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A letter to the editor is presented which focuses on the measurement of muscle activation and assessment of maximum voluntary activation through interpolated twitch technique (ITT).

Purpose: This study aimed to determine the best muscle size index of muscle strength by establishing if incorporating muscle architecture measurements improved the human muscle size-strength relationship. The influence of calculating muscle force and the location of anatomical cross-sectional area (ACSA) measurements on this relationship were also examined. Methods: Fifty-two recreationally active men completed unilateral isometric knee extension strength assessments and magnetic resonance imaging scans of the dominant thigh and knee to determine quadriceps femoris size variables (ACSA along the length of the femur, maximum ACSA (ACSAMAX), and volume (VOL)) and patellar tendon moment arm. Ultrasound images (two sites per constituent muscle) were analyzed to quantify muscle architecture (fascicle length, pennation angle) and, when combined with VOL (from magnetic resonance imaging), facilitated calculation of quadriceps femoris effective PCSA (EFFPCSA) as potentially the best muscle size determinant of strength. Muscle force was calculated by dividing maximum voluntary torque by the moment arm and addition of antagonist torque (derived from hamstring EMG). Results: The associations of EFFPCSA (r = 0.685), ACSAMAX (r = 0.697), or VOL (r = 0.773) with strength did not differ, although qualitatively VOL explained 59.8% of the variance in strength, ~11%-13% greater than EFFPCSA or ACSAMAX. All muscle size variables had weaker associations with muscle force than maximum voluntary torque. The association of strength--ACSA at 65% of femur length (r = 0.719) was greater than for ACSA measured between 10%-55% and 75%-90% (r = -0.042-0.633) of femur length. Conclusions: In conclusion, using contemporary methods to assess muscle architecture and calculate EFFPCSA did not enhance the muscle strength--size association. For understanding/monitoring muscle size, the major determinant of strength, these findings support the assessment of muscle volume, which is independent of architecture measurements and was most highly correlated with strength. [ABSTRACT FROM AUTHOR]

Hamstrings muscle fiber composition may be predominantly fast-twitch and could explain the high incidence of hamstrings strain injuries. However, hamstrings muscle composition in vivo, and its influence on knee flexor muscle function, remains unknown. We investigated biceps femoris long head ( BFlh) myosin heavy chain ( MHC) composition from biopsy samples, and the association of hamstrings composition and hamstrings muscle volume (using MRI) with knee flexor maximal and explosive strength. Thirty-one young men performed maximal (concentric, eccentric, isometric) and explosive (isometric) contractions. BFlh exhibited a balanced MHC distribution [mean ± SD (min-max); 47.1 ± 9.1% (32.6-71.0%) MHC-I, 35.5 ± 8.5% (21.5-60.0%) MHC- IIA, 17.4 ± 9.1% (0.0-30.9%) MHC- IIX]. Muscle volume was correlated with knee flexor maximal strength at all velocities and contraction modes ( r = 0.62-0.76, P < 0.01), but only associated with late phase explosive strength (time to 90 Nm; r = −0.53, P < 0.05). In contrast, BFlh muscle composition was not related to any maximal or explosive strength measure. BFlh MHC composition was not found to be 'fast', and therefore composition does not appear to explain the high incidence of hamstrings strain injury. Hamstrings muscle volume explained 38-58% of the inter-individual differences in knee flexor maximum strength at a range of velocities and contraction modes, while BFlh muscle composition was not associated with maximal or explosive strength. [ABSTRACT FROM AUTHOR]

Purpose: Whilst neural and morphological adaptations following resistance training (RT) have been investigated extensively at a group level, relatively little is known about the contribution of specific physiological mechanisms, or pre-training strength, to the individual changes in strength following training. This study investigated the contribution of multiple underpinning neural [agonist EMG (QEMGMVT), antagonist EMG (HEMGANTAG)] and morphological variables [total quadriceps volume (QUADSVOL), and muscle fascicle pennation angle (QUADSθ p)], as well as pre-training strength, to the individual changes in strength after 12 weeks of knee extensor RT.Methods: Twenty-eight healthy young men completed 12 weeks of isometric knee extensor RT (3/week). Isometric maximum voluntary torque (MVT) was assessed pre- and post-RT, as were simultaneous neural drive to the agonist (QEMGMVT) and antagonist (HEMGANTAG). In addition QUADSVOL was determined with MRI and QUADSθ p with B-mode ultrasound.Results: Percentage changes (∆) in MVT were correlated to ∆QEMGMVT (r = 0.576, P = 0.001), ∆QUADSVOL (r = 0.461, P = 0.014), and pre-training MVT (r = -0.429, P = 0.023), but not ∆HEMGANTAG (r = 0.298, P = 0.123) or ∆QUADSθ p (r = -0.207, P = 0.291). Multiple regression analysis revealed 59.9% of the total variance in ∆MVT after RT to be explained by ∆QEMGMVT (30.6%), ∆QUADSVOL (18.7%), and pre-training MVT (10.6%).Conclusions: Changes in agonist neural drive, quadriceps muscle volume and pre-training strength combined to explain the majority of the variance in strength changes after knee extensor RT (~60%) and adaptations in agonist neural drive were the most important single predictor during this short-term intervention. [ABSTRACT FROM AUTHOR]

Purpose: This study examined the association of muscle size and strength for the quadriceps and hamstrings, the relationship between the size of these muscles, and whether the H:Q size ratio influenced reciprocal strength balance-widely regarded as a risk factor for hamstrings injury.Methods: Knee extensor and flexor isometric, concentric and eccentric (50 and 350° s(-1)) strength were measured in 31 healthy, recreationally active young men. Muscle volume was measured with magnetic resonance imaging.Results: The knee flexors achieved higher concentric and eccentric torques (normalised to isometric values) than the extensors. Muscle volume explained a significant part of the inter-individual differences in strength in both extensors (isometric 71%, concentric 30-31%) and flexors (isometric 38%, concentric 50-55%). Notably, muscle size was related to knee flexor eccentric strength (r = 0.69-0.76; R (2) = 48-58%) but not extensor eccentric strength. Quadriceps and hamstrings volumes were moderately correlated (r = 0.64), with the majority of the variance in the size of one muscle (59%) not explained by the size of the other muscle. The hamstrings-to-quadriceps (H:Q) volume ratio was correlated with the isometric (r = 0.45) and functional strength ratios (350° s(-1), r = 0.56; 50° s(-1), r = 0.34).Conclusions: Muscle size exhibited a differential influence on knee extensor and flexor eccentric strength. Quadriceps and hamstrings muscle size was related, and the H:Q size ratio contributed to their strength ratios. Muscle size imbalances contribute to functional imbalances and these findings support the use of hamstrings strength training with an emphasis on hypertrophic adaptations for reducing injury risk. [ABSTRACT FROM AUTHOR]

Purpose: A disproportionately small biceps femoris long head (BFlh) proximal aponeurosis has been suggested as a risk factor for hamstring strain injury by concentrating mechanical strain on the surrounding muscle tissue. However, the size of the BFlh aponeurosis relative to BFlh muscle size, or overall knee flexor strength, has not been investigated. This study aimed to examine the relationship of BFlh proximal aponeurosis area with muscle size (maximal anatomical cross-sectional area (ACSAmax)) and knee flexor strength (isometric and eccentric). Methods: Magnetic resonance images of the dominant thigh of 30 healthy young males were analyzed to measure BFlh proximal aponeurosis area and muscle ACSAmax. Participants performed maximum voluntary contractions to assess knee flexion maximal isometric and eccentric torque (at 50° s-1 and 350° s-1). Results: BFlh proximal aponeurosis area varied considerably between participants (more than fourfold, range = 7.5-33.5 cm², mean = 20.4 ± 5.4 cm², coefficient of variation = 26.6%) and was not related to BFlh ACSAmax (r = 0.04, P = 0.83). Consequently, the aponeurosis/muscle area ratio (defined as BFlh proximal aponeurosis area divided by BFlh ACSAmax) exhibited sixfold variability, being 83% smaller in one individual than another (0.53 to 3.09, coefficient of variation = 32.5%). Moreover, aponeurosis size was not related to isometric (r = 0.28, P = 0.13) or eccentric knee flexion strength (r = 0.24, P > 0.20). Conclusion: BFlh proximal aponeurosis size exhibits high variability between healthy young men, and it was not related to BFlh muscle size or knee flexor strength. Individuals with a relatively small aponeurosis may be at increased risk of hamstring strain injury. [ABSTRACT FROM AUTHOR]

We have investigated the action of exogenous nitric oxide (NO) on the strength and contractile properties of human skeletal muscle working in vivo. Maximum isometric voluntary contraction force (MVC) of the quadriceps was measured and superimposed electrical stimulation was used to estimate the level of activation and ‘true maximum force’ (TMF). Force–frequency relationships were determined to assess changes in contractile properties of the muscle. Subjects in the experimental group (E, n=10) were measured before and during two separate periods of treatment with different doses of glyceryl trinitrate, a NO donor, delivering 100 (GTN100) or 200 (GTN200) μg h–1 as a trans-dermal patch. A control group (C, n=6) was measured during two similar periods whilst taking an oral placebo. There was a significant increase in strength with GTN200 (MVC: +5.15%; TMF: +3.87%). There was no change in the strength of group C. There was a trend towards reduced forces at submaximal frequencies with GTN administration but the most notable change was a decline in twitch force (approximately 12%, P < 0.05) with GTN100 treatment and this remained depressed throughout the study. No changes were seen in the contractile properties of the control group C. The present results show that GTN treatment increased maximum voluntary strength but decreased twitch tension. The time course and dose–response characteristics indicate that these are two separate actions of NO on human muscle working in vivo. [ABSTRACT FROM AUTHOR]

The authors offer a response to a query made on their article "Changes in agonist neural drive, hypertrophy and pre-training strength all contribute to the individual strength gains after resistance training." The study aims to investigate whether there was a relationship between putative predictor/variables adaptations after resistance training (RT). The strength changes after RT were found to be correlated with hypertrophy and the changes in agonist neural drive and pre-training strength.

This study compared the muscle and tendon morphology of an extraordinarily strong individual, a World's Strongest Man and deadlift champion (WSM), with that of various other athletic, trained, and untrained populations. The WSM completed the following: 1 ) 3.0-T MRI scans, to determine the volume of 22 individual lower limb muscles, 5 functional muscle groups, patellar tendon (PT) cross-sectional area (CSA), and PT moment arm; and 2 ) countermovement jumps (CMJ) and isometric midthigh pull (IMTP) contractions. The WSM was compared with previously assessed groups from our laboratory (muscle and tendon) and the wider research literature (CMJ and IMTP). The WSM's CMJ peak power (9,866 W) and gross (9,171 N) and net (7,480 N) IMTP peak forces were higher than any previously published values. The WSM's overall measured leg muscle volume was approximately twice that of untrained controls (+96%) but with pronounced anatomical variability in the extent of muscular development. The plantar flexor group (+120%) and the guy rope muscles (sartorius, gracilis, and semitendinosus: +140% to +202%), which stabilize the pelvis and femur, demonstrated the largest differences relative to that of untrained controls. The WSM's pronounced quadriceps size (greater than or equal to twofold vs. untrained) was accompanied by modest PT moment arm differences and, notably, was not matched by an equivalent difference in PT CSA (+30%). These results provide novel insight into the musculotendinous characteristics of an extraordinarily strong individual, which may be toward the upper limit of human variation, such that the WSM's very pronounced lower limb muscularity also exhibited distinct anatomical variability and with muscle size largely uncoupled from tendon size. NEW & NOTEWORTHY Lower-body muscle size of an extraordinarily strong individual, a World's Strongest Man and deadlift champion (WSM), was approximately twice that of controls but was underpinned by pronounced anatomical variability in the extent of muscular development (+23-202%): the plantar flexor group and guy rope muscles demonstrating the largest differences. The WSM's quadriceps size (more than or equal to twice that of controls) contrasted with modest differences in patella tendon moment arm (+18%) and was uncoupled from patellar tendon size (+30%).

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) vs lengthened state eccentric training (LSET, isoinertial weight stack resistance in an accentuated hip-flexed position) vs habitual activity (no training controls: CON)., Methods: Forty-two healthy young males completed 34 sessions of NHT or LSET over 12 wk or served as CON ( n = 14/group). Magnetic resonance imaging-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%) inbetween., 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., (Copyright © 2024 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the American College of Sports Medicine.)

Introduction: Increases in skeletal muscle size occur in response to prolonged exposure to resistance training that is typically ascribed to increased muscle fiber size. Whether muscle fiber number also changes remains controversial, and a paucity of data exists about myofibrillar structure. This cross-sectional study compared muscle fiber 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 magnetic resonance imaging in 16 LRT (5.9 ± 3.5 yr' experience) and 13 UNT males. A muscle biopsy was taken from the biceps brachii to measure muscle fiber area, myofibril area, and myosin spacing. Muscle fiber number, and myofibril number in total and per fiber were estimated by dividing ACSAmax by muscle fiber area or myofibril area, and muscle fiber area by myofibril area, respectively., Results: Compared with UNT, LRT individuals had greater ACSAmax (+70%, P < 0.001), fiber area (+29%, P = 0.028), fiber number (+34%, P = 0.013), and myofibril number per fiber (+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 toward smaller myofibril area (-16%, P = 0.074). ACSAmax was positively correlated with fiber area ( r = 0.526), fiber number ( r = 0.445), and myofibril number (in total r = 0.873 and per fiber 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 fibers in cross-section and larger muscle fibers, which contained substantially more total myofibrils and more packed myofilaments than UNT participants, suggesting plasticity of muscle ultrastructure., (Copyright © 2024 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the American College of Sports Medicine.)

How the neuromechanics of the lower limb functional muscle groups change with running speed remains to be fully elucidated, with implications for our understanding of human locomotion, conditioning, and injury prevention. This study compared the neuromechanics (ground reaction and joint kinetics, kinematics and muscle activity) of middle-distance athletes running on an instrumented treadmill at six wide-ranging speeds (2.78-8.33 m·s -1 ). Ground reaction forces and kinematics were analyzed using inverse dynamics to calculate flexor and extensor joint torques, and positive and negative work done by these torques. Contributions of each functional muscle group to the total positive and negative work done by the limb during stance, swing, and the whole stride were quantified. During stance, the ankle plantar flexors were the major energy generator and absorber (>60%) at all speeds, but their contribution to whole stride energy generation and absorption declined with speed. Positive work by the hip extensors rose superlinearly with speed during stance (3-fold) and especially during swing (12-fold), becoming the biggest energy generator across the whole stride at >5 m·s -1 . Knee flexor and extensor negative work also rose superlinearly with speed during swing, with the knee flexors becoming the greatest energy absorber over the whole stride at >7.22 m·s -1 . Across speeds, plantar flexor peak moment and positive work accounted for 97% and 96% of the variance in step length, and swing hip extension peak moment and positive work accounted for 98% and 99% of the variance in step frequency. There were pronounced speed, phase (stance/swing), and work (positive/negative) dependent contributions of the different functional muscle groups during running, with extensive implications for conditioning and injury prevention., (© 2024 The Author(s). Scandinavian Journal of Medicine & Science In Sports published by John Wiley & Sons Ltd.)

Introduction: Running economy (RE) deteriorates during prolonged running, although the effect of measuring energy cost (EC) or oxygen cost (OC) on the magnitude of these changes has not been investigated. Similarly, it is unknown if runners' performance level may influence the deterioration of RE during prolonged running. The aims of this study were to compare changes in EC and OC measurements of RE during a prolonged run in a large cohort of well-trained male runners, and to compare changes between runners of high and low performance standard., Methods: Forty-four male runners (maximal oxygen uptake (V̇O2max) 62.4 ml·kg-1·min-1; 10 km time 35:50 ± 4:40 mm:ss) completed an incremental test determining lactate threshold 1 (LT1) and V̇O2max, and on a separate occasion, a 90 min run at LT1. Respiratory gases were collected at 15 min intervals. Subsequently, sub-groups of high- (HP, 10 km 31:20 ± 01:00 mm:ss) and low-performing (LP, 10 km 41:50 ± 01:20 mm:ss) runners were compared., Results: RE deterioration was only fractionally larger when expressed as OC than EC (0.1% greater from 30-90 min; p < 0.001), perhaps due to the small change in RER (-0.01) in this study. For the HP group increases were lower than LP after 90 min in both EC (+2.3 vs +4.3%; p < 0.01) and OC (+2.4 vs +4.5%; p < 0.01). Similarly, at standardized distances, changes were lower for HP vs LP e.g. at 16.7 km +1.0 vs +3.2% for EC (p < 0.01), and + 1.2 vs +3.4% for OC (p < 0.001)., Conclusions: The deterioration of RE was dependent on athlete's performance level, with HP runners displaying superior RE durability. The use of EC or OC had only a fractional influence on RE durability, although this may gain importance with larger shifts in substrate metabolism., Competing Interests: Conflict of Interest and Funding Source: No external funding has been provided for this research project. The authors do not have any conflict of interest to disclose., (Copyright © 2024 by the American College of Sports Medicine.)

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 (PMM EF , 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 PMM EF , relative hip extensor volume, but not stature or percent body fat, differentiated for performance (PMM EF 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.

Background: How the physical metrics, especially physical intensity, and possession interact with each other, and subsequently combine to influence performance remains opaque. Therefore, we investigated the interrelationship of possession, physical metrics, and team performance in elite soccer., Methods: Four seasons of a top European league were used to derive 80 team league performances (points), together with possession and physical data. Physical metrics were absolute distances (m) during the whole match and ball-in-play, and rates of distance covered (m⋅min -1 ) as the index of physical intensity, notably when in-possession/out-of-possession, in total and within five speed categories. Interrelationships of possession, physical metrics, possession, and performance were assessed with Pearson's correlations and mediation analysis., Results: Overall possession (r = 0.794) and time out-of-possession within the defensive third (r = -0.797) were most strongly correlated with performance. The strong relationships between in-possession distances and performance appeared coincidental due to greater time in-possession. Physical intensity had a complex relationship with possession and performance, with opposite relationships according to possession status: lower physical intensity when in-possession and higher physical intensity when out-of-possession were associated with possession and performance. Mediation analysis revealed the direct, independent importance of possession for team performance; however, the association of physical intensity with performance was largely (>79%) mediated by possession., Conclusion: Based on these findings, we propose a novel model of the interrelationships between possession, physical intensity, and performance, whereby higher possession is the largest, direct contributor toward enhanced team performance, with lower physical intensity in-possession a consequence of higher possession, but greater physical intensity when out-of-possession a cause of increased possession., (© 2023 The Authors. Scandinavian Journal of Medicine & Science In Sports published by John Wiley & Sons Ltd.)

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 a week). 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., (Copyright © 2023 by the American College of Sports Medicine.)

Aging involves a marked decline in physical function and especially muscle power. Thus, optimal resistance exercise (RE) to improve muscle power is required for exercise prescription. An eccentric lowering phase immediately before a concentric lift (ECC-CON) may augment concentric power production, due to various proposed mechanisms (e.g., elastic recoil, pre-activation, stretch reflex, contractile history), when compared with a concentric contraction alone (CON-Only). This study compared the effect of a prior eccentric lowering phase on older adult concentric power performance (ECC-CON vs. CON-Only) during a common multiple joint isoinertial RE (i.e., leg press) with a range of loads. Twelve healthy older adult males completed two measurement sessions, consisting of ECC-CON and CON-Only contractions, performed in a counterbalanced order using 20-80% of one repetition maximum [% 1RM] loads on an instrumented isoinertial leg press dynamometer that measured power, force, and velocity. Muscle activation was assessed with surface electromyography (sEMG). For mean power ECC-CON>CON-Only, with a pronounced effect of load on the augmentation of power by ECC-CON (+19 to +55%, 35-80% 1RM, all p < 0.032). Similarly, for mean velocity ECC-CON>CON-Only, especially as load increased (+15 to 54%, 20-80% 1RM, all p < 0.005), but mean force showed more modest benefits of ECC-CON (+9 to 14%, 50-80% 1RM, all p < 0.05). In contrast, peak power and velocity were similar for ECC-CON and CON-Only with all loads. Knee and hip extensor sEMG were similar for both types of contractions. In conclusion, ECC-CON contractions produced greater power, and velocity performance in older adults than CON-Only and may provide a superior stimulus for chronic power development., (© 2023 The Authors. Scandinavian Journal of Medicine & Science In Sports published by John Wiley & Sons Ltd.)

The physical demands of soccer match-play have typically been assessed using a low-resolution whole match approach ignoring whether the ball is in or out of play (BIP/BOP) and during these periods which team has possession. This study investigated the effect of fundamental match structure variables (BIP/BOP, in/out of possession) on the physical demands, and especially intensity, of elite match-play. For 1083 matches from a major European league, whole match duration, and player physical tracking data, were divided into BIP/BOP, and in/out of possession periods throughout the match, using on-ball event data. These distinct phases were used to derive absolute (m) and rate (m·min -1 ) of distance covered in total and within six speed categories during BIP/BOP and in/out possession. The rate of distance covered, an index of physical intensity, was >2-fold greater during BIP vs BOP. Whole match total distance covered was confounded by BIP time and poorly associated with physical intensity during BIP ( r  = 0.36). Whole match rates of distance covered substantially underestimated those during BIP, particularly for higher running speeds (∼-62%). Ball possession markedly effected physical intensity, with the rates of distance covered running (+31%), at high-speed (+30%) and in total (+7%) greater out than in possession. Whole match physical metrics underestimated the physical intensity during BIP, and thus the rate(s) of distance covered during BIP are recommended for accurate measurement of physical intensity in elite soccer. The greater demands of being out of possession support a possession-based tactical approach to minimise fatigue and its negative consequences.

Because of the biophysical relation between muscle fibre diameter and the propagation velocity of action potentials along the muscle fibres, motor unit conduction velocity could be a non-invasive index of muscle fibre size in humans. However, the relation between motor unit conduction velocity and fibre size has been only assessed indirectly in animal models and in human patients with invasive intramuscular EMG recordings, or it has been mathematically derived from computer simulations. By combining advanced non-invasive techniques to record motor unit activity in vivo, i.e. high-density surface EMG, with the gold standard technique for muscle tissue sampling, i.e. muscle biopsy, here we investigated the relation between the conduction velocity of populations of motor units identified from the biceps brachii muscle, and muscle fibre diameter. We demonstrate the possibility of predicting muscle fibre diameter (R 2  = 0.66) and cross-sectional area (R 2  = 0.65) from conduction velocity estimates with low systematic bias (∼2% and ∼4% respectively) and a relatively low margin of individual error (∼8% and ∼16%, respectively). The proposed neuromuscular interface opens new perspectives in the use of high-density EMG as a non-invasive tool to estimate muscle fibre size without the need of surgical biopsy sampling. The non-invasive nature of high-density surface EMG for the assessment of muscle fibre size may be useful in studies monitoring child development, ageing, space and exercise physiology, although the applicability and validity of the proposed methodology need to be more directly assessed in these specific populations by future studies. KEY POINTS: Because of the biophysical relation between muscle fibre size and the propagation velocity of action potentials along the sarcolemma, motor unit conduction velocity could represent a potential non-invasive candidate for estimating muscle fibre size in vivo. This relation has been previously assessed in animal models and humans with invasive techniques, or it has been mathematically derived from simulations. By combining high-density surface EMG with muscle biopsy, here we explored the relation between the conduction velocity of populations of motor units and muscle fibre size in healthy individuals. Our results confirmed that motor unit conduction velocity can be considered as a novel biomarker of fibre size, which can be adopted to predict muscle fibre diameter and cross-sectional area with low systematic bias and margin of individual error. The proposed neuromuscular interface opens new perspectives in the use of high-density EMG as a non-invasive tool to estimate muscle fibre size without the need of surgical biopsy sampling., (© 2023 The Authors. The Journal of Physiology published by John Wiley & Sons Ltd on behalf of The Physiological Society.)

Purpose: Adjustments in motor unit (MU) discharge properties have been shown after short-term resistance training; however, MU adaptations in long-term resistance-trained (RT) individuals are less clear. Here, we concurrently assessed MU discharge characteristics and MU conduction velocity in long-term RT and untrained (UT) men., Methods: Motor unit discharge characteristics (discharge rate, recruitment, and derecruitment threshold) and MU conduction velocity were assessed after the decomposition of high-density electromyograms recorded from vastus lateralis (VL) and vastus medialis (VM) of RT (>3 yr; n = 14) and UT ( n = 13) during submaximal and maximal isometric knee extension., Results: Resistance-trained men were on average 42% stronger (maximal voluntary force [MVF], 976.7 ± 85.4 N vs 685.5 ± 123.1 N; P < 0.0001), but exhibited similar relative MU recruitment (VL, 21.3% ± 4.3% vs 21.0% ± 2.3% MVF; VM, 24.5% ± 4.2% vs 22.7% ± 5.3% MVF) and derecruitment thresholds (VL, 20.3% ± 4.3% vs 19.8% ± 2.9% MVF; VM, 24.2% ± 4.8% vs 22.9% ± 3.7% MVF; P ≥ 0.4543). There were also no differences between groups in MU discharge rate at recruitment and derecruitment or at the plateau phase of submaximal contractions (VL, 10.6 ± 1.2 pps vs 10.3 ± 1.5 pps; VM, 10.7 ± 1.6 pps vs 10.8 ± 1.7 pps; P ≥ 0.3028). During maximal contractions of a subsample population (10 RT, 9 UT), MU discharge rate was also similar in RT compared with UT (VL, 21.1 ± 4.1 pps vs 14.0 ± 4.5 pps; VM, 19.5 ± 5.0 pps vs 17.0 ± 6.3 pps; P = 0.7173). Motor unit conduction velocity was greater in RT compared with UT individuals in both VL (4.9 ± 0.5 m·s -1 vs 4.5 ± 0.3 m·s -1 ; P < 0.0013) and VM (4.8 ± 0.5 m·s -1 vs 4.4 ± 0.3 m·s -1 ; P < 0.0073)., Conclusions: Resistance-trained and UT men display similar MU discharge characteristics in the knee extensor muscles during maximal and submaximal contractions. The between-group strength difference is likely explained by superior muscle morphology of RT as suggested by greater MU conduction velocity., (Copyright © 2022 by the American College of Sports Medicine.)

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., (© 2022 The Authors. Acta Physiologica published by John Wiley & Sons Ltd on behalf of Scandinavian Physiological Society.)

Introduction: A paucity of research exists examining the importance of muscle morphological and functional characteristics for elite female sprint performance., Purpose: This study aimed to compare lower body muscle volumes and vertical jumping power between elite and subelite female sprinters and assess the relationships of these characteristics with sprint race and acceleration performance., Methods: Five elite (100 m seasons best [SBE 100 ], 11.16 ± 0.06 s) and 17 subelite (SBE 100 , 11.84 ± 0.42 s) female sprinters underwent: 3T magnetic resonance imaging to determine the volume of 23 individual leg muscles/compartments and five functional muscle groups; countermovement jump and 30 m acceleration tests., Results: Total absolute lower body muscle volume was higher in elite versus subelite sprinters (+15%). Elite females exhibited greater muscle volume of the hip flexors (absolute, +28%; relative [to body mass], +19%), hip extensors (absolute, +22%; relative, +14%), and knee extensors (absolute, +21%), demonstrating pronounced anatomically specific muscularity, with relative hip flexor volume alone explaining 48% of sprint performance variability. The relative volume of five individual muscles (sartorius, gluteus maximus, adductor magnus, vastus lateralis, illiopsoas) were both distinct between groups (elite > subelite) and related to SBE 100 ( r = 0.553-0.639), with the combination of the sartorius (41%) and the adductor magnus (17%) explaining 58% of the variance in SBE 100 . Elite female sprinters demonstrated greater absolute countermovement jump power versus subelite, and absolute and relative power were related to both SBE 100 ( r = -0.520 to -0.741) and acceleration performance ( r = 0.569 to 0.808)., Conclusions: This investigation illustrates the distinctive, anatomically specific muscle volume distribution that facilitates elite sprint running in females, and emphasizes the importance of hip flexor and extensor relative muscle volume., (Copyright © 2022 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of the American College of Sports Medicine.)

Maximal rate of force development in adult humans is determined by the maximal motor unit discharge rate, however, the origin of the underlying synaptic inputs remains unclear. Here, we tested a hypothesis that the maximal motor unit discharge rate will increase in response to a startling cue, a stimulus that purportedly activates the pontomedullary reticular formation neurons that make mono- and disynaptic connections to motoneurons via fast-conducting axons. Twenty-two men were required to produce isometric knee extensor forces "as fast and as hard" as possible from rest to 75% of maximal voluntary force, in response to visual (VC), visual-auditory (VAC; 80 dB), or visual-startling cue (VSC; 110 dB). Motoneuron activity was estimated via decomposition of high-density surface electromyogram recordings over the vastus lateralis and medialis muscles. Reaction time was significantly shorter in response to VSC compared with VAC and VC. The VSC further elicited faster neuromechanical responses including a greater number of discharges per motor unit per second and greater maximal rate of force development, with no differences between VAC and VC. We provide evidence, for the first time, that the synaptic input to motoneurons increases in response to a startling cue, suggesting a contribution of subcortical pathways to maximal motoneuron output in humans. NEW & NOTEWORTHY Motor unit discharge characteristics are a key determinant of rate of force development in humans, but the neural substrate(s) underpinning such output remains unknown. Using decomposition of high-density electromyogram, we show greater number of discharges per motor unit per second and greater rate of force development after a startling auditory stimulus. These observations suggest a possible subcortical contribution to maximal in vivo motor unit discharge rate in adult humans.

Objective. High-density surface electromyography (HD-sEMG) allows the reliable identification of individual motor unit (MU) action potentials. Despite the accuracy in decomposition, there is a large variability in the number of identified MUs across individuals and exerted forces. Here we present a systematic investigation of the anatomical and neural factors that determine this variability. Approach . We investigated factors of influence on HD-sEMG decomposition, such as synchronization of MU discharges, distribution of MU territories, muscle-electrode distance (MED-subcutaneous adipose tissue thickness), maximum anatomical cross-sectional area (ACSA max ), and fiber cross-sectional area. For this purpose, we recorded HD-sEMG signals, ultrasound and magnetic resonance images, and took a muscle biopsy from the biceps brachii muscle from 30 male participants drawn from two groups to ensure variability within the factors-untrained-controls (UT = 14) and strength-trained individuals (ST = 16). Participants performed isometric ramp contractions with elbow flexors (at 15%, 35%, 50% and 70% maximum voluntary torque-MVT). We assessed the correlation between the number of accurately detected MUs by HD-sEMG decomposition and each measured parameter, for each target force level. Multiple regression analysis was then applied. Main results. ST subjects showed lower MED (UT = 5.1 ± 1.4 mm; ST = 3.8 ± 0.8 mm) and a greater number of identified MUs (UT: 21.3 ± 10.2 vs ST: 29.2 ± 11.8 MUs/subject across all force levels). The entire cohort showed a negative correlation between MED and the number of identified MUs at low forces ( r = -0.6, p = 0.002 at 15% MVT). Moreover, the number of identified MUs was positively correlated to the distribution of MU territories ( r = 0.56, p = 0.01) and ACSA max ( r = 0.48, p = 0.03) at 15% MVT. By accounting for all anatomical parameters, we were able to partly predict the number of decomposed MUs at low but not at high forces. Significance. Our results confirmed the influence of subcutaneous tissue on the quality of HD-sEMG signals and demonstrated that MU spatial distribution and ACSA max are also relevant parameters of influence for current decomposition algorithms., (© 2022 IOP Publishing Ltd.)

Purpose: Neuromuscular power is critical for healthy ageing. Conventional older adult resistance training (RT) guidelines typically recommend lifting slowly (2-s; CONV), whereas fast/explosive contractions performed either non-ballistically (FAST-NB) or ballistically (FAST-B, attempting to throw the load) may involve greater acute power production, and could ultimately provide a greater chronic power adaptation stimulus. To compare the neuromechanics (power, force, velocity, and muscle activation) of different types of concentric isoinertial RT contractions in older adults., Methods: Twelve active older adult males completed three sessions, each randomly assigned to one type of concentric contraction (CONV or FAST-NB or FAST-B). Each session involved lifting a range of loads (20-80%1RM) using an instrumented isoinertial leg press dynamometer that measured power, force, and velocity. Muscle activation was assessed with surface electromyography (sEMG)., Results: Peak and mean power were markedly different, according to the concentric contraction explosive intent FAST-B > FAST-NB > CONV, with FAST-B producing substantially more power (+ 49 to 1172%, P ≤ 0.023), force (+ 10 to 136%, P < 0.05) and velocity (+ 55 to 483%, P ≤ 0.025) than CONV and FAST-NB contractions. Knee and hip extensor sEMG were typically higher during FAST-B than CON (all P < 0.02) and FAST-NB (≤ 50%1RM, P ≤ 0.001)., Conclusions: FAST-B contractions produced markedly greater power, force, velocity and muscle activation across a range of loads than both CONV or FAST-NB and could provide a more potent RT stimulus for the chronic development of older adult power., (© 2022. The Author(s).)

The purpose of this cross-sectional study was to compare explosive strength and underpinning contractile, hypertrophic, and neuromuscular activation characteristics of long-term maximum strength-trained (LT-MST; ie, ≥3 years of consistent, regular knee extensor training) and untrained individuals. Sixty-three healthy young men (untrained [UNT] n = 49, and LT-MST n = 14) performed isometric maximum and explosive voluntary, as well as evoked octet knee extension contractions. Torque, quadriceps, and hamstring surface EMG were recorded during all tasks. Quadriceps anatomical cross-sectional area (QACSA MAX ; via MRI) was also assessed. Maximum voluntary torque (MVT; +66%) and QACSA MAX (+54%) were greater for LT-MST than UNT ([both] p < 0.001). Absolute explosive voluntary torque (25-150 ms after torque onset; +41 to +64%; [all] p < 0.001; 1.15≤ effect size [ES]≤2.36) and absolute evoked octet torque (50 ms after torque onset; +43, p < 0.001; ES = 3.07) were greater for LT-MST than UNT. However, relative (to MVT) explosive voluntary torque was lower for LT-MST than UNT from 100 to 150 ms after contraction onset (-11% to -16%; 0.001 ≤ p ≤ 0.002; 0.98 ≤ ES ≤ 1.11). Relative evoked octet torque 50 ms after onset was lower (-10%; p < 0.001; ES = 1.14) and octet time to peak torque longer (+8%; p = 0.001; ES = 1.18) for LT-MST than UNT indicating slower contractile properties, independent from any differences in torque amplitude. The greater absolute explosive strength of the LT-MST group was attributable to higher evoked explosive strength, that in turn appeared to be due to larger quadriceps muscle size, rather than any differences in neuromuscular activation. In contrast, the inferior relative explosive strength of LT-MST appeared to be underpinned by slower intrinsic/evoked contractile properties., (© 2022 The Authors. Scandinavian Journal of Medicine & Science In Sports published by John Wiley & Sons Ltd.)

Despite the known positive effects of acute exercise on cognition, the effects of a competitive team sport match are unknown. In a randomized crossover design, 20 female and 17 male field hockey players (19.7 ± 1.2 years) completed a battery of cognitive tests (Visual Search, Stroop, Corsi Blocks, and Rapid Visual Information Processing) prior to, at half-time, and immediately following a competitive match (or control trial of seated rest); with effect sizes (ES) presented as raw ES from mixed effect models. Blood samples were collected prior to and following the match and control trial, and analyzed for adrenaline, noradrenaline, brain derived neurotrophic factor (BDNF), cathepsin B, and cortisol. The match improved response times for a simple perception task at full-time (ES = -14 ms; P < 0.01) and response times on the complex executive function task improved at half-time (ES = -44 ms; P < 0.01). Working memory declined at full-time on the match (ES = -0.6 blocks; P < 0.01). The change in working memory was negatively correlated with increases in cortisol ( r = -0.314, P = 0.01; medium), as was the change in simple perception response time and the change in noradrenaline concentration ( r = -0.284, P = 0.01; small to medium). This study is the first to highlight the effects a competitive hockey match can have on cognition. These findings have implications for performance optimization, as understanding the influence on specific cognitive domains across a match allows for the investigation into strategies to improve these aspects., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Malcolm, Cooper, Folland, Tyler and Sunderland.)

Neural and morphological adaptations combine to underpin the enhanced muscle strength following prolonged exposure to strength training, although their relative importance remains unclear. We investigated the contribution of motor unit (MU) behavior and muscle size to submaximal force production in chronically strength-trained athletes (ST) versus untrained controls (UT). Sixteen ST (age: 22.9 ± 3.5 yr; training experience: 5.9 ± 3.5 yr) and 14 UT (age: 20.4 ± 2.3 yr) performed maximal voluntary isometric force (MViF) and ramp contractions (at 15%, 35%, 50%, and 70% MViF) with elbow flexors, whilst high-density surface electromyography (HDsEMG) was recorded from the biceps brachii (BB). Recruitment thresholds (RTs) and discharge rates (DRs) of MUs identified from the submaximal contractions were assessed. The neural drive-to-muscle gain was estimated from the relation between changes in force (ΔFORCE, i.e. muscle output) relative to changes in MU DR (ΔDR, i.e. neural input). BB maximum anatomical cross-sectional area (ACSA MAX ) was also assessed by MRI. MViF (+64.8% vs. UT, P < 0.001) and BB ACSA MAX (+71.9%, P < 0.001) were higher in ST. Absolute MU RT was higher in ST (+62.6%, P < 0.001), but occurred at similar normalized forces. MU DR did not differ between groups at the same normalized forces. The absolute slope of the ΔFORCE - ΔDR relationship was higher in ST (+66.9%, P = 0.002), whereas it did not differ for normalized values. We observed similar MU behavior between ST athletes and UT controls. The greater absolute force-generating capacity of ST for the same neural input demonstrates that morphological, rather than neural, factors are the predominant mechanism for their enhanced force generation during submaximal efforts. NEW & NOTEWORTHY In this study, we observed that recruitment strategies and discharge characteristics of large populations of motor units identified from biceps brachii of strength-trained athletes were similar to those observed in untrained individuals during submaximal force tasks. We also found that for the same neural input, strength-trained athletes are able to produce greater absolute muscle forces (i.e., neural drive-to-muscle gain). This demonstrates that morphological factors are the predominant mechanism for the enhanced force generation during submaximal efforts.

This study compared elbow flexor (EF; experiment 1 ) and knee extensor (KE; experiment 2 ) maximal compound action potential (M max ) amplitude between long-term resistance trained (LTRT; n = 15 and n = 14, 6 ± 3 and 4 ± 1 yr of training) and untrained (UT; n = 14 and n = 49) men, and examined the effect of normalizing electromyography (EMG) during maximal voluntary torque (MVT) production to M max amplitude on differences between LTRT and UT. EMG was recorded from multiple sites and muscles of EF and KE, M max was evoked with percutaneous nerve stimulation, and muscle size was assessed with ultrasonography (thickness, EF) and magnetic resonance imaging (cross-sectional area, KE). Muscle-electrode distance (MED) was measured to account for the effect of adipose tissue on EMG and M max . LTRT displayed greater MVT (+66%-71%, P < 0.001), muscle size (+54%-56%, P < 0.001), and M max amplitudes (+29%-60%, P ≤ 0.010) even when corrected for MED ( P ≤ 0.045). M max was associated with the size of both muscle groups ( r  ≥ 0.466, P ≤ 0.011). Compared with UT, LTRT had higher absolute voluntary EMG amplitude for the KE ( P < 0.001), but not the EF ( P = 0.195), and these differences/similarities were maintained after correction for MED; however, M max normalization resulted in no differences between LTRT and UT for any muscle and/or muscle group ( P ≥ 0.652). The positive association between M max and muscle size, and no differences when accounting for peripheral electrophysiological properties (EMG/M max ), indicates the greater absolute voluntary EMG amplitude of LTRT might be confounded by muscle morphology, rather than providing a discrete measure of central neural activity. This study therefore suggests limited agonist neural adaptation after LTRT. NEW & NOTEWORTHY In a large sample of long-term resistance-trained individuals, we showed greater maximal M-wave amplitude of the elbow flexors and knee extensors compared with untrained individuals, which appears to be at least partially mediated by differences in muscle size. The lack of group differences in voluntary EMG amplitude when normalized to maximal M-wave suggests that differences in muscle morphology might impair interpretation of voluntary EMG as an index of central neural activity.

Purpose: There are suggestions that isometric resistance training (RT) produces highly angle-specific changes in strength with the greatest changes at the training angle, but these effects remain controversial with limited rigorous evidence, and the possible underpinning physiological mechanism(s) remain opaque. This study investigated the extent of angle-specific changes in strength and neuromuscular activation after RT in comparison to a control group., Methods: A RT group (n = 13) performed 14 isometric RT sessions at a knee-joint angle of 65° (0° is anatomical position) over a 4-week period, whilst a control group (CON, n = 9) maintained their habitual activity. Pre- and post-test sessions involved voluntary and evoked isometric knee extension contractions at five knee-joint angles (35°, 50°, 65°, 80° and 95°), while electromyography was recorded., Results: RT group increased maximum voluntary torque (MVT) at the training angle (65°; + 12%) as well as 80° (+ 7%), 50° (+ 11%) and 35° (+ 5%). Joint-angle specificity was demonstrated within the RT group (MVT increased more at some angles vs. others), and also by more rigorous between-group comparisons (i.e., larger improvements after RT vs. CON at some angles than others). For the RT group, normalized EMG increased at three of the same joint angles as strength, but not for CON. Importantly, however, neither within- or between-group analyses provided evidence of joint angle-specific changes in activation., Conclusion: In conclusion, this study provides robust evidence for joint angle-specific strength gains after isometric RT, with weaker evidence that changes in neuromuscular activation may contribute to these adaptations.

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.)

Explosive torque has been demonstrated to relate to static balance. However, sports injuries occur dynamically and unpredictably, yet the relationship between explosive torque and balance response to dynamic perturbations is unknown. This study investigated the relationship between explosive torque of the plantar flexors and knee extensors and the centre of mass (COM) response to unexpected perturbations. Thirty-three healthy subjects (17 females, 16 males) were assessed for maximal and explosive isometric knee extension (KE) and plantar flexion (PF) torque and COM response (velocity (COMV), displacement (COMD)) to unexpected platform translations. Relationships between explosive torque and balance measures were investigated using Pearson's correlation and multiple regression. A negative relationship between PF explosive torque at 50, 100, and 150 ms and COMV at 300, 400, and 500 ms (r = -0.363 to -0.508, p ≤ 0.049), and COMD at 400 and 500 ms (r = -0.349 to -0.416, p ≤ 0.046) was revealed. A negative relationship between KE explosive torque at 50, 100, and 150 ms and COMV at 400 ms (r = -0.381 to -0.411, p ≤ 0.029) but not COMD was also revealed. Multiple regression found PF 100 ms predicted 17.3% of variability in COMD at 500 ms and 25.8% of variability in COMV at 400 ms. These results suggest that producing torque rapidly may improve COM response to unexpected perturbation., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

The influence of running speed and sex on running economy is unclear and may have been confounded by measurements of oxygen cost that do not account for known differences in substrate metabolism, across a limited range of speeds, and differences in performance standard. Therefore, this study assessed the energy cost of running over a wide range of speeds in high-level and recreational runners to investigate the effect of speed (in absolute and relative terms) and sex (men vs women of equivalent performance standard) on running economy. To determine the energy cost (kcal · kg -1  · km -1 ) of submaximal running, speed at lactate turn point (sLTP), and maximal rate of oxygen uptake, 92 healthy runners (high-level men, n = 14; high-level women, n = 10; recreational men, n = 35; recreational women, n = 33) completed a discontinuous incremental treadmill test. There were no sex-specific differences in the energy cost of running for the recreational or high-level runners when compared at absolute or relative running speeds (P > .05). The absolute and relative speed-energy cost relationships for the high-level runners demonstrated a curvilinear U shape with a nadir reflecting the most economical speed at 13 km/h or 70% sLTP. The high-level runners were more economical than the recreational runners at all absolute and relative running speeds (P < .05). These findings demonstrate that there is an optimal speed for economical running, there is no sex-specific difference, and high-level endurance runners exhibit better running economy than recreational endurance runners.

Despite an intuitive relationship between technique and both running economy (RE) and performance, and the diverse techniques used by runners to achieve forward locomotion, the objective importance of overall technique and the key components therein remain to be elucidated., Purpose: This study aimed to determine the relationship between individual and combined kinematic measures of technique with both RE and performance., Methods: Ninety-seven endurance runners (47 females) of diverse competitive standards performed a discontinuous protocol of incremental treadmill running (4-min stages, 1-km·h increments). Measurements included three-dimensional full-body kinematics, respiratory gases to determine energy cost, and velocity of lactate turn point. Five categories of kinematic measures (vertical oscillation, braking, posture, stride parameters, and lower limb angles) and locomotory energy cost (LEc) were averaged across 10-12 km·h (the highest common velocity < velocity of lactate turn point). Performance was measured as season's best (SB) time converted to a sex-specific z-score., Results: Numerous kinematic variables were correlated with RE and performance (LEc, 19 variables; SB time, 11 variables). Regression analysis found three variables (pelvis vertical oscillation during ground contact normalized to height, minimum knee joint angle during ground contact, and minimum horizontal pelvis velocity) explained 39% of LEc variability. In addition, four variables (minimum horizontal pelvis velocity, shank touchdown angle, duty factor, and trunk forward lean) combined to explain 31% of the variability in performance (SB time)., Conclusions: This study provides novel and robust evidence that technique explains a substantial proportion of the variance in RE and performance. We recommend that runners and coaches are attentive to specific aspects of stride parameters and lower limb angles in part to optimize pelvis movement, and ultimately enhance performance.

Purpose: Females are known to exhibit a greater risk of ACL injury compared to males. Lower explosive hamstrings-to-quadriceps (H/Q) force ratio in the first 150 ms from activation onset could reflect an impaired capacity for knee joint stabilisation and increased risk of ACL injury. However, the explosive H/Q force ratio has not been compared between the sexes., Methods: The neuromuscular performance of untrained males and females (20 of each) was assessed during a series of isometric knee flexor and extensor contractions, specifically explosive and maximum voluntary contractions of each muscle group. Force, in absolute terms and normalised to body mass, and surface EMG of the hamstrings and quadriceps were recorded. Hamstrings force was expressed relative to quadriceps force to produce ratios of explosive H/Q force and H/Q maximum voluntary force (MVF). For the explosive contractions, agonist electromechanical delay (EMD) and agonist neural activation were also assessed., Results: The H/Q MVF ratio was greater in males (56 %) than females (50 %; P < 0.001). However, the explosive H/Q force ratio was similar between the sexes at each time point (25-150 ms) from activation onset. Explosive hamstrings, but not quadriceps, force relative to body mass was greater for males compared to females. There were no sex differences in EMD or agonist activation for either of the muscle groups., Conclusions: The lack of a sex difference in early phase isometric explosive H/Q force ratio suggests other factors might be more important in determining the substantially higher knee injury rates of females.

Purpose: Dietary nitrate supplementation positively affects cardiovascular function at rest and energy metabolism during exercise in humans and has recently also been reported to markedly enhance the in vitro contractile properties of mouse fast-twitch muscle. The aim of this study was to investigate the effects of short-term nitrate supplementation on the in vivo contractile properties of the skeletal muscle and voluntary muscle function of humans., Methods: In a double-blind, randomized, crossover design, 19 healthy untrained men (21 ± 3 yr) ingested a nitrate-rich concentrated beetroot juice (NIT; nitrate dosage, approximately 9.7 mmol·d) and a placebo (PLA) for seven consecutive days. After the last supplementation dose, force was recorded while participants completed a series of voluntary and involuntary (electrically evoked) unilateral isometric contractions of the knee extensors., Results: NIT enhanced the peak force response to low-frequency electrical stimulation, as follows: maximal twitch (NIT, 149 ± 41 N, vs PLA, 138 ± 37 N; P = 0.008; effect size, r (ES) = 0.56) and submaximal 1- to 20-Hz contractions (5%-10%, ES = 0.53-0.63). Whereas explosive (rising phase) force production during the first 50 ms of evoked maximal twitch and octet contractions (eight electrical impulses at 300 Hz) was also 3%-15% greater after NIT compared with that after PLA (P = 0.023-0.048, ES = 0.52-0.59), explosive voluntary force remained similar (P = 0.510, ES = 0.16). Maximum voluntary force was also unchanged after NIT (P = 0.539, ES = 0.15)., Conclusions: These results indicate that 7 d of dietary nitrate supplementation enhanced the in vivo contractile properties of the human skeletal muscle. Specifically, nitrate supplementation improved excitation-contraction coupling at low frequencies of stimulation and enhanced evoked explosive force production but did not affect maximum or explosive voluntary force production in untrained individuals.

Purpose: The influence of muscle/tendon vibration on maximal muscle performance is unclear. This study examined the effect of a prolonged tendon vibration stimulus on maximum voluntary contraction (MVC) and explosive voluntary contraction (EVC) performance., Methods: Eighteen young healthy males (nine strength trained and nine untrained) completed a series of isometric unilateral knee extensions (EVCs, electrically evoked octet responses, MVCs, ramp contractions) pre and post two separate 30-min intervention trials; infrapatellar tendon vibration (80 Hz), and quiet sitting (control). H max and M max were measured at the start and end of each series of contractions, both pre- and post-intervention (i.e., at four time points). Knee extensor force and both quadriceps and hamstrings EMG were measured throughout each series of contractions., Results: Vibration had no effect on either maximum force (ANOVA, trial × time interaction P = 0.92), explosive force (P ≥ 0.36), or the associated agonist EMG amplitude during these tasks (P ≥ 0.23). Octet responses were also unaffected by vibration (P ≥ 0.39). Conversely, post-intervention H max/M max was 60 % lower in the vibration trial vs. control, and remained 38 % lower at the end of the post-intervention measurements (t test, both P < 0.01). Individual H max/M max depression did not correlate to changes in either maximum or explosive force (Spearman's Rank, P ≥ 0.54), and training status had no influence on the effect of vibration., Conclusion: Prolonged infrapatellar tendon vibration depressed H-reflex amplitude, but did not affect either maximal or explosive isometric force production of the quadriceps.

Purpose: Whilst skeletal muscle hypertrophy is considered an important adaptation to resistance training (RT), it has not previously been found to explain the inter-individual changes in strength after RT. This study investigated the contribution of hypertrophy to individual gains in isometric, isoinertial and explosive strength after 12 weeks of elbow flexor RT., Methods: Thirty-three previously untrained, healthy men (18-30 years) completed an initial 3-week period of elbow flexor RT (to facilitate neurological responses) followed by 6-week no training, and then 12-week elbow flexor RT. Unilateral elbow flexor muscle strength [isometric maximum voluntary force (iMVF), single repetition maximum (1-RM) and explosive force], muscle volume (V(m)), muscle fascicle pennation angle (θ(p)) and normalized agonist, antagonist and stabilizer sEMG were assessed pre and post 12-week RT., Results: Percentage gains in V(m) correlated with percentage changes in iMVF (r = 0.527; P = 0.002) and 1-RM (r = 0.482; P = 0.005) but not in explosive force (r ≤ 0.243; P ≥ 0.175). Percentage changes in iMVF, 1-RM, and explosive force did not correlate with percentage changes in agonist, antagonist or stabilizer sEMG (all P > 0.05). Percentage gains in θ(p) inversely correlated with percentage changes in normalized explosive force at 150 ms after force onset (r = 0.362; P = 0.038)., Conclusions: We have shown for the first time that muscle hypertrophy explains a significant proportion of the inter-individual variability in isometric and isoinertial strength gains following 12-week elbow flexor RT in healthy young men.

Purpose: To compare the effects of short-term maximal (MST) vs. explosive (EST) strength training on maximal and explosive force production, and assess the neural adaptations underpinning any training-specific functional changes., Methods: Male participants completed either MST (n = 9) or EST (n = 10) for 4 weeks. In training participants were instructed to: contract as fast and hard as possible for ~1 s (EST); or contract progressively up to 75% maximal voluntary force (MVF) and hold for 3 s (MST). Pre- and post-training measurements included recording MVF during maximal voluntary contractions and explosive force at 50-ms intervals from force onset during explosive contractions. Neuromuscular activation was assessed by recording EMG RMS amplitude, normalised to a maximal M-wave and averaged across the three superficial heads of the quadriceps, at MVF and between 0-50, 0-100 and 0-150 ms during the explosive contractions., Results: Improvements in MVF were significantly greater (P < 0.001) following MST (+21 ± 12%) than EST (+11 ± 7%), which appeared due to a twofold greater increase in EMG at MVF following MST. In contrast, early phase explosive force (at 100 ms) increased following EST (+16 ± 14%), but not MST, resulting in a time × group interaction effect (P = 0.03), which appeared due to a greater increase in EMG during the early phase (first 50 ms) of explosive contractions following EST (P = 0.052)., Conclusions: These results provide evidence for distinct neuromuscular adaptations after MST vs. EST that are specific to the training stimulus, and demonstrate the independent adaptability of maximal and explosive strength.

Background and Aims: Whilst extensive research has detailed the loss of muscle strength with ageing for isolated single joint actions, there has been little attention to power production during more functionally relevant multiple joint movements. The extent to which force or velocity are responsible for the loss in power with ageing is also equivocal. The aim of this study was to evaluate the contribution of force and velocity to the differences in power with age by comparing the force-velocity and power-velocity relationships in young and older men during a multiple joint leg press movement., Methods: Twenty-one older men (66 ± 3 years) and twenty-three young men (24 ± 2 years) completed a series of isometric (maximum and explosive) and dynamic contractions on a leg press dynamometer instrumented to record force and displacement., Results: The force-velocity relationship was lower for the older men as reflected by their 19 % lower maximum isometric strength (p < 0.001). Explosive isometric strength (peak rate of force development) was 21 % lower for the older men (p < 0.05) but was similar between groups when normalised to maximum strength (p = 0.58). The power-velocity relationship was lower for the older men as shown by reduced maximum power (-28 %, p < 0.001) and lower force (-20 %, p < 0.001) and velocity (-11 %, p < 0.05)., Conclusions: Whilst force and velocity were lower in older men, the decrement in force was greater and therefore the major explanation for the attenuation of power during a functionally relevant multiple joint movement.

Introduction: There is little evidence as to whether exercise can increase BMD in older men with no investigation of high impact exercise. Lifestyle changes and individual variability may confound exercise trials but can be minimised using a within-subject unilateral design (exercise leg [EL] vs. control leg [CL]) that has high statistical power., Purpose: This study investigated the influence of a 12month high impact unilateral exercise intervention on femoral neck BMD in older men., Methods: Fifty, healthy, community-dwelling older men commenced a 12month high impact unilateral exercise intervention which increased to 50 multidirectional hops, 7days a week on one randomly allocated leg. BMD of both femurs was measured using dual energy X-ray absorptiometry (DXA) before and after 12months of exercise, by an observer blind to the leg allocation. Repeated measures ANOVA with post hoc tests was used to detect significant effects of time, leg and interaction., Results: Thirty-five men (mean±SD, age 69.9±4.0years) exercised for 12months and intervention adherence was 90.5±9.1% (304±31 sessions completed out of 336 prescribed sessions). Fourteen men did not complete the 12month exercise intervention due to: health problems or injuries unrelated to the intervention (n=9), time commitments (n=2), or discomfort during exercise (n=3), whilst BMD data were missing for one man. Femoral neck BMD, BMC and cross-sectional area all increased in the EL (+0.7, +0.9 and +1.2 % respectively) compared to the CL (-0.9, -0.4 and -1.2%); interaction effect P<0.05. Although the interaction term was not significant (P>0.05), there were significant main effects of time for section modulus (P=0.044) and minimum neck width (P=0.006). Section modulus increased significantly in the EL (P=0.016) but not in the CL (P=0.465); mean change +2.3% and +0.7% respectively, whereas minimum neck width increased significantly in the CL (P=0.004) but not in the EL (P=0.166); mean changes being +0.7% and +0.3% respectively., Conclusion: A 12month high impact unilateral exercise intervention was feasible and effective for improving femoral neck BMD, BMC and geometry in older men. Carefully targeted high impact exercises may be suitable for incorporation into exercise interventions aimed at preventing fractures in healthy community-dwelling older men., (Copyright © 2013 Elsevier Inc. All rights reserved.)

The influence of contraction type on the human ability to use the torque capacity of skeletal muscle during explosive efforts has not been documented. Fourteen male participants completed explosive voluntary contractions of the knee extensors in four separate conditions: concentric (CON) and eccentric (ECC); and isometric at two knee angles (101°, ISO101 and 155°, ISO155). In each condition, torque was measured at 25 ms intervals up to 150 ms from torque onset, and then normalized to the maximum voluntary torque (MVT) specific to that joint angle and angular velocity. Explosive voluntary torque after 50 ms in each condition was also expressed as a percentage of torque generated after 50 ms during a supramaximal 300 Hz electrically evoked octet in the same condition. Explosive voluntary torque normalized to MVT was more than 60 per cent larger in CON than any other condition after the initial 25 ms. The percentage of evoked torque expressed after 50 ms of the explosive voluntary contractions was also greatest in CON (ANOVA; p < 0.001), suggesting higher concentric volitional activation. This was confirmed by greater agonist electromyography normalized to M(max) (recorded during the explosive voluntary contractions) in CON. These results provide novel evidence that the ability to use the muscle's torque capacity explosively is influenced by contraction type, with concentric contractions being more conducive to explosive performance due to a more effective neural strategy.