International Sports Sciences Association (ISSA) Trainer Practice Exam 2025 - Free Trainer Practice Questions and Study Guide

The size principle is fundamental in understanding how the nervous system recruits motor units for muscle contraction. According to this principle, motor units are recruited in a specific order based on their size and the threshold needed to activate them. Smaller motor units, which generally have lower thresholds and finer control, are recruited first. As the demand for force increases, larger motor units, which have higher thresholds and generate more force, are subsequently recruited.

This orderly recruitment allows for more precise and efficient control of movement, starting with lighter loads and progressively increasing intensity. It fundamentally explains why when lifting weights, for instance, lighter weights primarily activate smaller, slower-twitch muscle fibers first, while larger, fast-twitch fibers are recruited for heavier lifts or explosive movements. Understanding the size principle is crucial for trainers as it informs how they design training programs that maximize strength gains and enhance performance.