Role of the nervous system in muscle fatigue in humans
Muscle fatigue develops with any kind of sustained or repetitive exercise. That is, people become progressively less able to produce force with the exercised muscles. In healthy people, muscle fatigue limits performance of strenuous tasks, and in patients with many kinds of disorders, it can impair performance of everyday tasks. While some of the processes that lead to muscle fatigue lie within the muscle itself, processes in the nervous system can also contribute. We have found that under some circumstances, the nervous system is responsible for more than half of the fatigue resulting from exercise. It is likely that processes at different sites in the brain and spinal cord contribute to fatigue. Current studies in healthy volunteers are investigating processes that affect the motoneurones in the spinal cord. These are the nerve cells that receive signals from the brain and then drive the muscles. During voluntary muscle activity, the motoneurones fire repeatedly and this firing controls the timing and strength of muscle contractions. If the firing of motoneurones is impeded then this contributes to fatigue.
Current and proposed studies address the following questions. i) Does fatigue of the quadriceps muscles make the motoneurones that innervate the muscle hard to activate? Do such changes occur because of repeated firing of the motoneurones? Does sensory feedback from the fatigued quadriceps muscle contribute to such changes? ii) Does sensory feedback from one fatigued muscle in the leg affect the motoneurones of other muscles in the legs or arms? iii) Is there a neuromodulator that is released in association with voluntary contraction and that makes motoneurones hard to activate?