Motor Point Targeting for Stimulation of Upper Extremity Muscles Results in High Selectivity and Low Fatigue

Introduction: Advancements in brain-machine interface (BMI) technology have increased potential for use as a voluntary motor control center for patients with SCI. Functional electrical stimulation (FES) has shown potential for control of the distal upper extremities. In BMI-FES, motor activation signals are decoded from the brain and used to activate specific muscles in the upper extremity. Current FES systems which achieve muscle control by placement into target muscle bellies, are limited by high fatiguability and limited range of motion. Placement of FES leads into the point of nerve entry into the muscle (motor point) may improve performance.

Methods: Sixteen bipolar leads were implanted into forearm of rhesus macaque, targeting motor points of muscles controlling hand movement. Nerve block of was performed in proximal arm, to temporarily paralyze central muscle control and allow FES control. Leads were implanted into flexor digitorum profundus, extensor digitorum communis, flexor digitorum superficialis, flexor carpi radialis and ulnaris, and extensor carpi radialis. Testing was used to confirm placement. Selectivity and fatigue were measured and compared to muscle belly FES.

Results: A range of stimulation was used to characterize muscle responses of these leads. High definition camera system was used to quantify the amount of joint movement. All 16 leads were able to evoke the expected hand movements based on intraoperative stimulation and implant location. A lower threshold of activation was needed for motor point vs muscle belly placement. After 100 stimulations, motor point group experienced 14% fatigue in range of motion, muscle belly group experienced 32% reduction.

Conclusion : Motor point targeting for FES lead placement is a feasible and effective strategy for control of distal upper extremity. Less muscle fatigue and lower stimulation threshold was shown utilizing motor point lead placement, compared to traditional muscle belly placement. Motor point lead placement is feasible and generates more sustainable muscle contractions. Combining motor point FES with brain machine interface system may allow for implantation of a system for distal extremity control in conditions such as tetraplegia.