Impact of length during repetitive contractions on fatigue in rat skeletal muscle

The magnitude of fatigue resulting from repeated contractions at a short length has been reported to be less than that which occurs with contractions at a long length. However, there have been what appear to be contradictory reports; the rate of fatigue is greater at a short length. The purpose of this study was to evaluate the impact of length on the magnitude and the rate of fatigue resulting from a series of repetitive stimulations. Experiments were done with anesthetized rats and the medial gastrocnemius muscle was stimulated via the sciatic nerve. Submaximal force-length relationships were obtained prior to and 45 min after repeated contractions (50 Hz, 300 ms) at short or long length. Stimulation was applied at 1 Hz or 0.5 Hz for 5 min at the long length or 1 Hz at the short length (difference=3.6mm). This approach permitted evaluation of the impact of rate of muscle activation as well as length on subsequent contractile response. Repetitive stimulation at a short length resulted in more potentiation and a greater (relative) rate of fatigue but after 5 min the depression of relative active force was similar between the series at long and short length. The submaximal force-length relationship obtained after 45 min of recovery revealed that depression of force was greater after 1 Hz contractions at the long length. These results are consistent with both sides of the apparent contradiction in the literature; rate of fatigue is greater at a short length and magnitude of fatigue is greater at a long length.