EMG-biofeedback and load sharing problem in assistive and rehabilitation orthotic devices

Biofeedback signals have been frequently used for rehabilitation purposes, and in design and calibration of orthotic and prosthetic devices. Whenever one or a couple of muscles of a joint are chosen for rehabilitation or control of a device, it's assumed that a specific load sharing or activation pattern exists among them for each individual and for each specific joint demand. Indeterminacy or a load sharing problem arises from having more muscles crossing a joint than needed to perform all possible movements. It's proven that muscle activation patterns depend on fatigue, the task (isometric/isokinetic, concentric/eccentric), load type, mental demands, etc. The most used biofeedbacks are electromyogram of one of the muscles or the joint torque signal. An important question is if they can be used interchangeably. This study investigated if the choice of biofeedback can also change the activation pattern in the two main elbow flexors. The results of this experiment on six healthy subjects and seven activation levels, indicated that change in biofeedback type had a significant effect on the activation ratio of these two muscles.