Activation of human quadriceps femoris muscle during dynamic contractions:effects of load on fatigue

Muscle fatigue is both multifactorial and task dependent. Electrical stimul ation may assist individuals with paralysis to perform functional activitie s [functional electrical stimulation (FES), e.g., standing or walking], but muscle fatigue is a limiting factor. One method of optimizing force is to use stimulation patterns that exploit the catchlike property of skeletal mu scle [catchlike-inducing trains (CITs)]. Although nonisometric (dynamic) co ntractions are important parts of both normal physiological activation of s keletal muscles and FES, no previous studies have attempted to identify the effect that the load being lifted by a muscle has on the fatigue produced. This study examined the effects of load on fatigue during dynamic contract ions and the augmentation produced by CITs as a function of load. Knee exte nsion in healthy subjects was electrically elicited against three different loads. The highest load produced the least excursion, work, and average po wer, but it produced the greatest fatigue. CIT augmentation was greatest at the highest load and increased with fatigue. Because CITs were effective d uring shortening contractions for a variety of loads, they may be of benefi t during FES applications.