Changes in muscle afferents, motoneurons and motor drive during muscle fatigue

Fatigue is a reduction of maximal muscle force or power that occurs with ex ercise. It is accompanied by changes at multiple levels in the motor pathwa y and also by changes in the discharge patterns of muscle afferents. Change s in afferent firing can lead to altered perceptions and can also act on th e efferent pathway. Changes in the motor pathway include slowing of motor u nit firing rates during sustained maximal voluntary contractions (MVCs). Mu scle responses to stimulation at different levels of the motor pathway also change. Transcranial magnetic stimulation of the motor cortex and stimulat ion of descending tracts in the spinal cord in human subjects show an incre ase in the response of the cortex and a decrease in response of the motoneu ron pool during sustained MVCs. In addition, the silent period following ma gnetic stimulation is prolonged. During relaxation after fatiguing exercise , muscle responses to stimulation of the motor cortex are initially facilit ated and are then depressed for many minutes, whereas responses to descendi ng tract stimulation are initially depressed but recover over about 2 min. Although some of the loss of force of fatigue does occur through inadequate drive to the muscle, it is not clear which, if any, of the changes describ ed in the cortex or the motoneurons are responsible for loss of maximal vol untary force and thus contribute to fatigue. Changes may be associated with muscle fatigue without causing it.