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Individuals with anterior cruciate ligament reconstruction (ACLR) have quadriceps dysfunction that contributes to physical disability and posttraumatic knee osteoarthritis. Quadriceps function in the ACLR limb is commonly evaluated relative to the contralateral uninjured limb. Bilateral quadriceps dysfunction is common in individuals with ACLR, potentially biasing these evaluations.   To compare quadriceps function between individuals with ACLR and uninjured control participants.   Cross-sectional study.   Research laboratory.   Twenty individuals with unilateral ACLR (age = 21.1 ± 1.7 years, mass = 68.3 ± 14.9 kg, time since ACLR = 50.7 ± 21.3 months; females = 14; Tegner Score = 7.1 ± 0.3; 16 patellar tendon autografts, 3 hamstrings autografts, 1 allograft) matched to 20 control participants (age = 21.2 ± 1.2 years, mass = 67.9 ± 11.3 kg; females = 14; Tegner Score = 7.1 ± 0.4) on age, sex, body mass index, and Tegner Activity Scale.   Maximal voluntary isometric knee extension was performed on an isokinetic dynamometer. Peak torque (PT), rate of torque development (RTD), electromyographic (EMG) amplitude, central activation ratio (CAR), and hamstrings EMG amplitude were assessed during maximal voluntary isometric knee extension and compared between groups using independent-samples t tests. Relationships between hamstrings co-activation and quadriceps function were assessed using Pearson correlations.   Participants with anterior cruciate ligament reconstruction displayed lesser quadriceps PT (1.86 ± 0.74 versus 2.56 ± 0.37 Nm/kg, P = .001), RTD (39.4 ± 18.7 versus 52.9 ± 16.4 Nm/s/kg, P = .03), EMG amplitude (0.25 ± 0.12 versus 0.37 ± 0.26 mV, P = .04), and CAR (83.3% ± 11.1% versus 93.7% ± 3.2%, P = .002) and greater hamstrings co-activation (27.2% ± 12.8% versus 14.3% ± 3.7%, P < .001) compared with control participants. Correlations were found between hamstrings co-activation and PT (r = -0.39, P = .007), RTD (r = -0.30, P = .03), and EMG amplitude (r = -0.30, P = .03).   Individuals with ACLR possessed deficits in PT, RTD, and CAR compared with control participants. Peak torque is the net result of all agonist and antagonist activity, and lesser PT in individuals with ACLR is partially attributable to greater hamstrings co-activation.

Declines in muscle size and strength are commonly reported as a consequence of aging; however, few studies have investigated the influence of aging on the rate of muscle activation and rapid force characteristics across the lifespan. This study aims to investigate the effects of aging on the rate of muscle activation and rapid force characteristics of the plantar flexors. Plantar flexion peak force (PF), absolute (peak, 50, and 100–200 ms), and relative (10 %, 30 %, and 50 %) rate of force development (RFD), the rapid to maximal force ratio (RFD/PF), and the rate of electromyography rise (RER) were examined during an isometric maximal voluntary contraction (MVC) in young (age = 22 ± 2 years), middle-aged (43 ± 2 years), and old (69 ± 5 years) men. The old men exhibited lower PF (30.7 % and 27.6 % lower, respectively) and absolute (24.4–55.1 %) and relative (16.4–28.9 %) RFD values compared to the young and middle-aged men ( P  ≤ 0.03). RER values were similar between the young and old men ( P  ≥ 0.30); however, RER values were greater for the middle-aged men when compared to the young and old men for the soleus ( P  < 0.01) and the old men for the medial gastrocnemius ( P  ≤ 0.02). Likewise, RFD/PF ratios were similar between young and old men ( P  ≥ 0.26); however, these ratios were greater for the middle-aged men at early ( P  ≤ 0.03), but not later ( P  ≥ 0.10), time intervals. The lower PF and absolute and relative RFD values for the old men may contribute to the increased functional limitations often observed in older adults. Interestingly, higher rates of muscle activation and greater early RFD/PF ratios in middle-aged men may be a reflection of physiological alterations in the neuromuscular system occurring in the fifth decade.

The purpose of this study was to examine the acute effects of static versus dynamic stretching on peak torque (PT) and electromyographic (EMG), and mechanomyographic (MMG) amplitude of the biceps femoris muscle (BF) during isometric maximal voluntary contractions of the leg flexors at four different knee joint angles. Fourteen men ((mean ± SD) age, 25 ± 4 years) performed two isometric leg flexion maximal voluntary contractions at knee joint angles of 41°, 61°, 81°, and 101° below full leg extension. EMG (μV) and MMG (m·s) signals were recorded from the BF muscle while PT values (Nm) were sampled from an isokinetic dynamometer. The right hamstrings were stretched with either static (stretching time, 9.2 ± 0.4 minutes) or dynamic (9.1 ± 0.3 minutes) stretching exercises. Four repetitions of three static stretching exercises were held for 30 seconds each, whereas four sets of three dynamic stretching exercises were performed (12-15 repetitions) with each set lasting 30 seconds. PT decreased after the static stretching at 81° (p = 0.019) and 101° (p = 0.001) but not at other angles. PT did not change (p > 0.05) after the dynamic stretching. EMG amplitude remained unchanged after the static stretching (p > 0.05) but increased after the dynamic stretching at 101° (p < 0.001) and 81° (p < 0.001). MMG amplitude increased in response to the static stretching at 101° (p = 0.003), whereas the dynamic stretching increased MMG amplitude at all joint angles (p ≤ 0.05). These results suggested that the decreases in strength after the static stretching may have been the result of mechanical rather than neural mechanisms for the BF muscle. Overall, an acute bout of dynamic stretching may be less detrimental to muscle strength than static stretching for the hamstrings.

Background Citrulline is an increasingly common dietary supplement that is thought to enhance exercise performance by increasing nitric oxide production. In the last 5 years, several studies have investigated the effects of citrulline supplements on strength and power outcomes, with mixed results reported. To date, the current authors are unaware of any attempts to systematically review this emerging body of literature. Objective The current study sought to conduct a systematic review and meta-analysis of the literature describing the effects of citrulline supplementation on strength and power outcomes. Methods A comprehensive, systematic search of three prominent research databases was performed to find peer-reviewed, English language, original research studies evaluating the effects of citrulline supplementation on indices of high-intensity exercise performance in healthy men and women. Outcomes included strength and power variables from performance tests involving multiple repetitive muscle actions of large muscle groups, consisting of either resistance training sets or sprints lasting 30 s or less. Tests involving isolated actions of small muscle groups or isolated attempts of single-jump tasks were not included for analysis due to differences in metabolic requirements. Studies were excluded from consideration if they lacked a placebo condition for comparison, were carried out in clinical populations, provided a citrulline dose of less than 3 g, provided the citrulline dose less than 30 min prior to exercise testing, or combined the citrulline ingredient with creatine, caffeine, nitrate, or other ergogenic ingredients. Results Twelve studies, consisting of 13 total independent samples ( n  = 198 participants), met the inclusion criteria. Between-study variance, heterogeneity, and inconsistency across studies were low (Cochrane’s Q  = 6.9, p  = 0.86; τ 2  = 0.0 [0.0, 0.08], I 2  = 0.0 [0.0, 40.0]), and no funnel plot asymmetry was present. Results of the meta-analysis identified a significant benefit for citrulline compared to placebo treatments ( p  = 0.036), with a small pooled standardized mean difference (SMD; Hedges’ G ) of 0.20 (95% confidence interval 0.01–0.39). Conclusion The effect size was small (0.20), and confidence intervals for each individual study crossed the line of null effect. However, the results may be relevant to high-level athletes, in which competitive outcomes are decided by small margins. Further research is encouraged to fully elucidate the effects of potential moderating study characteristics, such as the form of citrulline supplement, citrulline dose, sex, age, and strength versus power tasks.

The purpose of this study was to investigate the acute effects of passive stretching on the electromechanical delay (EMD), peak twitch force (PTF), rate of force development (RFD), and compound muscle action potential (M-wave) amplitude during evoked twitches of the plantar flexor muscles. 16 men (mean age ± SD = 21.1 ± 1.7 years; body mass = 75.9 ± 11.4 kg; height = 176.5 ± 8.6 cm) participated in this study. A single, square-wave, supramaximal transcutaneous electrical stimulus was delivered to the tibial nerve before and after passive stretching. The stretching protocol consisted of nine repetitions of passive assisted stretching designed to stretch the calf muscles. Each repetition was held for 135 s separated by 5–10 s of rest. Dependent-samples t tests (pre- vs. post-stretching) were used to analyze the EMD, PTF, RFD, and M-wave amplitude data. There were significant changes ( P  ≤ 0.05) from pre- to post-stretching for EMD (mean ± SE = 4.84 ± 0.31 and 6.22 ± 0.34 ms), PTF (17.2 ± 1.3 and 15.6 ± 1.5), and RFD (320.5 ± 24.5 and 279.8 ± 28.2), however, the M-wave amplitude did not change ( P  > 0.05). These findings suggested that passively stretching the calf muscles affected the mechanical aspects of force production from the onset of the electrically evoked twitch to the peak twitch force. These results may help to explain the mechanisms underlying the stretching-induced force deficit that have been reported as either “mechanical” or “electrical” in origin.

Few investigations have evaluated the validity of current body composition technology among racially and ethnically diverse populations. This study assessed the validity of common body composition methods in a multi-ethnic sample stratified by race and ethnicity. One hundred and ten individuals (55 % female, age: 26·5 (sd 6·9) years) identifying as Asian, African American/Black, Caucasian/White, Hispanic, Multi-racial and Native American were enrolled. Seven body composition models (dual-energy X-ray absorptiometry (DXA), air displacement plethysmography (ADP), two bioelectrical impedance devices (BIS, IB) and three multi-compartment models) were evaluated against a four-compartment criterion model by assessing total error (TE) and standard error of the estimate. For the total sample, measures of % fat and fat-free mass (FFM) from multi-compartment models were all excellent to ideal (% fat: TE = 0·94–2·37 %; FFM: TE = 0·72–1·78 kg) compared with the criterion. % fat measures were very good to excellent for DXA, ADP and IB (TE = 2·52–2·89 %) and fairly good for BIS (TE = 4·12 %). For FFM, single device estimates were good (BIS; TE = 3·12 kg) to ideal (DXA, ADP, IB; TE = 1·21–2·15 kg). Results did not vary meaningfully between each race and ethnicity, except BIS was not valid for African American/Black, Caucasian/White and Multi-racial participants for % fat (TE = 4·3–4·9 %). The multi-compartment models evaluated can be utilised in a multi-ethnic sample and in each individual race and ethnicity to obtain highly valid results for % fat and FFM. Estimates from DXA, ADP and IB were also valid. The BIS may demonstrate greater TE for all racial and ethnic cohorts and results should be interpreted cautiously.

The acute effects of stretching on peak force ( F peak ), percent voluntary activation (%VA), electromyographic (EMG) amplitude, maximum range of motion (MROM), peak passive torque, the passive resistance to stretch, and the percentage of ROM at EMG onset (%EMGonset) were examined in 18 young and 19 old men. Participants performed a MROM assessment and a maximal voluntary contraction of the plantarflexors before and immediately after 20 min of passive stretching. F peak (−11 %), %VA (−6 %), and MG EMG amplitude (−9 %) decreased after stretching in the young, but not the old ( P  > 0.05). Changes in F peak were related to reductions in all muscle activation variables ( r  = 0.56–0.75), but unrelated to changes in the passive resistance to stretch ( P  ≥ 0.24). Both groups experienced increases in MROM and peak passive torque and decreases in the passive resistance to stretch. However, the old men experienced greater changes in MROM ( P  < 0.001) and passive resistance ( P  = 0.02–0.06). Changes in MROM were correlated to increases in peak passive torque ( r  = 0.717), and the old men also experienced a nonsignificant greater ( P  = 0.08) increase in peak passive torque. %EMGonset did not change from pre- to post-stretching for both groups ( P  = 0.213), but occurred earlier in the old ( P  = 0.06). The stretching-induced impairments in strength and activation in the young but not the old men may suggest that the neural impairments following stretching are gamma-loop-mediated. In addition, the augmented changes in MROM and passive torque and the lack of change in %EMGonset for the old men may be a result of age-related changes in muscle-tendon behavior.

Baseball pitching load is linked to injury in adolescent baseball athletes; however, it is unclear if pitch counts are a good indicator of total upper extremity load during baseball pitching. The purpose of this study is to determine (1) the recovery time-course of musculoskeletal variables after a single live pitching bout and (2) the association between pitch counts, rating of perceived exertion (RPE), and arm-specific session RPE on musculoskeletal changes after live game pitching in adolescent baseball athletes. Cross-sectional study. Competitive baseball games. Thirty-six adolescent baseball pitchers (16.1 ± 0.9 years, 178.2 ± 10.4 cm, 71.5 ± 10.2 kg). Internal and external shoulder range of motion, internal and external shoulder rotation peak force, and infraspinatus cross-sectional area and echo intensity were collected before pitching, immediately after pitching, and on days 1, 3, and 5 after pitching. Pitch count and RPE were collected during the pitching bout, and an arm-specific session RPE score was calculated as the product of pitch count and RPE. Linear mixed models were used to determine the recovery time-course on both arms and to determine the association between the load variables (pitching count, RPE, arm-specific session RPE) and the change in the musculoskeletal variables on the dominant arm. Internal shoulder range of motion was highest on day 3 (mean difference: 3.31, = 3.12, = .019), and external shoulder rotation peak force decreased immediately after pitching (-11.53, = 3.51, = .005) and increased at day 5 (14.8, = 4.52, < .001). Internal shoulder rotation peak force was lowest immediately after pitching and highest at day 5 (19.14, = 4.18, < .001). No significant ( > .057) association was found between load variables and musculoskeletal variables. Baseball-specific pitching load metrics did not predict musculoskeletal changes after live game pitching. Future researchers should investigate pitching load variables that better predict musculoskeletal changes.

The purposes of the present study were to (1) characterize viscoelastic creep in vivo in the human skeletal muscle–tendon unit and (2) to examine the consistency of these responses during a single 30-s stretch. Twelve volunteers (mean ± SD = 22 ± 3 years; height = 169 ± 11 cm; mass = 70 ± 17 kg) participated in two separate experimental trials. Each trial consisted of a 30-s constant-torque stretch of the plantar flexor muscles. Position (°) values were quantified at every 5-s period (0, 5, 10, 15, 20, 25, and 30 s) and the percent change in position was quantified for each 5-s epoch (0–5, 5–10, 10–15, 15–20, 20–25, and 25–30 s) relative to the total increase in the range of motion. In addition, the intraclass correlation coefficient (ICC) and standard errors of the measurement (SEM) were calculated for test–retest reliability. These results indicated that position increased over the entire 30-s stretch ( P  < 0.05), while the majority of the increases in position (73–85%) occurred during the first 15–20 s. ICC values were > 0.994 and SEM values (expressed as percentage of the mean) were < 1.54%. In conclusion, these results demonstrate viscoelastic creep in vivo in the human skeletal muscle–tendon unit and suggest that these responses may be reliable for future studies.

Understanding the effects of acute feeding on body composition and metabolic measures is essential to the translational component and practical application of measurement and clinical use. To investigate the influence of acute feeding on the validity of dual-energy X-ray absorptiometry (DXA), a four-compartment model (4C) and indirect calorimetry metabolic outcomes, thirty-nine healthy young adults (n 19 females; age: 21·8 (sd 3·1) years, weight; 71·5 (sd 10·0) kg) participated in a randomised cross-over study. Subjects were provided one of four randomised meals on separate occasions (high carbohydrate, high protein, ad libitum or fasted baseline) prior to body composition and metabolic assessments. Regardless of macronutrient content, acute feeding increased DXA percent body fat (%fat) for the total sample and females (average constant error (CE):–0·30 %; total error (TE): 2·34 %), although not significant (P = 0·062); the error in males was minimal (CE: 0·11 %; TE: 0·86 %). DXA fat mass (CE: 0·26 kg; TE: 0·75 kg) and lean mass (LM) (CE: 0·83 kg; TE: 1·23 kg) were not altered beyond measurement error for the total sample. 4C %fat was significantly impacted from all acute feedings (avg CE: 0·46 %; TE: 3·7 %). 4C fat mass (CE: 0·71 kg; TE: 3·38 kg) and fat-free mass (CE: 0·55 kg; TE: 3·05 kg) exceeded measurement error for the total sample. RMR was increased for each feeding condition (TE: 1666·9 kJ/d; 398 kcal/d). Standard pre-testing fasting guidelines may be important when evaluating DXA and 4C %fat, whereas additional DXA variables (fat mass and LM) may not be significantly impacted by an acute meal. Measuring body composition via DXA under less stringent pre-testing guidelines may be valid and increase feasibility of testing in clinical settings.