Coactivation Does Not Contribute to Fatigue-Induced Decreases in Isokinetic Forearm Flexion and Extension Torque

Purpose: The present study examined the effects of a reciprocal, slow velocity forearm flexion and extension task on fatigue-induced changes in isokinetic torque, agonist and antagonist muscle activation, and coactivation ratios at slow and moderate velocities. Methods: Nine women (mean ± SD: age = 21.0 ± 1.7 years; body mass = 68.1 ± 8.2 kg; height = 167.4 ± 7.2 cm) completed pre-testing for forearm flexion and extension isokinetic peak torque at 60 and 180°/s, a fatiguing task of 50 maximal, reciprocal, isokinetic muscle actions at 60°/s, and post-testing. The amplitude (AMP) of the electromyographic (EMG) signals from the biceps and triceps were simultaneously recorded. Torque and EMG AMP were normalized to the corresponding values from the pre-testing peak torque movements. Repeated measures ANOVAs and pairwise comparisons were used to identify mean changes in torque, EMG AMP, and coactivation ratios. Results: The torque analyses indicated significant decreases from pre- to post-testing for forearm flexion (14.1% ± 5.0%; P< 0.001) and extension (25.4% ± 12.2%; P< 0.001) at 60°. At 180°/s there was a significant decrease, collapsed across the forearm movements (24.7% ± 11.7%; P< 0.001). For EMG AMP and coactivation ratios, there were no changes (P> 0.05) from pre- to post-testing for either velocity or movement. Conclusions: The torque responses were velocity-specific, with greater fatigability exhibited for forearm flexion versus extension at 60°/s, but no differences at 180°/s. The parallel EMG AMP responses between the agonist and antagonist muscles for both velocities supported the lack of fatigue-induced changes in coactivation ratios. Thus, our results demonstrated that fatigue-induced decreases in torque were not attributable to increases in antagonist activation or coactivation.