Effects of activation pattern on human skeletal muscle fatigue.

Variable-frequency stimulation trains (VFTs) that take advantage of the catchlike property of skeletal muscle have been shown to augment the force production of fatigued muscles compared with constant-frequency trains (CFTs). The present study is the first to report the force augmentation produced by VFTs after fatiguing the muscle with VFTs versus fatiguing the muscle with CFTs. Data were obtained from the human quadriceps femoris muscles of 12 healthy subjects. Each subject participated in three experimental sessions. Each session fatigued the muscle with one of three protocols: CFTs with 70-ms interpulse intervals (CFT70); CFTs with 55.5-ms interpulse intervals (CFT55.5); or VFTs. Following each fatiguing protocol the muscles were tested with all three stimulation patterns (i.e., CFT55.5, CFT70, and VFT). At the end of the fatiguing protocol the VFT produced force-time integrals and peak forces approximately 18% and 32% greater than the CFT70, respectively. The testing trains showed that the VFT produced approximately 25-35% greater force-time integrals than either CFT and approximately 35-47% greater peak forces than the CFT70. For each testing train, approximately 10-15% greater force-time integrals were seen when the muscles were fatigued with the CFTs than when fatigued with the VFTs. These results support suggestions that VFTs may be useful during clinical applications of electrical stimulation.