Relation between force and fatigability of red and pale skeletal muscles in man.

Experiments were performed to test the effect of muscular fatigue on the relationship between smoothed rectified electrical activity (SREMG) and force in human muscles of different fiber compositions. Fatigue was shown to be produced by failure of a mechanism distal to the neuromuscular junction in three muscles tested: biceps brachii, triceps brachii, and adductor pollicis. Fatigue produced changes in SREMG in first dorsal interosseous, suggesting an effect at or proximal to the neuromuscular junction. Fatigue produced changes in the SREMG-force relationship only at high force levels in the pale triceps and biceps brachii, but produced a change throughout the force range in the red adductor pollicis. It is suggested that muscle units composed of pale fibers fatigue selectively in mixed muscles containing many such units, while fatigue effects are more difficult to produce and are distributed more uniformly in predominantly red muscles.