Motor Point Displacement: Optimising Electrode Positioning in FES

Neuromuscular electrical stimulation (NMES) induces muscle contractions by delivering electrical impulses to motor points (MPs), the skin areas nearest to terminal nerve branches. Proper mapping of MPs is critical for optimizing NMES and FES, as it reduces activation thresholds and discomfort while enhancing muscle contraction. The effect of joint movement on MP displacement, especially in the lower limbs, has not been thoroughly examined.This study investigates MP displacement of the rectus femoris proximal MP during knee extension at three angles (90°, 120°, and 180°) in 13 healthy subjects - corresponding to a range of motion observed in FES-assisted movements such as standing up, walking; cycling and rowing. MP locations were identified using an electrical stimulator and tracked via anthropometric references. Results revealed a significant MP displacement averaging 6.89 cm, approximately 15% of the thigh length. The displacement predominantly occurred medially and distally. Activation thresholds were also assessed, indicating variability in current intensity requirements among subjects.Such motor point displacement during movement can mis-align electrodes, reducing contraction and stimulation efficacy and limiting FES-based rehabilitation, recreation, and sport.