Diminished fatigue at reduced muscle length in human skeletal muscle.

Understanding muscle fatigue properties at different muscle lengths is essential to improve electrical stimulation applications in which impaired muscle is activated to produce function or to serve as an orthotic assist. This study examined the effects of muscle length on fatigue in human quadriceps muscle. Twelve healthy subjects were tested at short and long muscle lengths (15 degrees and 90 degrees of knee flexion, respectively) before and after a fatigue-producing protocol using low-, high-, and variable-frequency testing trains. Greater fatigue was observed at the longer muscle length, supporting the notion that fatigue is largely dependent upon metabolic factors. Fatigue, however, was characterized by greater attenuation of low- than high-frequency responses (i.e., low-frequency fatigue, LFF) at the long length. This observation, accompanied by the fact that variable-frequency trains produced greater augmentations in force production than comparable low-frequency trains at the longer length, suggests that excitation-contraction coupling impairment is also a contributing factor to fatigue and plays a greater role at the more fatigue-susceptible longer muscle length.