Co-contraction of antagonist muscles during knee extension against gravity: insights for functional electrical stimulation control design.

Functional electrical stimulation (FES) involves electrically stimulating the neuromuscular system to generate skeletal muscle contractions in paralyzed muscles. Several new FES applications have been proposed that require closed-loop control systems. Co-contraction of antagonist muscle groups has been postulated as a promising approach for closed-loop control of FES systems. However, this control approach has not yet been used in practical FES applications, in part due to a lack of information concerning how able-bodied subjects use co-contraction of antagonist muscles during standard control tests such as unit step and sinusoidal responses. The purpose of this work is to elucidate how able-bodied individuals use co-contraction by analyzing the EMG activity of antagonist muscles during voluntary knee extension against gravity. The results clearly demonstrate that able-bodied subjects use a co-contraction strategy when executing standard control performance tests, and strengthen the argument for using a co-contraction strategy for closed-loop FES control algorithms. These data will inform the development of new and effective controllers for FES applications.