A Mechanistic Perspective on Post-Activation Performance Enhancement Responsiveness: A Randomized Controlled Study of Acute Changes in Muscle Architecture, Contractile Property Kinetics, and Muscle Excitability

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.