Neuromuscular fatigue differs with biofeedback type when performing a submaximal contraction

The aim of the study was to examine alterations in contractile and neural processes in response to an isometric fatiguing contraction performed with EMG feedback (constant-EMG task) when exerting 40% of maximal voluntary contraction (MVC) torque with the knee extensor muscles. A task with a torque feedback (constant-torque task) set at a similar intensity served as a reference task. Thirteen men (26 ± 5 yr) attended two experimental sessions that were randomized across days. Endurance time was greater for the constant-EMG task compared with the constant-torque task (230 ± 156 s vs. 101 ± 32 s, P < 0.01). Average EMG activity for the knee extensor muscles increased from 33.5 ± 4.5% to 54.7 ± 21.7% MVC EMG during the constant-torque task (P < 0.001), whereas the torque exerted during the constant-EMG task decreased from 42.8 ± 3.0% to 17.9 ± 5.6% MVC torque (P < 0.001). Comparable reductions in knee extensors MVC (−15.7 ± 8.7% for the constant-torque task vs. −17.5 ± 9.8% for the constant-EMG task, P > 0.05) and voluntary activation level were observed at exhaustion. In contrast, excitation–contraction coupling process, assessed with an electrically evoked twitch and doublet, was altered significantly more at the end of the constant-EMG task despite the absence of M-wave changes for both tasks. Present results suggest that prolonged contractions using EMG biofeedback should be used cautiously in rehabilitation programs.