Motor Point Displacement: Optimising Electrode Positioning in FES.

Area of Science:

• Biomechanics
• Neurophysiology
• Rehabilitation Engineering

Background:

• Neuromuscular electrical stimulation (NMES) relies on accurate motor point (MP) identification for optimal muscle activation.
• Functional electrical stimulation (FES) applications in rehabilitation and assistive devices require precise electrode placement.
• The impact of joint movement on lower limb MP displacement remains under-investigated.

Purpose of the Study:

• To quantify the displacement of the rectus femoris proximal motor point during knee extension.
• To assess the influence of joint angle on MP location and displacement.
• To evaluate the implications of MP displacement for NMES and FES applications.

Main Methods:

• Thirteen healthy subjects participated in the study.
• Motor point locations were identified using an electrical stimulator and tracked using anthropometric references.
• Knee extension was performed at 90°, 120°, and 180° angles to measure MP displacement during simulated FES-relevant movements.

Main Results:

• Significant motor point displacement averaging 6.89 cm (approx. 15% of thigh length) was observed during knee extension.
• MP displacement primarily occurred in the medial and distal directions.
• Variability in activation thresholds was noted across subjects, highlighting individual differences.

Conclusions:

• Joint movement causes substantial motor point displacement in the rectus femoris, potentially leading to electrode mis-alignment.
• This displacement can reduce the efficacy of NMES and FES, impacting their effectiveness in rehabilitation, recreation, and sports.
• Accurate, movement-compensated MP mapping strategies are crucial for advancing FES technology.