar tendon CSA (mean or regional) occurred for CP or PLA, a modest overall time effect (n = 39) was observed for mean (+1.4%) and proximal region (+2.4%) patellar tendon CSA (ANOVA, 0.017 = P = 0.048). Conclusions: In conclusion, CP supplementation did not enhance RT-induced tendinous tissue remodeling (either size or mechanical properties) compared with PLA within a population of healthy young men. [ABSTRACT FROM AUTHOR]

Aim: Bioactive collagen peptides (CP) have been suggested to augment the functional, structural (size and architecture), and contractile adaptations of skeletal muscle to resistance training (RT), but with limited evidence. This study aimed to determine if CP vs. placebo (PLA) supplementation enhanced the functional and underpinning structural, and contractile adaptations after 15 weeks of lower body RT. Methods: Young healthy males were randomized to consume either 15 g of CP (n = 19) or PLA (n = 20) once every day during a standardized program of progressive knee extensor, knee flexor, and hip extensor RT 3 times/wk. Measurements pre‐ and post‐RT included: knee extensor and flexor isometric strength; quadriceps, hamstrings, and gluteus maximus volume with MRI; evoked twitch contractions, 1RM lifting strength, and architecture (with ultrasound) of the quadriceps. Results: Percentage changes in maximum strength (isometric or 1RM) did not differ between‐groups (0.684 ≤ p ≤ 0.929). Increases in muscle volume were greater (quadriceps 15.2% vs. 10.3%; vastus medialis (VM) 15.6% vs. 9.7%; total muscle volume 15.7% vs. 11.4%; [all] p ≤ 0.032) or tended to be greater (hamstring 16.5% vs. 12.8%; gluteus maximus 16.6% vs. 12.9%; 0.089 ≤ p ≤ 0.091) for CP vs. PLA. There were also greater increases in twitch peak torque (22.3% vs. 12.3%; p = 0.038) and angle of pennation of the VM (16.8% vs. 5.8%, p = 0.046), but not other muscles, for CP vs. PLA. Conclusions: CP supplementation produced a cluster of consistent effects indicating greater skeletal muscle remodeling with RT compared to PLA. Notably, CP supplementation amplified the quadriceps and total muscle volume increases induced by RT. [ABSTRACT FROM AUTHOR]

Supplemental digital content is available in the text. The influence of muscle morphology and strength characteristics on sprint running performance, especially at elite level, is unclear. Purpose: This study aimed to investigate the differences in muscle volumes and strength between male elite sprinters, sub-elite sprinters, and untrained controls and to assess the relationships of muscle volumes and strength with sprint performance. Methods: Five elite sprinters (100-m season's best equivalent [SBE100], 10.10 ± 0.07 s), 26 sub-elite sprinters (SBE100, 10.80 ± 0.30 s), and 11 untrained control participants underwent 1) 3-T magnetic resonance imaging scans to determine the volume of 23 individual lower limb muscles/compartments and 5 functional muscle groups and 2) isometric strength assessment of lower body muscle groups. Results: Total lower body muscularity was distinct between the groups (controls < sub-elite +20% < elite +48%). The hip extensors exhibited the largest muscle group differences/relationships (elite, +32% absolute and +15% relative [per kg] volume, vs sub-elite explaining 31%–48% of the variability in SBE100), whereas the plantarflexors showed no differences between sprint groups. Individual muscle differences showed pronounced anatomical specificity (elite vs sub-elite absolute volume range, +57% to −9%). Three hip muscles were consistently larger in elite vs sub-elite (tensor fasciae latae, sartorius, and gluteus maximus; absolute, +45%–57%; relative volume, +25%–37%), and gluteus maximus volume alone explained 34%–44% of the variance in SBE100. The isometric strength of several muscle groups was greater in both sprint groups than controls but similar for the sprint groups and not related to SBE100. Conclusions: These findings demonstrate the pronounced inhomogeneity and anatomically specific muscularity required for fast sprinting and provides novel, robust evidence that greater hip extensor and gluteus maximus volumes discriminate between elite and sub-elite sprinters and are strongly associated with sprinting performance. [ABSTRACT FROM AUTHOR]

The purpose of this study was to compare the effect of resistance training (RT) duration, including years of exposure, on agonist and antagonist neuromuscular activation throughout the knee extension voluntary torque range. Fifty‐seven healthy men (untrained [UNT] n = 29, short‐term RT [12WK] n = 14, and long‐term RT [4YR] n = 14) performed maximum and sub‐maximum (20%‐80% maximum voluntary torque [MVT]) unilateral isometric knee extension contractions with torque, agonist and antagonist surface EMG recorded. Agonist EMG, including at MVT, was corrected for the confounding effects of adiposity (ie, muscle‐electrode distance; measured with ultrasonography). Quadriceps maximum anatomical cross‐sectional area (QACSAMAX; via MRI) was also assessed. MVT was distinct for all three groups (4YR +60/+39% vs UNT/12WK; 12WK +15% vs UNT; 0.001 < P ≤ 0.021), and QACSAMAX was greater for 4YR (+50/+42% vs UNT/12WK; [both] P < 0.001). Agonist EMG at MVT was +44/+33% greater for 4YR /12WK ([both] P < 0.001) vs. UNT, but did not differ between RT groups. The torque‐agonist EMG relationship of 4YR displayed a right/down shift with lower agonist EMG at the highest common torque (196 Nm) compared to 12WK and UNT (0.005 ≤ P ≤ 0.013; Effect size [ES] 0.90 ≤ ES ≤ 1.28). The torque‐antagonist EMG relationship displayed a lower slope with increasing RT duration (4YR < 12WK < UNT; 0.001 < P ≤ 0.094; 0.56 ≤ ES ≤ 1.31), and antagonist EMG at the highest common torque was also lower for 4YR than UNT (−69%; P < 0.001; ES = 1.18). In conclusion, 4YR and 12WK had similar agonist activation at MVT and this adaptation may be maximized during early months of RT. In contrast, inter‐muscular coordination, specifically antagonist coactivation was progressively lower, and likely continues to adapt, with prolonged RT. [ABSTRACT FROM AUTHOR]

Voluntary surface electromyography (sEMG) amplitude is known to be influenced by both electrode position and subcutaneous adipose tissue thickness, and these factors likely compromise both between‐ and within‐individual comparisons. Normalization of voluntary sEMG amplitude to evoked maximum M‐wave parameters (MMAX peak‐to‐peak [P‐P] and Area) may remove the influence of electrode position and subcutaneous tissue thickness. The purpose of this study was to: (a) assess the influence of electrode position on voluntary, evoked (MMAX P‐P and Area), and normalized sEMG measurements across the surface of the vastus lateralis (VL; experiment 1: n = 10); and (b) investigate if MMAX normalization removes the confounding influence of subcutaneous tissue thickness [muscle‐electrode distance (MED) from ultrasound imaging] on sEMG amplitude (experiment 2; n = 41). Healthy young men performed maximum voluntary contractions (MVCs) and evoked twitch contractions during both experiments. Experiment 1: voluntary sEMG during MVCs was influenced by electrode location (P ≤ 0.046, ES≥1.49 "large"), but when normalized to MMAX P‐P showed no differences between VL sites (P = 0.929) which was not the case when normalized to MMAX Area (P < 0.004). Experiment 2: voluntary sEMG amplitude was related to MED, which explained 31%‐38% of the variance. Normalization of voluntary sEMG amplitude to MMAX P‐P or MMAX Area reduced but did not consistently remove the influence of MED which still explained up to 16% (MMAX P‐P) and 23% (MMAX Area) of the variance. In conclusion, MMAX P‐P was the better normalization parameter for removing the influence of electrode location and substantially reduced but did not consistently remove the influence of subcutaneous adiposity. [ABSTRACT FROM AUTHOR]

Biceps femoris long head (BF LH ) aponeurosis size was compared between legs with and without prior hamstring strain injury (HSI) using within-group (injured vs. uninjured legs of previous unilateral HSI athletes) and between-group (previously injured legs of HSI athletes vs. legs of No prior HSI athletes) approaches. Currently healthy competitive male athletes with Prior HSI history ( n =23; ≥1 verified BF LH injury; including a sub-group with unilateral HSI history; most recent HSI 1.6 ± 1.2 years ago) and pair-matched athletes with No prior HSI history ( n =23) were MRI scanned. Anonymised axial images were manually segmented to quantify BF LH aponeurosis and muscle size. Prior unilateral HSI athletes' BF LH aponeurosis maximum width, aponeurosis area, and aponeurosis:muscle area ratio was 14.0-19.6% smaller in previously injured vs. contralateral uninjured legs (paired t-test, 0.008≤ p ≤0.044). BF LH aponeurosis maximum width and area were also 9.4-16.5% smaller in previously injured legs ( n =28) from prior HSI athletes vs. legs ( n =46) of No prior HSI athletes (unpaired t-test, 0.001≤ p ≤0.044). BF LH aponeurosis size was smaller in legs with Prior HSI vs. those without prior HSI. These findings suggest BF LH aponeurosis size, especially maximum width, could be a potential cause or consequence of HSI, with prospective evidence needed to support or refute these possibilities., Competing Interests: The authors declare that they have no conflict of interest., (Thieme. All rights reserved.)

Introduction: Increases in skeletal muscle size occur in response to prolonged exposure to resistance training that is typically ascribed to increased muscle fibre size. Whether muscle fibre number also changes remains controversial, and a paucity of data exists about myofibrillar structure. This cross-sectional study compared muscle fibre and myofibril characteristics in long-term resistance-trained (LRT) versus untrained (UNT) individuals., Methods: The maximal anatomical cross-sectional area (ACSAmax) of the biceps brachii muscle was measured by MRI in 16 LRT (5.9 ± 3.5 years' experience) and 13 UNT males. A muscle biopsy was taken from the biceps brachii to measure muscle fibre area, myofibril area and myosin spacing. Muscle fibre number, myofibril number in total and per fibre were estimated by dividing ACSAmax by muscle fibre area or myofibril area, and muscle fibre area by myofibril area, respectively., Results: Compared to UNT, LRT individuals had greater ACSAmax (+70%, P < 0.001), fibre area (+29%, P = 0.028), fibre number (+34%, P = 0.013), and myofibril number per fibre (+49%, P = 0.034) and in total (+105%, P < 0.001). LRT individuals also had smaller myosin spacing (-7%, P = 0.004; i.e. greater packing density) and a tendency towards smaller myofibril area (-16%, P = 0.074). ACSAmax was positively correlated with fibre area ( r = 0.526), fibre number ( r = 0.445) and myofibril number (in total r = 0.873 and per fibre r = 0.566), and negatively correlated with myofibril area ( r = -0.456) and myosin spacing ( r = -0.382) (all P < 0.05)., Conclusions: The larger muscles of LRT individuals exhibited more fibres in cross-section and larger muscle fibres, which contained substantially more total myofibrils and more packed myofilaments than UNT participants, suggesting plasticity of muscle ultrastructure., Competing Interests: Conflict of Interest and Funding Source: This work was partly supported by JSPS KAKENHI grant 21H03335 (SM). The authors declare no competing interests., (Copyright © 2024 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the American College of Sports Medicine.)

Introduction: The hamstring muscles play a crucial role in sprint running, but are also highly susceptible to strain injuries, particularly within the biceps femoris long head (BFlh). This study compared the adaptations in muscle size and strength of the knee flexors, as well as BFlh muscle and aponeurosis size, after two eccentrically focused knee flexion training regimes: Nordic hamstring training (NHT) or lengthened state eccentric training (LSET, isoinertial weight-stack resistance in an accentuated hip-flexed position), to habitual activity (no training controls: CON)., Methods: 42 healthy young males completed 34 sessions of NHT or LSET over 12 weeks or served as CON (n = 14/group). MRI-measured muscle volume of seven individual knee flexors and BFlh aponeurosis area, and maximum knee flexion torque during eccentric, concentric and isometric contractions were assessed pre- and post-training., Results: LSET induced greater increases in hamstrings (+18% vs +11%) and BFlh (+19% vs +5%) muscle volumes and BFlh aponeurosis area (+9% vs +3%) than NHT (all P ≤ 0.001), with no changes after CON. There were distinctly different patterns of hypertrophy between the two training regimes, largely due to the functional role of the muscles; LSET was more effective for increasing the size of knee flexors that also extend the hip (2.2-fold vs NHT), whereas NHT increased the size of knee flexors that do not extend the hip (1.9-fold vs LSET; both P ≤ 0.001). Changes in maximum eccentric torque differed only between LSET and CON (+17% vs +4%; P = 0.009), with NHT (+11%) in-between., Conclusions: These results suggest that LSET is superior to NHT in inducing overall hamstrings and BFlh hypertrophy, potentially contributing to better sprint performance improvements and protection against hamstring strain injuries than NHT., Competing Interests: Conflict of Interest and Funding Source: This study was funded by the National Institute of Education Academic Research Fund (RI 4/15 KPW), Singapore., (Copyright © 2024 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the American College of Sports Medicine.)

Voluntary surface electromyography (sEMG) amplitude is known to be influenced by both electrode position and subcutaneous adipose tissue thickness, and these factors likely compromise both between- and within-individual comparisons. Normalization of voluntary sEMG amplitude to evoked maximum M-wave parameters (M MAX peak-to-peak [P-P] and Area) may remove the influence of electrode position and subcutaneous tissue thickness. The purpose of this study was to: (a) assess the influence of electrode position on voluntary, evoked (M MAX P-P and Area), and normalized sEMG measurements across the surface of the vastus lateralis (VL; experiment 1: n = 10); and (b) investigate if M MAX normalization removes the confounding influence of subcutaneous tissue thickness [muscle-electrode distance (MED) from ultrasound imaging] on sEMG amplitude (experiment 2; n = 41). Healthy young men performed maximum voluntary contractions (MVCs) and evoked twitch contractions during both experiments. Experiment 1: voluntary sEMG during MVCs was influenced by electrode location (P ≤ 0.046, ES≥1.49 "large"), but when normalized to M MAX P-P showed no differences between VL sites (P = 0.929) which was not the case when normalized to M MAX Area (P < 0.004). Experiment 2: voluntary sEMG amplitude was related to MED, which explained 31%-38% of the variance. Normalization of voluntary sEMG amplitude to M MAX P-P or M MAX Area reduced but did not consistently remove the influence of MED which still explained up to 16% (M MAX P-P) and 23% (M MAX Area) of the variance. In conclusion, M MAX P-P was the better normalization parameter for removing the influence of electrode location and substantially reduced but did not consistently remove the influence of subcutaneous adiposity., (© 2018 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

New Findings: What is the central question of this study? Do tendon and/or muscle-tendon unit stiffness influence rate of torque development? What is the main finding and its importance? In our experimental conditions, some measures of relative (to maximal voluntary torque and tissue length) muscle-tendon unit stiffness had small correlations with voluntary/evoked rate of torque development over matching torque increments. However, absolute and relative tendon stiffness were unrelated to voluntary and evoked rate of torque development. Therefore, the muscle aponeurosis but not free tendon influences the relative rate of torque development. Factors other than tissue stiffness more strongly determine the absolute rate of torque development. The influence of musculotendinous tissue stiffness on contractile rate of torque development (RTD) remains opaque. In this study, we examined the relationships between both patellar tendon (PT) and vastus lateralis muscle-tendon unit (MTU) stiffness and the voluntary and evoked knee-extension RTD. Fifty-two healthy untrained men completed duplicate laboratory sessions. Absolute and relative RTD were measured at 50 N m or 25% maximal voluntary torque (MVT) increments from onset and sequentially during explosive voluntary and evoked octet isometric contractions (supramaximal stimulation; eight pulses at 300 Hz). Isometric MVT was also assessed. Patellar tendon and MTU stiffness were derived from simultaneous force and ultrasound recordings of the PT and vastus lateralis aponeurosis during constant RTD ramp contractions. Absolute and relative (to MVT and resting tissue length) stiffness (k) was measured over identical torque increments as RTD. Pearson's correlations tested relationships between stiffness and RTD measurements over matching absolute/relative torque increments. Absolute and relative PT k were unrelated to equivalent voluntary/evoked (r = 0.020-0.255, P = 0.069-0.891). Absolute MTU k was unrelated to voluntary or evoked RTD (r ≤ 0.191, P ≥ 0.184), but some measures of relative MTU k were related to relative voluntary/evoked RTD (e.g. RTD for 25-50% MVT, r = 0.374/0.353, P = 0.007/0.014). In conclusion, relative MTU k explained a small proportion of the variance in relative voluntary and evoked RTD (both ≤19%), despite no association of absolute MTU k or absolute/relative PT k with equivalent RTD measures. Therefore, the muscle-aponeurosis component but not free tendon was associated with relative RTD, although it seems that an overriding influence of MVT negated any relationship of absolute MTU k and absolute RTD., (© 2017 The Authors. Experimental Physiology © 2017 The Physiological Society.)