contraction force, up to the maximal force. The maximal force depends on the contractile properties of the muscle fibers. Within a motor unit, all the muscle fibers are of the same type (e.g. type I (slow twitch) or Type II fibers (fast twitch)), and motor units of multiple types make up a given muscle. Motor units of a given muscle are collectively referred to as a motor pool.

The force produced in a given muscle thus depends on: 1) How many motor neurons are active, and their spike rates; 2) the contractile