"Size Principle" of motor unit recruitment
Use small first the progressively larger are recruited.
Motor neurons that control certain muscles are controlled by interneurons in CNS
Interneurons begin to fire, and motor neurons with smaller cell bodies will reach -55 mV (threshold) before (few EPSPs) motor neurons with larger (more EPSPs) cell bodies
small motor units are innervated by smaller neurons with smaller cell bodies, thus fire first in sequence