Central fatigue in continuous and intermittent contractions of triceps brachii.

Conflicting results have been found across studies concerning the effect of rest periods on the development of central fatigue during prolonged muscle activity. Thus, the aim of the present study was to assess differences in the development of central fatigue between continuous and intermittent elbow extension fatigue tasks in the same subjects. Force and electromyographic data were collected on eight healthy volunteers. The ability to maximally activate the triceps brachii muscle was assessed by delivering trains of electrical stimulation during maximal voluntary efforts. This was done before, during, and after three fatigue tasks involving a maximal contraction in elbow extension. One short-duration ( approximately 55-s) and two long-duration (3-min) fatigue tasks were performed by all subjects on separate sessions. One 3-min task was intermittent (5-s rests every 30 s) and the other was continuous. The main findings were that the development and extent of central fatigue were task-dependent, with a greater decrease in the ability to maximally activate triceps brachii observed for the 3-min continuous task. Also, the voluntary activation (VA) ratio was found to be a more sensitive index of central fatigue than the central activation ratio (CAR). These results suggest that, when assessing central fatigue in patients, conclusions may vary depending on the continuous/intermittent nature of the task performed and the estimate of voluntary activation used.