ABSTRACT
It is well known that muscle strength declines during exercise. This is evident from the continuous fall in force output during sustained maximal voluntary or tetanically stimulated contractions (Fig. Ia) (Merton, 1954; Jones et al ., 1979; Hultman and Sjoholm, 1983). During submaximal voluntary exercise a decrease in muscle strength is not necessarily reflected in the performance. Instead, force loss can be determined from test contractions elicited at given intervals during the exercise (see Fig. lb). Here fatigue is defined as any reduction in the force-generating capacity of the muscle, as assessed by the force elicited when the muscle is maximally activated. The schematic example in Figure I b shows that the muscle strength, tested periodically by brief maximal voluntary contractions (MVCs), starts to decline from the onset of exercise. Moreover, at the time when the MVC force falls below the target force level, subjects can no longer generate the required force. The definition used in many studies, that fatigue occurs when there is "an inability to maintain the required or expected force" (Edwards, 1981), ignores the continuous fall in maximum muscle contractile strength that occurs before this point. Instead, we describe the condition when a subject is no longer able to maintain the target force as exhaustion, with fatigue accruing from exercise onset (see Fig. lb) . Therefore, we prefer to define fatigue as "a reduction in the force-generating
Figure 1 Schematic representation of fatigue protocols with (a) sustained MVC and (b) repeated contractions at 45% of the control MVC. Fatigue was defined as the reduction in the MVC force,
capacity" regardless of the task required (V~llestad and Sejersted, 1988; V~llestad et al., 1988). With this definition, we can quantify fatigue from the fall in MVC force or in the force response to electrically stimulated tetanic contractions. The latter definition of fatigue will, therefore, be used here to present novel information concerning changes in mechanical properties and activation processes, which challenges some of the generally accepted facts about events underlying fatigue.
