Muscle fatigue and efficiency in relation to interval duration of successive contractions.

The work output, energy consumption and efficiency during repetitive dynamic contractions were calculated for rat extensor digitorum longus muscle. The muscles performed 40 successive dynamic contractions at 37 degrees C (with occluded bloodflow) with interval durations of either 500, 250 or 167 ms. The muscle-tendon complexes were allowed to shorten at the velocity at which they could exert their highest power output (50 mm.s-1). Work output in the first contraction was the same among the three groups with different interval durations. The reduction in work output during the series of contractions differed among the groups, mainly in the last part of the exercise period. In the group with the longest interval duration, work output steadily decreased over the whole contraction period and at the end was approximately 72% of the output in the first contraction. In contrast, after the 30th contraction, work output decreased at a significantly higher rate of approximately 3% of each contraction in the groups with the intermediate and the shortest interval duration. After the last contraction, work output in these groups was approximately 52% of the work output in the first contraction. These differences in fatigue coincided with differences in the reduction in adenosine 5'-triphosphate and the production of inosine-5'-monophosphate. Total work output was not significantly different among the three groups with different interval durations, indicating that the different reductions in work output in the last contractions only had a minor influence on total work output of all 40 contractions. Also high-energy phosphate consumption and efficiency were not significantly different over these three exercise periods. Thus with the protocol used no interval dependent pattern of efficiency could be detected.