Calibration Protocol for Functional Electrical Stimulation-Induced Ankle Dorsiflexion towards gait rehabilitation of patients with Foot Drop Syndrome

Abstract

Introduction:
Post-stroke foot drop syndrome (FDS) disrupts ankle dorsiflexion (ADF) and gait biomechanics. Functional Electrical Stimulation (FES) of the peroneal nerve and tibialis anterior (TA) can restore ADF. However, suitable electrode placement and stimulation parameters vary greatly due to individual anatomy. Suboptimal FES configurations may lead to excessive inversion/eversion (I/E), insufficient ADF angles and cutaneous discomfort.

Objective:
To report a FES protocol for setting parameters and electrode positions to induce natural, comfortable ADF movements, and a proof of concept (PoC) to evaluate its suitability using kinematic analysis.

Methodology:
Participants were sitting with legs hanging free and knees bent at 90-100° angle. The tibialis anterior (TA) muscle, lateral femoral epicondyle (LFM) and lateral external malleolus (LEM) of the stimulated leg (left) were located. The distance from the LFE to the LEM was measured (DEM). Optical markers were placed at the LFM, the LEM, and second foot finger.
The Rehastim 2 (HASOMED GmbH) stimulator was used to deliver biphasic, current-controlled electrical pulses, configured with main stimulation interval (33 ms), pulsewidth (400 µs), and trapezoidal burst timing: 2.5 s ON, 2.5 s OFF.
For baseline assessment, transcutaneous electrodes (5 cm) were placed at 35% and 55% of the DEM (from the LFE), to set three stimulation(amplitude) thresholds: sensory (ST), motor (MT), functional (FT). Then, a six-electrode (3.2) array (arranged in 3 columns by 2 rows) was centered at 35% DEM distance from the LFM. Stimulation was delivered to each electrode to identify which induced more natural (complete ADF, minimal I/E) and comfortable (lower Wong-Baker Faces scale score) ADF. Amplitude was set at 80% of FT.
Movements were recorded with two webcams (30 fps, 880x440 resolution) at frontal and side views. Ankle goniometry (ADF and I/E) was obtained from the videos using the KINOVEA software.

Results:
Five healthy volunteers participated in the study (four females, one male), with mean(STD) age of 36.2(13.04) years. ST, MT and FT from baseline assessment were 4.2(2.48) mA, 11.2(4.08) mA, and 21.4(3.43) mA, respectively, with mean(SD) discomfort scores of 0.2(0.45), 1.8(1.30), and 5(1.87).
From kinematic analysis, two electrodes induced high ADF angles: Electrode 4(E4), with 24.96(5.25) °, and E2 with 24.30(9.52) °. However, E4 was slightly more comfortable than E2, with 4.6(1.67) score and 6.4(2.5), respectively, and had lower I/E angles than E2: -8.97(8.56) and -10.05(5.48), respectively. Then, E4 is considered the overall suitable stimulation position among subjects. For reference, E4 was placed 5 cm lateral to the tibia, at 35% of the DEM, and E2 was placed just 3.7 cm above E4.

Conclusions:
The proposed protocol and PoC enabled enhanced electrode placement and improved movement quality, showing feasibility for FES-based gait interventions.
It is remarkable that, despite low sample, gender and age variability, the overall optimal position (E4) aligned with reported high-probability locations of motor points of the TA.
In future works, usability and repeatability of the protocol will be studied in both healthy subjects and post-stroke patients stratified by gender and age.