ABSTRACT
While length and velocity affect muscle force, so does muscle activation. The human nervous system uses two mechanisms to vary muscle force (Milner -Brown et al ., 1973a, 1973b). It activates motor units within the muscle in order of size, enlisting small units at low force levels and increasingly big ones at high force levels (‘recruitment’). It also sets the rate at which motor units fire, driving them slowly when they are first recruited, then faster as more force is needed (‘rate coding’) (see review by Clamann, 1993).
