Interleaved stimulation in foot drop treatment: the issue of stimulation electrode positioning and its effect on foot kinematics

Foot drop is a general term referring to the difficulty flexing dorsally of the foot. It is commonly observed in subjects with muscle weakness related to upper motor neuron disease or injury, including stroke survivors and individuals with multiple sclerosis. Foot drop may significantly affect the gait as it decreases toe clearance during the swing phase thus increasing the risk of falls. Functional electrical stimulation (FES) of tibialis anterior (TA) has been proposed to assist ankle dorsiflexion (DF) during the swing phase in patients affected by foot drop, such devices are referred to as peroneal stimulators. Although peroneal stimulators were shown to improve the gait in subjects with foot drop, their diffusion has been limited by a common issue affecting all FES devices: the increased muscle fatigability due to non-physiological motor unit (MU) activation during electrically-induced contractions. In order to overcome this limitation, interleaved stimulation (iStim) has been proposed. By interleaving electrical pulses between the muscle (mStim) and the nerve (nStim), iStim distributes the stimuli among different MU groups, thus reducing the activation frequency of each group. The practical applicability of this approach to peroneal stimulators requires that both mStim and nStim induce foot DFs with minimal in-/eversion and ab-/adduction for all the knee angles within the swing phase. While for mStim knee rotations are not critical, changes in knee angle may modify the relative position between nStim electrodes and peroneal nerve, possibly inducing foot rotations outside the sagittal plane. In this study we investigated the effect of five nStim electrodes positioning along the course of the peroneal nerve on induced foot rotations in eleven healthy subjects. The main objective was to identify the configuration leading to foot DF maximally similar to the voluntary ones for the entire knee range of motion (0?, 20?, 40? and 60?, being 0? full knee extension). Foot rotations were quantified through an IMU sensor placed under the foot while surface electromyograms (EMG) were detected from both TA and peroneus longus (PL) muscles. By comparing electrically-induced foot rotations with those measured during voluntary DFs, it was possible to identify two electrode configurations inducing, for the considered knee angles, DFs similar to the voluntary ones. For the identified configurations the ratio between EMG amplitude detected from TA and PL was maximum, which confirmed our initial hypothesis concerning the importance of the stimulation selectivity in peroneal stimulators. These findings suggest that it is possible to find a common nStim electrode configuration that induces, in the majority of the participants, foot DFs with minimal eversion and inversion for the knee angles within the swing phase. This is a necessary condition to ensure the applicability of stimulation strategies based on distributed nerve/muscle stimulation (interleaved stimulation) aimed at reducing muscle fatigue in FES-based foot drop treatments.