Coactivation does not contribute to fatigue-induced decreases in torque during reciprocal, isokinetic muscle actions

BACKGROUND: Studies of coactivation have typically utilized single movement isometric or isokinetic fatiguing muscle actions. OBJECTIVE: The purpose of the current study was to examine coactivation of the biceps brachii (BB) and triceps brachii (TB) in response to a maximal, reciprocal, isokinetic fatiguing task of the forearm flexors and extensors at slow (60∘/s) and moderate (180∘/s) isokinetic velocities in men. METHODS: Ten men (mean ± SD: age = 21.6 ± 1.3 years) completed 50 consecutive, maximal, reciprocal, isokinetic muscle actions of the right forearm flexors and extensors at 60 and 180∘/s. The amplitude (AMP) and mean power frequency (MPF) contents of the electromyographic (EMG) and mechanomyographic (MMG) signals from the BB and TB were recorded simultaneously throughout the fatiguing task. Repeated measures ANOVAs with Tukey post hocs were used to determine mean differences for the torque and neuromuscular parameters across repetitions. RESULTS: The torque analyses indicated greater fatigability at 180∘/s, compared to 60∘/s (p= 0.02). There were no significant changes in EMG AMP for either muscle during flexion or extension at 60∘/s (p> 0.05). At 180∘/s, there were significant increases in agonist EMG AMP (p= 0.01 to 0.004), however, no changes in antagonist EMG AMP (p> 0.05). For EMG MPF, there were significant decreases during flexion and extension (p< 0.001 to p= 0.02) at both velocities, collapsed across muscle. There were no significant (p> 0.05) changes across repetition for MMG AMP or MPF. CONCLUSIONS: This study indicated velocity-specific responses to fatigue, with a greater magnitude of fatigability at 180∘/s. Furthermore, despite increases in EMG AMP of the agonist muscles at 180∘/s only, it was not sufficient to alter the ratio of coactivation, likely due to common neural drive between muscles. Thus, the decreases in torque in the present study were not attributable to increases in coactivation.