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

Variable-frequency trains that exploit the catchlike property of skeletal m uscle can augment force production in fatigued skeletal muscle. The present study is the first to examine the effect of such trains during recovery. T he 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 fatigu e and at 2, similar to 13, and similar to 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. Th e fatiguing protocol also induced low-frequency fatigue (LFF); the time cou rses of the degree of LFF and the amount of variable-frequency train force augmentation were inversely related (r = 0.629; F = 38.024; P less than or equal to 0.001), suggesting a common mechanism between the two phenomena. T hese 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 ef fects of LFF, (C) 1999 John Wiley & Sons, Inc.