Effects of length and stimulation frequency on fatigue of the human tibialis anterior muscle.

It has been suggested that the reduced fatigability of muscles exercised at short length may result from a decrease in the metabolic cost of contractions in the shortened position. We compared the fatigue properties and metabolic cost of stimulated isometric tetanic contractions in the tibialis anterior of 10 normal subjects at the optimum length (Lo) for force production and when the muscle was shortened (Ls). Six 15-s ischemic contractions at Ls caused force to decline to 53% of the fresh value when tested at Lo, whereas muscles exercised and tested at Lo declined to 40% of fresh force. However, the extent of fatigue was proportionately greater if the muscle was exercised and tested at Ls. The apparent fatigue resistance of short muscles was found to be a consequence of recovery occurring under ischemic conditions while the muscle was changed from Ls to Lo-. 31P-nuclear magnetic resonance spectroscopy showed similar metabolic changes associated with 5-s stimulated contractions at Lo and Ls, indicating that any differences in fatigability at the two lengths were not caused by altered metabolic costs. The similarity between the force profiles of muscles fatigued at Ls and those fatigued by high-frequency stimulation supports the hypothesis that an enhanced activation failure occurs at Ls, possibly resulting from failure of sarcolemmal action potential propagation in the transverse tubules.