Muscle force augmentation by low-intensity electrical stimulation.

In cases of muscle partial deficiency, force augmentation can be achieved by hybrid activation, i.e., by combining electrical stimulation (ES) with volitional activation. In the present study the volitional and electrically-induced torque components are resolved under visual-feedback activation. Isometric contraction of the tibialis anterior (TA) muscle was studied on 5 healthy subjects, using an activation protocol combining ES alone, volitional activation alone and hybrid activation. Ankle torque and TA EMG were measured. A computational algorithm was developed to dissociate the volitional from the overall torque, based on EMG filtering and on pre-measured calibration curves of volitional torque versus EMG. Based on a defined facilitation factor, the results indicate that within the range of stimulation intensities, there exist regions of increased facilitation of the volitional activation of the TA muscle, in which the torque contribution due to the induced activation is higher compared that of the recruitment curve.