A Joint Power Approach to Define Countermovement Jump Phases Using Force Platforms Conflicting methodologies are used to define certain countermovement vertical jump (CMVJ) phases, which limits identification of performance-enhancing factors (e.g., rate of force development). Purpose We (a) utilized a joint power approach to define CMVJ phases that accurately describe body weight unloading (i.e., unweighting) and eccentric (i.e., braking) actions, which were combined with the robustly-defined concentric (i.e., propulsion) phase, and (b) determined whether the phases can be identified using only ground reaction force (GRF) data. Methods Twenty-one men performed eight maximal CMVJs while kinematic and GRF data were obtained. Hip, knee, and ankle joint powers were calculated using inverse dynamics. The net sum of the joint powers (JPSUM) was calculated to define phases by the preeminence of negative (i.e., net eccentric actions) or positive (i.e., net concentric actions) power where appropriate. Unloading, eccentric, and concentric phases were identified using JPSUM and linked to GRF and center of mass (COM) velocity features. Results Bland & Altman plots of the bias and 95% confidence intervals for the limits of agreement (LOA), intra-class correlation coefficients (ICC), and coefficients of variation (CV) indicated precise agreement for detecting the unloading (bias: 0.060 s; LOA: -0.110 to 0.229 s) and eccentric (bias: 0.012 s; LOA: -0.010 to 0.040 s) phases with moderate (ICC: 0.578; CV: 40.72%) and excellent (ICC: 0.993; CV: 2.18%) reliability, respectively. The eccentric phase should be divided into yielding (eccentric actions while accelerating downward) and braking (eccentric actions while decelerating downward) sub-phases for detailed assessments. Conclusion CMVJ phases defined by combining joint and COM mechanics can be detected using only force platform data, enabling functionally relevant CMVJ assessments using instrumentation commonly available to practitioners. Corresponding Author: John R. Harry, Ph.D., CSCS, Human Performance & Biomechanics Laboratory, Department of Kinesiology & Sport Management, Texas Tech University, 3204 Main Street, Lubbock, TX 79409, Tel: 806-835-7836, Fax: 806-742-1688, Email: john.harry@ttu.edu The project was partially supported by a grant from the National Strength and Conditioning Association (NSCA) Foundation. The NSCA Foundation did not contribute to the study design, the collection, analysis, and interpretation of the data, the writing of the manuscript, or the decision to publish. The contents of this project are the sole responsibility of the authors and do not necessarily represent the views of the NSCA. The authors have no conflicts of interest to disclose. The results of the present study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The results of the present study do not constitute endorsement by ACSM. Accepted for Publication: 11 October 2019 © 2019 American College of Sports Medicine

Evaluation of Early Submaximal Exercise Tolerance in Adolescents with Symptomatic Sport-related Concussion Purpose To compare the cardiorespiratory response to a graded aerobic exercise challenge between adolescents with symptomatic sport-related concussion (SSRC) and healthy control subjects. Methods A quasi-experimental non-randomized study at a multi-disciplinary pediatric concussion program was conducted. Thirty-four adolescents with SSRC (19 males and 15 females) and 40 healthy control subjects (13 males and 27 females) completed Buffalo Concussion Treadmill testing (BCTT) until either symptom exacerbation or volitional fatigue. Main outcome measures included heart rate (HR), oxygen consumption (V˙O2), carbon dioxide production (V˙CO2), and minute ventilation (V˙E) at rest and at test termination, and change from rest in variables (ΔHR, ΔV˙O2, ΔV˙CO2, and ΔV˙E) during the first 5 stages of the BCTT. Main outcomes were analyzed using three-way mixed model ANOVAs, with group status (control vs. SSRC) and sex (male vs. female) as between-subjects factors, and time (BCTT stage) as the within-subjects factor. Results No group differences in resting HR, systolic and diastolic blood pressure (BP), ΔV˙O2, V˙CO2, and V˙E were observed. During the first 5 stages of the BCTT, no group differences in ΔV˙O2, V˙CO2, and V˙E were observed, however SSRC patients demonstrated higher ratings of perceived exertion (p < .0005) compared to control subjects. No sex-based differences were observed among SSRC patients on measures collected at rest and during early stages of BCTT. Conclusions Although SSRC patients exhibited higher ratings of perceived exertion during a graded aerobic exercise challenge, no differences in cardiorespiratory response were observed compared to control subjects exercising at equivalent workloads. Further work is needed to elucidate the physiological mechanisms underlying exercise intolerance following SSRC. Corresponding author: Marc P. Morissette, 75 Poseidon Bay, Winnipeg, Manitoba, R3M 3E4, Tel: 204-925-1558, Fax: 204-927-2775, E-mail: mmorissette2@panamclinic.com The authors acknowledge the Pan Am Clinic Foundation for their financial support of this project. M. P. Morissette was supported by a Canadian Institutes of Health Research Frederick Banting and Charles Best Canada Graduate Scholarship. The authors have no personal or financial conflicts of interest to disclose. The results from the present study do not constitute endorsement by the American College of Sports Medicine. The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. Accepted for Publication: 14 October 2019 © 2019 American College of Sports Medicine

Planned Aerobic Exercise Increases Energy Intake at the Preceding Meal Purpose Effects of exercise on subsequent energy intake are well documented, but whether pre-exercise energy intake is affected by future planned exercise is unknown. This study investigated the effect of planned late-afternoon exercise on appetite and energy intake before (breakfast and lunch) and after (evening meal/snacks) exercise. Methods Twenty healthy, active participants (10 male; age 23 ± 5 y, BMI 23.7 ± 3.2 kg/m2, VO2peak 44.1 ± 5.4 ml/kg/min) completed randomised, counterbalanced exercise (EX) and resting (REST) trials. After trial notification, participants were provided ad libitum breakfast (0800 h) and lunch (1200 h) in the laboratory, before completing 1-h exercise (30 min cycling, 30 min running) at 75-80% maximal HR (EX; 2661 ± 783 kJ) or 1-h supine rest (REST; 310 ± 58 kJ) 3-h post-lunch. Participants were provided a food pack (pasta meal/snacks) for consumption post-exercise (outside laboratory). Appetite was measured regularly and meal and 24-hour energy intake quantified. Results Ad-libitum energy intake was greater during EX at lunch (EX 3450 ± 1049 kJ; REST 3103 ± 927 kJ; P=0.004), but similar between trials at breakfast (EX 2656 ± 1291 kJ; REST 2484 ± 1156 kJ; P=0.648) and dinner (EX 6249 ± 2216 kJ; REST 6240 ± 2585 kJ; P=0.784). Total 24-hour energy intake was similar between trials (P=0.388), meaning relative energy intake (24-h energy intake minus EX/REST energy expenditure), was reduced during EX (EX 9694 ± 3313 kJ; REST 11517 ± 4023 kJ; P=0.004). Conclusion Energy intake appears to be increased in anticipation of, rather than in response to, aerobic exercise, but the increase was insufficient to compensate for energy expened during exercise, meaning aerobic exercise reduced energy balance relative to rest. Corresponding author: Lewis J. James, L.James@lboro.ac.uk. School of Sport, Exercise and Health Sciences, Loughborough University, Leicestershire, United Kingdom LJJ is part of the National Institute for Health Research (NIHR) Leicester Biomedical Research Centre, which is a partnership between University Hospitals of Leicester NHS Trust, Loughborough University and the University of Leicester. This report is independent research by the National Institute for Health Research. The views expressed in this publication are those of the authors and not necessarily those of the NHS, the National Institute for Health Research or the Department of Health. No other funding was received for this study. The authors declare no conflict of interest. The results of the present study do not constitute endorsement by the American College of Sports Medicine. Accepted for Publication: 19 September 2019. © 2019 American College of Sports Medicine

The Effect of Shortening-induced Torque Depression on Fatigue-related Sex Differences Residual torque depression (rTD) is the decrease in isometric torque following active shortening of skeletal muscle, compared to a purely isometric (ISO) contraction performed at the same muscle length and level of activation. Performance fatigability is defined as any exercise-induced reduction in voluntary force or power, and females are typically more fatigue resistant than males at low intensity isometric contractions. Purpose This study investigated performance fatigability in males and females during ISO contractions and isometric contractions following active shortening (rTD). Methods 14 females (22 ± 2 years) and 14 males (23 ± 2 years) performed three baseline maximum voluntary contractions (MVCs) of the dorsiflexors. The MVCs were used to determine a 30% submaximal torque target, which participants matched as steadily as possible until task failure. The ISO fatigue task was performed at 10° plantar flexion. In the rTD session, the participants’ ankle was rotated from 40° to 10° plantar flexion prior to performing the same fatigue task. MVCs were performed immediately after task failure, 30-s, 1, 2, 3, 4, 5, 10, 20, and 30-mins after task failure to track recovery. Results The baseline MVC torque amplitude for males (32.1 ± 6.6 N·m) was 31% greater than females (22.3 ± 3.1 N·m) (p < 0.001, ηp2 = 0.490). Female’s time to task failure was 44% longer than males in the ISO state (p = 0.032, ηp2 = 0.164). However, there was no sex-difference in the rTD state (p = 0.142). Conclusion It appears that the sex-differences in fatigue resistance observed in a low intensity ISO task are abolished in the isometric state following an active shortening contraction. Correspondence: Geoffrey A. Power PhD, Neuromechanical Performance Research Laboratory, Department of Human Health and Nutritional Sciences, College of Biological Sciences, University of Guelph, Ontario, Canada, Telephone: 1-519-824-4120 x53752, Email: gapower@uoguelph.ca The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The results of the present study do not constitute endorsement by ACSM. This project was supported by the Natural Science and Engineering Research Council of Canada (NSERC). Infrastructure was provided by the University of Guelph start-up funding. No conflicts of interest, financial or otherwise, are declared by the authors. Accepted for Publication: 16 October 2019 © 2019 American College of Sports Medicine

Age-related Deficits in Voluntary Activation: A Systematic Review and Meta-analysis Whether there are age-related differences in neural drive during maximal effort contractions is not clear. This review determined the effect of age on voluntary activation during maximal voluntary isometric contractions (MVC). The literature was systematically reviewed for studies reporting voluntary activation quantified with the interpolated twitch technique (ITT) or central activation ratio (CAR) during isometric contractions in young (18-35 years) and old adults (>60 years, mean ≥65 years). Of the 2,697 articles identified, 54 were eligible for inclusion in the meta-analysis. Voluntary activation was assessed with electrical stimulation (ES) and transcranial magnetic stimulation (TMS) on 5 different muscle groups. Random-effects meta-analysis revealed lower activation in old compared with young adults (d=-0.45, 95% CI [-0.62 – -0.29], p<0.001), with moderate heterogeneity (52.4%). To uncover the sources of heterogeneity, subgroup analyses were conducted for muscle group, calculation method (ITT or CAR), and stimulation type (ES or TMS) and number (single, paired or train stimulations). The age-related reduction in voluntary activation occurred for all muscle groups investigated except the ankle dorsiflexors. Both ITT and CAR demonstrated an age-related reduction in voluntary activation of the elbow flexors, knee extensors and plantar flexors. ITT performed with paired and train stimulations showed lower activation for old than young adults with no age difference for the single electrical stimulation. Together, the meta-analysis revealed that healthy older adults have a reduced capacity to activate some upper and lower limb muscles during MVCs; however, the effect was modest and best assessed with at least paired stimulations to detect the difference. Sandra K. Hunter and Ashleigh E. Smith share senior authorship Corresponding Author: Sandra K Hunter, PhD, Exercise Science Program, Department of Physical Therapy, Marquette University, P.O. Box 1881. Milwaukee, 53201, WI, USA This work was supported by a National Institute of Aging R01 (AG048262) to Sandra Hunter and Robert Fitts, a National Institute of Aging Ruth L. Kirschstein predoctoral fellowship (F31 AG052313) to Christopher Sundberg, and a NHRMC-ARC Dementia Research Development Fellowship (APP1097397) to Ashleigh E. Smith. For the remaining authors none were declared. Authors have no professional relationships with companies or manufacturers who will benefit from the results of the present study. Results of the present study do not constitute endorsement by ACSM. Results of the study are presented clearly, honestly and without fabrication, falsification, or inappropriate data manipulation. Accepted for Publication: 29 August 2019. © 2019 American College of Sports Medicine

Baseline and Exercise Predictors of VO2peak in Systolic Heart Failure Patients: Results from SMARTEX-HF Purpose To investigate baseline, exercise testing, and exercise training-mediated predictors of change in peak oxygen uptake (VO2peak) from baseline to 12-week follow-up (ΔVO2peak) in a post-hoc analysis from the SMARTEX Heart Failure trial. Methods We studied 215 patients with heart failure with left ventricular ejection fraction (LVEF) <35%, and NYHA class II-III, who were randomized to either supervised high intensity interval training (HIIT) with exercise target intensity 90-95% of peak heart rate (HRpeak), supervised moderate continuous training (MCT) with target intensity 60-70% of HRpeak, or who received a recommendation of regular exercise on their own (RRE). Predictors of ΔVO2peak were assessed in two models; A logistic regression model comparing highest and lowest tertile (baseline parameters) and a multivariate linear regression model (test/training/clinical parameters). Results The change in VO2peak in response to the interventions (ΔVO2peak) varied substantially, from -8.50 to +11.30 mL·kg-1·min-1. Baseline NYHA (class II gave higher odds vs III, odds ratio (OR) 7.1 (2.0, 24.9), p=0.002), LVEF OR per % 1.1 (1.0, 1.2), p = 0.005), age (OR per 10 years 0.5 (0.3, 0.8)), p=0.003) were associated with ΔVO2peak. In the multivariate linear regression, 34% of the variability in [INCREMENT]VO2peak was explained by the increase in exercise training workload, [INCREMENT]HRpeak between baseline and 12-wk post-testing, age, and ever having smoked. Conclusion Exercise training response (ΔVO2peak) correlated negatively with age, LVEF and NYHA class. The ability to increase workload during the training period, and increased ΔHRpeak between baseline and the 12-week test were associated with a positive outcome. Trine Karlsen and Vibeke Videm share first authorship on the publication Corresponding author: Dean Trine Karlsen, MSc PhD, Faculty of Nursing and Health Sciences, P.O.Box 1490, NO-8049 Bodø, Norway, Telephone. +47 92644422. Email: trine.karlsen@nord.no Funding Sources This work was supported by St. Olavs Hospital; Faculty of Medicine and Health Sciences, NTNU – Norwegian University of Science and Technology; Norwegian Health Association; Danish Research Council; Central Norwegian Health Authorities/NTNU; Western Norway Health Authorities; Simon Fougner Hartmanns Familiefond; Else-Kröner-Fresenius-Stiftung, and Société Luxembourgeoise pour la recherche sur les maladies cardio-vasculaires. Conflict of Interests Disclosures MH reports grants from the Else-Kröner-Fresenius Foundation for the present work and is on the advisory board of Novartis, Sanofi-Aventis and MSD outside of the present study. AL reports grants and personal fees from Medtronic and from Claret Medical, and personal fees from Edwards, SJM, Bard, and Symetis, all outside of the present study. The results of this study do not constitute endorsement by ACSM. The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. Accepted for Publication: 23 July 2019. © 2019 American College of Sports Medicine

Neuromuscular Electrical Stimulation Preserves Leg Lean Mass in Geriatric Patients Aim To examine changes in lean mass during hospitalization in geriatric patients and the effect of muscle activation by neuromuscular electrical stimulation. Methods Thirteen patients (69-94 years) at a geriatric ward completed tests at hospital admission (day 2-3) and discharge (day 8-10). One leg received daily stimulation of the knee extensors while the other leg served as a control leg. Lean mass was evaluated by DEXA scans and muscle thickness by ultrasound scans. Muscle biopsies were collected from both legs at admission and discharge in 9 patients and analyzed for fibre size, satellite cell number and activation and expression of genes associated with muscle protein synthesis and breakdown, connective tissue and cellular stress. Results The relative decline in leg lean mass and midthigh region lean mass was larger in the control (-2.8±1.5%) vs. the stimulated leg (-0.5±1.4%, p<0.05). Although there were no changes in fibre size or satellite cell number, the mRNA data revealed that, compared to control, the stimulation resulted in a downregulation of Myostatin (p<0.05) and a similar trend for MAFbx (p=0.099), together with an upregulation of Collagen I (p<0.001), TenascinC (p<0.001), CD68 (p<0.01) and Ki67 (p<0.05) mRNA. Conclusion These findings demonstrate a moderate decline in leg lean mass during a hospital stay in geriatric patients, while leg lean mass was preserved with daily neuromuscular electrical muscle activation. At the cellular level the stimulation had a clear influence on suppression of atrophy signaling pathways in parallel with a stimulation of connective tissue and cellular remodeling processes. Corresponding author: Anders Karlsen, Institute of Sports Medicine Copenhagen, Department of Orthopaedic Surgery M, Bispebjerg Hospital, Copenhagen, Denmark. ak@anderskarlsen.dk This work was supported by The Nordea Foundation (Healthy Aging Grant), The Danish Council for Independent Research, Bispebjerg Hospital, The A.P. Møller Foundation for the Advancement of Medical Science, and The Faculty of Health and Medical Sciences at the University of Copenhagen, is gratefully acknowledged. No conflicts of interest, financial or otherwise, are declared by the authors. The results of the present study do not constitute endorsement by ACSM. All authors declare that the results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. Accepted for Publication: 7 October 2019. © 2019 American College of Sports Medicine

Association of Fish Oil and Physical Activity on Mobility Disability in Older Adults Purpose To examine whether long-term fish oil (FO) supplementation is associated with a lower risk of mobility disability and enhances benefits of physical activity. Methods 1635 sedentary adults aged 70 to 89 years from the Lifestyle Interventions and Independence for Elders (LIFE) single-blinded randomized, multi-center clinical trial, which compared a structured physical activity (PA) program to a health education (HE) program. Primary outcome was incident major mobility disability (MMD), defined by loss of ability to walk 400m, measured every 6 months for an average of 2.6 years. Secondary outcomes included persistent mobility disability (PMD), Short Physical Performance Battery (SPPB), 400m walk speed, and grip strength. Results A third of participants reported using FO at baseline (456, 28%; mean age, 78.5 years; 70.5% women). MMD was experienced by 131 participants (28.7%) in the FO group and 405 (34.4%) participants in the nonuser group. After adjusting for confounders, FO supplementation was associated with a lower risk (hazard ratio [HR], 0.78; 95% CI, 0.64-0.96) of incident MMD. However, there was no interaction (P= .19) between FO supplementation and PA intervention for MMD. For the secondary outcome of PMD, the intervention association differed by supplementation (P= .002) with PA intervention associations of (HR, 1.36; 95% CI, 0.83-2.23) for users and (HR, 0.61; 95% CI, 0.46-0.81) for nonusers. Changes in physical performance outcomes were not modified by baseline FO supplementation or combination with PA. Conclusions FO supplementation was associated with a lower risk of major mobility disability in low to moderate functioning older adults. However, supplementation did not enhance the benefit of physical activity on risk of mobility disability. These results are hypothesis generating and need to be confirmed in randomized trials. Corresponding Author: Todd Manini, PhD, Associate Professor, Department of Aging and Geriatric Research, Institute on Aging, 2004 Mowry Road, Gainesville, FL 32611, PO Box 100107, Gainesville, FL 32610. Email: tmanini@ufl.edu Funding/Support The Lifestyle Interventions and Independence for Elders Study is funded by a National Institutes of Health/National Institute on Aging Cooperative Agreement #UO1 AG22376 and a supplement from the National Heart, Lung and Blood Institute 3U01AG022376-05A2S, and sponsored in part by the Intramural Research Program, National Institute on Aging, NIH. The research is partially supported by the Claude D. Pepper Older Americans Independence Centers at the University of Florida (1 P30 AG028740), Wake Forest University (1 P30 AG21332), Tufts University (1P30AG031679), University of Pittsburgh (P30 AG024827), and Yale University (P30AG021342) and the NIH/NCRR CTSA at Stanford University (UL1 RR025744). Tufts University is also supported by the Boston Rehabilitation Outcomes Center (1R24HD065688-01A1).LIFE investigators are also partially supported by the following: Dr. Thomas Gill (Yale University) is the recipient of an Academic Leadership Award (K07AG3587) from the National Institute on Aging. Dr. Carlos Fragoso (Spirometry Reading Center, Yale University) is the recipient of a Career Development Award from the Department of Veterans Affairs. Dr. Roger Fielding (Tufts University) is partially supported by the U.S. Department of Agriculture, under agreement No. 58-1950-0-014. Role of the Funder/Sponsor: The funders had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication. The results of this study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The results of the present study do not constitute endorsement by ACSM. Conflicts of Interests: DG is the Chief Scientific Officer at Blue Star Nutraceuticals. DG proposed the study concept when he was a postdoctoral fellow at the University of Florida. No other disclosures are reported. Accepted for Publication: 30 September 2019. © 2019 American College of Sports Medicine

A Limb-specific Strategy across a Range of Running Speeds in Transfemoral Amputees Purpose This study investigated the vertical ground reaction force (vGRF) variables and spatiotemporal parameters related to running speed across a range of speeds in sprinters with unilateral transfemoral amputation who used running-specific prostheses (RSPs). Methods Ten sprinters with unilateral transfemoral amputation ran on an instrumented treadmill at incremental speeds of 30%, 40%, 50%, 60%, and 70% of the average speed of their 100-m personal best (100%), while using their RSPs. The vGRF data were collected at 1000 Hz during each trial. We calculated the vGRF variables and spatiotemporal parameters, including the stance average vGRF (Favg), step frequency (Freqstep), and contact length (Lc: the length travelled by a runner’s body during the stance phase). Results All 3 mechanical variables related to speed (Favg, Freqstep, and Lc) were similar for both the unaffected and affected limbs at relatively slower speeds, and these variables increased with speed for each limb. Although Freqstep remained similar between the limbs at relatively faster speeds, the affected limb exerted 11% smaller Favg and showed 12% longer Lc than the unaffected limb. Conclusion These results suggest that, in order to achieve a faster running speed, runners with unilateral transfemoral amputation using RSPs likely adopt limb-specific biomechanical strategies for the unaffected and affected limbs, where the smaller Favg of the affected limb would be compensated by the longer Lc of the affected limb, without achieving a higher Freqstep. Corresponding author: Hiroaki Hobara, PhD, Artificial Intelligence Research Center, National Institute of Advanced Industrial Science and Technology, AIST Waterfront 3F, 2-3-26, Aomi, Koto-ku, Tokyo 135-0064, Japan. Tel.: +81 3 3599 8201, Fax: +81-3-5500-5233. E-mail: hobara-hiroaki@aist.go.jp This work was supported by JSPS KAKENHI grant number 26702027. None of the authors had any conflict of interest associated with the study. The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The results of the present study do not constitute endorsement by the American College of Sports Medicine. Accepted for Publication: 17 October 2019. © 2019 American College of Sports Medicine

Strength Training Increases Conduction Velocity of High-Threshold Motor Units Purpose Motor unit conduction velocity (MUCV) represents the propagation velocity of action potentials along the muscle fibres innervated by individual motor neurons and indirectly reflects the electrophysiological properties of the sarcolemma. In this study, we investigated the effect of a 4-week strength training intervention on the peripheral properties (MUCV and motor unit action potential amplitude, RMSMU) of populations of longitudinally tracked motor units (MUs). Methods The adjustments exhibited by 12 individuals who participated in the training (INT) were compared with 12 controls (CON). Strength training involved ballistic (4x10) and sustained (3x10) isometric ankle dorsi flexions. Measurement sessions involved the recordings of maximal voluntary isometric force (MViF) and submaximal isometric ramp contractions, while high-density surface EMG (HDsEMG) was recorded from the tibialis anterior. HDsEMG signals were decomposed into individual MU discharge timings and MUs were tracked across the intervention. Results MViF (+14.1%, P=0.003) and average MUCV (+3.00%, P=0.028) increased in the INT group, while normalized MUs recruitment threshold (RT) decreased (-14.9%, P=0.001). The slope (rate of change) of the regression between MUCV and MUs RT increased only in the INT group (+32.6%, P=0.028), indicating a progressive greater increase in MUCV for higher-threshold MUs. The intercept (initial value) of MUCV did not change following the intervention (P=0.568). The association between RMSMU and MUs RT was not altered by the training. Conclusion: The increase in the rate of change in MUCV as a function of MU recruitment threshold, but not the initial value of MUCV, suggests that short-term strength training elicits specific adaptations in the electrophysiological properties of the muscle fibre membrane in high-threshold motor units. Corresponding author: Dario Farina. Department of Bioengineering, Imperial College London, SW7 2AZ, London, UK. Tel: Tel: +44 (0)20 759 41387, Email: d.farina@imperial.ac.uk This work was funded by the University of Rome Foro Italico, Rome, Italy. The authors declare that they have no competing interests. All authors affirm that the results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation. The results of the study do not constitute endorsement by the American College of Sports Medicine. Accepted for Publication: 11 October 2019. © 2019 American College of Sports Medicine