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Variable‐frequency trains offset low‐frequency fatigue in human skeletal muscle

Variable‐frequency trains that exploit the catchlike property of skeletal muscle can augment force production in fatigued skeletal muscle. The present study is the first to examine the effect of such trains during recovery. The quadriceps femoris muscles of 12 healthy individuals were fatigued using six‐pulse, 14.3‐Hz trains delivered at a rate of 1/s for 3 min. The force‐generating ability of the muscle was tested with several constant‐frequency trains (8.3–100 Hz) and a variable‐frequency train before and after fatigue and at 2, ∼13, and ∼38 min of recovery. The variable‐frequency train produced significant augmentation of force versus the best constant‐frequency train (12.5 Hz) in acute fatigue and during recovery. The fatiguing protocol also induced low‐frequency fatigue (LFF); the time courses of the degree of LFF and the amount of variable‐frequency train force augmentation were inversely related (r = 0.629; F = 38.024; P ≤ 0.001), suggesting a common mechanism between the two phenomena. These results suggest that clinical use of variable‐frequency trains (e.g., functional electrical stimulation) will enable the muscle to generate more force during acute fatigue and offset, at least partially, the long‐term effects of LFF. © 1999 John Wiley & Sons, Inc. Muscle Nerve 22: 874–882, 1999