Figure 1.

Experimental setting required for the assessment of neuromuscular fatigue using transcranial magnetic stimulation (panel A) and determination of voluntary activation by transcranial magnetic stimulation (panel B). (A) A pulse is delivered to the brain area corresonding to the motor cortex (preliminarily determined) during a voluntary contraction and the evoked muscle strength is recorded with an appropriate ergometer (strain gauge). Strength measures may also be couple with EMG recordings. (B) The exposed case depicts the determination of voluntary activation of the (unfatigued) knee extensors by transcranial magnetic stimulation in a 67-year old man with obstructive sleep apnea (apnea-hypopnea index = 35 events/hour). Specifically, the participant are asked to perform a brief maximal voluntary contraction followed by two submaximal contractions at 75% and 50% of maximal voluntary contraction. A pulse is applied to the skull, corresponding to the motor cortex, at these different strength levels after obtaining a plateau. The linear relationship between the voluntary strength and the evoked muscle strength by the pulse (superimposed twitch) allows the estimated resting twitch to be determined. The superimposed twitch obtained at 100% of maximal voluntary contraction and the estimated resting twitch (theoretically obtained on the relaxed muscles) are then used for determining voluntary activation. The representative case shows a clear deficit in cortical voluntary activation in a fresh state since a submaximal value is obtained. Abbreviations. EMG, electromyography; MVC, maximal voluntary contraction; SIT, superimposed twitch; TMS, transcranial magnetic stimulation; VA, voluntary activation.