Chapter 16 Orderly Recruitment Tested Across Muscle Boundaries

Abstract

Publisher Summary The movements and postures of limb and body segments are achieved through partial activation of multiple muscles. In the cat, multiple muscles are coactive during a variety of movements, such as treadmill locomotion, landing, paw shake, head turning, and in humans, during movements such as hand grip and elbow flexion. There are tens to hundreds of heteronymous motor units belonging to different muscles, which are recruited together during discrete motor actions. This chapter discusses whether recruitment is orderly among heteronymous motor units that are activated together. Recruitment order can be assessed in relation to the motoneuron portion of the motor unit. Alpha motoneurons within a motor nucleus (the collection of motoneurons supplying a single muscle) are progressively recruited from slow to fast axonal conduction velocity. Recruitment that is rank-ordered by these properties is commonly referred to as “recruitment in order” by the size principle. The chapter describes the recruitment order with respect to either a muscle and its motor units or a motor nucleus and its motoneurons or motor axons.