Chapter 34 Task- and Age-Dependent Variations in Steadiness

Abstract

Human aging is typically accompanied by a loss of muscle mass and an associated decrease in muscle strength. The muscle atrophy is caused by the death of motoneurons in the spinal cord and, preferentially, involves the motoneurons that innervate the histochemically defined type IIb muscle fibers. Surviving motoneurons, however, are able to develop axonal sprouts and reinnervate some of the abandoned muscle fibers. The net result of this reorganization is a reduction in the number of motor units in a muscle but an increase in the innervation ratio of the motoneurons that remain in the muscle. According to the Size Principle, the order in which motor units are recruited progresses from the smallest to the largest motoneuron as the force that a muscle exerts is graded. In this scheme, differences in motoneuron size and those properties that co-vary with size are the major determinants in the spinal-cord control of the muscle force. Therefore, disturbances in the relative size of motor units within a population, such as the reorganization that occurs with aging, may have significant consequences for the ability of humans to perform tasks requiring subtle variations in the force exerted by the muscle. The studies described in this chapter characterize the ability of older adults to perform steady submaximal contractions and determine the role of changes in motor unit size on the observed impairments in performance.