As introduced in the text, motor unit recruitment is the progressive activa tion of more and more muscle fibers i by the successive recruitment of ad- ditional motor units. Recall that a motor unit consists of one motor neu- ron and all of the muscle fibers that it is responsible for activating. All mus in the body contain motor units, and the fibers belonging to a motor unit are spread out across the entire muscle. When a single motor unit is activated, all the muscle fibers innervated by the motor neuron are stimulated to contract. The activa tion of a single motor unit results in a weak muscle contraction (ie, limited force production). To increase muscle force production, more motor units must be recruited. This process of motor unit recruitment occurs in an orderly fashion, beginning with the smallest motor neurons and event ally activating larger and larger motor neurons (37). This concept was de veloped by Elwood Henneman and is known as the size principle. Henneman proposed that the mechanism responsible for the size principle was that the smaller motor neurons had a smaller surface area and.