2.1.5: Motor Unit Recruitment and the Perception of Effort

Since we cannot consciously select which motor units to recruit, the body relies on a proxy: effort. Motor unit recruitment is under voluntary control through the central motor command, and it correlates tightly with the perception of how hard a movement feels. As Beardsley explains, “the number of motor units that are recruited in any given contraction is determined voluntarily. We choose how much effort to exert, and this in turn determines how many motor units are recruited” [1]. When you lift a 5-repetition maximum (5RM) load, the external demand is so high that maximal effort is required from the very first rep, leading to full motor unit recruitment immediately. When you lift a 30RM load, the early reps feel easy because the demand is low, and only low-threshold motor units are recruited. However, as fatigue accumulates and the muscle fibers that are already active begin to fail, the central motor command must increase to maintain force output. This escalating effort progressively recruits higher-threshold motor units until, on the final grinding reps of a set taken to failure, full recruitment is eventually achieved.