Objectives: 

Objective: Previously we have shown that young human subjects experience a fast form of fatigue during the execution of rhythmic movements at their fastest rate, which develops in about 5 seconds. In our first approach such form of fatigue was not straightforwardly explained by fatigue of the cortico-spinal tract, evaluated by the size of the motor evoked potentials induced by transcranial magnetic stimulation (TMS) on M1. Here we evaluate whether fatigue of the cortico-cortical networks involving M1 could be responsible for the decrement in motor execution during the same task.

Materials: 

Methods: Cortico-cortical fatigue was evaluated by means of the length of the silent period (SP) in response to TMS-pulses on M1 while subjects performed maximal voluntary contractions. 10 healthy young subjects underwent a protocol of 12 consecutive sets of finger tapping (FT) including different frequencies and durations. SP was evaluated before and after the execution. Since the fatigue induced by isometric (ISO) contractions is known to have a cortico-cortical origin it was separately evaluated as control for the FT task.

Results: 

FT at maximal rate and ISO induced a comparable fast decay in the tapping frequency and force development during short duration tasks (10s); in both protocols a fast recovery was observed. Conversely, repetition of the sets prolonged recovery in long tasks (30s) in the case of ISO. However, the increase in the SP and its recovery during maximal execution were comparable between FT and ISO in all cases.

Conclusions: 

Conclusion: In young subjects, there is a fast form of central fatigue with origins at the cortico-cortical level seen during the execution of repetitive movements at the fastest rate; such a form of fatigue recovers fast after activity. The same mechanism applies for ISO, but additionally when the task gets longer a different form of fatigue than FT develops; this form is time-compatible with peripheral fatigue.