Low-frequency fatigue and neuromuscular performance after exercise-induced damage to elbow flexor muscles.

The purpose of this study was to quantify the association between low-frequency fatigue (LFF) and the increase in EMG and force fluctuations after eccentric exercise of elbow flexor muscles. Ten subjects performed two tasks involving voluntary isometric contractions of elbow flexors: a maximum voluntary contraction (MVC) and a constant-force task at five submaximal target forces (5, 10, 20, 40, 60% MVC) while EMG was recorded from biceps and triceps brachii. A third task involved electrical stimulation of biceps brachii at 12 frequencies (1-100 Hz). These tasks were performed before, after, and 2 h and 24 h after concentric or eccentric exercise. MVC force declined after eccentric exercise (34% decline) and remained depressed 24 h later (22% decline), whereas the reduced force following concentric exercise (32%) was recovered 2 h later. Biceps brachii EMG and force fluctuations during the submaximal voluntary contractions increased after eccentric exercise (both approximately 2x greater) with the greatest effect at low forces. LFF was equivalent immediately after both types of exercise (50-60% reduction in 20:100 Hz force) with a slower recovery following eccentric exercise. A significant association was found between the change in LFF and EMG (r(2) values up to 0.52), with the strongest correlations observed at low forces (20% MVC) and at 2 h after exercise. In contrast, there were no significant associations between LFF and force fluctuations during voluntary or electrically evoked contractions, suggesting that other physiological factors located within the muscle are likely to be playing a major role in the impaired motor performance after eccentric exercise.