Reduction of the fatigue-induced force decline in human skeletal muscle by optimized stimulation trains

Objective: To identify the stimulation pattern that optimizes the force-time integral produced during isometric contractions of fatigued human skeletal muscle. Design: Twelve healthy subjects with no history of lower extremity orthopedic problems voluntarily participated. Results: The primary findings were that (1) the optimized trains showed augmentation only from fatigued muscles and (2) a simple stimulation pattern, containing one brief (5msec) initial interpulse interval, produced the greatest force-time integrals and rates of rise of force. With muscle fatigue, the rate of rise of force of the constant-frequency train slowed, whereas the rate of rise of force of the optimized trains remained unchanged. This difference in the rate of rise of force may explain why the optimized trains, which take advantage of the catchlike property of skeletal muscle, are able to augment forces from fatigued muscles when compared with the constant-frequency train. Conclusions: These results may have important clinical implications when using brief trains of electric stimulation to aid patients in performing functional movements and contribute to our understanding of the relationship between the activation pattern of a muscle and the force output produced. (C) 1997 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation.