Mechanistic Drivers of Acute PAPE Responsiveness: Muscle Architecture, Contractile Kinetics, and Excitability in a Randomized Controlled Trial

The goal of this study is to find out if one short set of heavy half-squats can help football players jump higher right away-and to understand what happens inside their muscles and nerves to make that boost happen. Key questions * Will performing 2-3 half-squats at 90% of one-rep max give a bigger jump boost than jogging on a treadmill for five minutes? * After each warm-up, how do muscle speed and stiffness, muscle size and fiber angle, and nerve signals change over the next 12 minutes? * Does each player's contribution of fast and slow muscle fibers affect how much and how long their jump improves? Study Plan Researches will invite 44 healthy football players, ages 18-21, who train regularly and meet our health rules. No one will know which warm-up each player does until the end. Participants will: * Get baseline tests of jump height, muscle speed and stiffness (using a harmless electrical sensor), muscle size and fiber angle (using ultrasound), and nerve signals (using sticky pads on the skin). * Be randomly assigned to either: 1. Heavy-squat group: 2-3 half-squats at 90% of their one-rep max 2. Jogging group: easy jog or walk on a treadmill for five minutes * Repeat all tests right after the warm-up and again at 4, 6, 8, 10, and 12 minutes to see how jump height and all muscle and nerve measures change over time. * Have their muscle fiber mix estimated from the first muscle-speed test to see if fiber type explains who gets the biggest jump boost. All tests are safe, painless, and approved by an ethics board. Players can stop at any time without giving a reason. This study will help athletes and coaches choose the best warm-up to get stronger, faster jumps right before a game or practice.

Design and setting. The study will be a randomized, assessor-blinded, parallel-group trial conducted in university sport-science laboratories. Two arms will be used: a heavy half-squat conditioning activity and an active warm-up control. All procedures will follow written standard operating protocols. Participants. Healthy, 44 resistance-trained male football/soccer players will be screened and will provide written informed consent. Eligibility will require regular training and the absence of musculoskeletal, neurological, dermatological, or cardiovascular contraindications to high-intensity exercise or peripheral electrical stimulation. Testing will be scheduled on a consistent team microcycle day to control for prior load. Visit schedule. Visit 1 - Familiarization and strength assessment. Medical screening, anthropometrics, and a progressive half-squat session will determine individual one-repetition maximum (1RM). Squat depth will be standardized at approximately 90° knee flexion with rack pins and spotters. Visit 2 - Experimental session. After compliance checks (sleep, diet, caffeine), participants will complete a structured general warm-up. Baseline tensiomyography (TMG) of the dominant vastus lateralis (VL), surface electromyography (EMG) normalization to maximal voluntary contraction (MVC), and baseline countermovement-jump (CMJ) testing will be completed. The allocated intervention will be delivered, followed by repeated CMJ testing with concurrent EMG at one-minute intervals. Ultrasound imaging will be obtained at baseline and after the final CMJ series. Arms and interventions. Experimental arm (conditioning activity). One set of 2-3 free-weight half-squats at \~90% 1RM will be performed from a rack with spotter supervision. Control arm (active warm-up). Approximately five minutes of treadmill running at light-moderate intensity (0% incline) will be completed. Outcome measures and timing. Primary outcome. Change in CMJ height at each minute from 4 to 12 minutes after the intervention, relative to baseline (5 prespecified post-intervention time points at 2-min resolution). CMJ-derived flight time, take-off velocity (device-derived where available), and individualized responder categorization at each minute using a predefined smallest-detectable-change threshold anchored to familiarization reliability. Exploratory relationships between neuromuscular phenotype (TMG) and the magnitude/timing of CMJ responses. Exploratory changes in VL architecture (ultrasound thickness, pennation, derived fascicle length) across the session. Electromyography outcome (recorded for every jump). For every CMJ, surface EMG amplitude will be recorded and expressed as a percentage of the participant's maximal voluntary contraction (%MVC). The primary EMG variables will be RMS EMG (%MVC) across the propulsive phase and peak RMS (%MVC) within the final 200-250 ms before take-off. Following the intervention, one CMJ will be performed at minute 1 and subsequently at each exact minute through minute 12. If a jump is technically invalid, a single replacement attempt will be permitted within that minute. Standardized instructions and hand placement will be maintained for all trials. Instrumentation and standardization. CMJ. Jumps will be measured with a validated optical timing system on a non-slip surface. Foot placement and countermovement depth will be self-selected but replicated across trials. Electromyography. Wireless surface EMG will be recorded from VL (with optional rectus femoris and biceps femoris for context). Skin will be shaved, lightly abraded, and cleaned with alcohol; disposable Ag/AgCl electrodes with \~20-mm inter-electrode distance will be placed along the fiber direction following SENIAM guidance and skin-marked for repeatability. Signals will be sampled at ≥2,000 Hz, band-pass filtered 20-450 Hz, notch-filtered at mains frequency, full-wave rectified, and converted to RMS with a 50-ms window. EMG and CMJ timing will be synchronized via hardware trigger; take-off will be identified from the optical system to define the propulsive phase. Prior to experimental jumps, three 3-5 s isometric knee-extension MVCs at \~60° knee flexion will be performed against a fixed dynamometer or rigid strap with strong verbal encouragement and 2-3 min rests. The highest stable 500-ms RMS epoch will define 100% MVC for normalization. Tensiomyography. VL contractile properties (Td, Tc, Tr, Ts, Dm, and displacement-rate metrics) will be obtained with a displacement sensor at constant pre-tension. Participants will be supine with the knee supported at \~120°. Electrode positions will be standardized and skin-marked; single twitches will be delivered with inter-stimulus intervals ≥10 s. A validated multivariable model will estimate %MHC-I as a non-invasive phenotype proxy. Ultrasound. B-mode imaging with a linear-array transducer will quantify VL thickness and pennation at mid-thigh; probe alignment will follow fascicle direction with minimal gel and consistent pressure. Fascicle length will be estimated from thickness and pennation. Three longitudinal images per site will be acquired and averaged. Randomization and masking. Participants will be randomized using concealed, sequentially numbered envelopes prepared by personnel independent of data collection. CMJ/EMG assessors will be blinded to allocation and will operate in a separate space from the intervention station. Data files will contain coded identifiers only. Participant flow during Visit 2. Participants will arrive for compliance checks. A standardized warm-up will be performed. MVC procedures for EMG normalization will be completed. Baseline TMG and baseline CMJ with EMG will be recorded. The allocated intervention will be administered. CMJ with EMG will be recorded at minutes 1-12. Ultrasound imaging will be performed at the end of the series, followed by cool-down and debrief. Responder framework. At each minute, CMJ change will be classified for individual interpretation as positive, no-change, or negative using a priori smallest-detectable-change thresholds derived from familiarization reliability. Safety and risk mitigation. Heavy squats will be performed in racks with pins and spotters; participants will have been familiarized with high loads during Visit 1. TMG will use brief single twitches with conservative current progression and long inter-stimulus intervals. Surface EMG and ultrasound are non-invasive; electrode preparation and probe handling will follow hygienic and standardized procedures. Adverse events and discomfort will be documented, and testing will cease if safety concerns arise. Data capture and quality assurance. All devices will be calibrated before each session. Electrode and probe sites will be marked to ensure within-session consistency. Operators will complete training and inter-rater checks before data collection. Raw data will be stored on encrypted, access-controlled servers with audit trails. Predefined rules will govern artifact handling, trial acceptance, and protocol deviations. Confidentiality and data handling. Participant identities will be coded. Source documents and electronic records will be stored on encrypted drives with restricted access limited to authorized personnel. Discontinuation and withdrawal. Participants may withdraw at any time. Investigators will discontinue participation in the case of adverse signs, illness, or non-compliance. Data collected to the point of withdrawal will be retained unless the participant requests removal under the approved ethics framework.