Understanding the mechanisms of neuromuscular fatigue with paired‐pulse stimulation

Abstract

Transcranial magnetic stimulation (TMS), using both single and paired-pulse techniques, has provided insight in understanding the balance of excitation and inhibition within the corticomotor system during maximal and submaximal exercise. A single pulse of TMS produces a short-latency motor-evoked potential (MEP), indicating the net excitation of the motor pathway which includes the balance within supraspinal (cortical) and spinal sources. The MEP is followed by an interruption of ongoing electromyographic (EMG) activity known as the silent period. Classically, the silent period has been interpreted to represent the inhibition within the motor cortex (i.e. intracortical inhibition) which probably involves γ-aminobutyric acid (GABAB) receptors. Intracortical inhibition is clinically important to understand because chronic pain disorders (migraines) and psychiatric disorders (post-traumatic stress disorder) have demonstrated less cortical inhibition during a voluntary contraction while other disorders such as Parkinson's and Alzheimer's diseases have demonstrated abnormal corticospinal excitatory circuits at rest. Additionally, the fatigue-induced reduction in cortical inhibition may be related to the motor impairments during exercise. The alterations in the MEP and silent period during exercise indicate that the central nervous system (CNS) may necessitate increases in both excitatory and inhibitory activity during fatiguing contractions; however, the mechanisms underlying these effects are yet to be completely understood.