Transcranial magnetic stimulation and human muscle fatigue

During exercise, changes occur at many sites in the motor pathway, includin g the muscle fiber, motoneuron, motor cortex, and "upstream" of the motor c ortex. Some of the changes result in fatigue, which can be defined as a dec rease in ability to produce maximal muscle force voluntarily. Transcranial magnetic stimulation (TMS) over the human motor cortex reveals changes in b oth motor evoked potentials (MEPs) and the silent period during and after f atiguing voluntary contractions in normal subjects. The relationship of the se changes to loss of force or fatigue is unclear. However, during a sustai ned maximal contraction TMS evokes extra force from the muscle and thus dem onstrates the development of suboptimal output from the motor cortex, that is, fatigue at a supraspinal level. In some patients with symptoms of fatig ue, the response to TMS after exercise is altered, but the changed MEP beha vior is not yet linked to particular symptoms or pathology. (C) 2001 John W iley 8 Sons, Inc.