Meeting Abstract

S7-1.4.1  Jan. 6  The Neurobiology of Muscle Fatigue: 15 Years Later ENOKA, R.M.*; BARRY, B.K.; University of Colorado, Boulder; University of Colorado, Boulder enoka@colorado.edu

Fifteen years ago, Stuart and Enoka published a review paper (J Appl Physiol 72: 1631-1648, 1992) on the neurobiology of muscle fatigue. The review emphasized that fatigue could be caused by a number of physiological impairments and that the dominant mechanism depended on the details of the task; that is, fatigue is not caused by any single mechanism. In hindsight, this principle of task dependency has resulted in the demise of attempts to answer the question, what causes muscle fatigue? However, the field has recently been energized by a similar, but sufficiently different, question: what causes task failure? The strategy has been to compare two fatiguing contractions and to identify the mechanisms responsible for the difference in the time to task failure. For example, when subjects exert the same net muscle torque with limb muscles and the task is either to sustain a submaximal torque (force task) or to maintain a particular joint angle (position task) for as long as possible, the time to task failure can be twice as long for the force task. The briefer duration for the position task is associated with a more rapid recruitment of the motor unit pool, despite the two tasks requiring each subject to exert the same net muscle torque and there being no difference in the amount of coactivation of the antagonists. The difference in the time to failure for the two tasks, however, is not constant and varies across muscles and with limb position. The influence of limb position underscores the significance of postural activity and the reflex pathways between muscles that are synergistic for one action but antagonistic for another. The task-failure approach affords new opportunities to examine the neurobiology of muscle fatigue in functionally relevant contexts. Supported by NINDS NS043275.