NMES-assisted standing model from varied seated postures.

After spinal cord injury (SCI), intact lower motor neurons can be electrically activated to produce functional muscular contractions and enhance one's capabilities beyond seated activities. Even with neuromuscular electrical stimulation (NMES), significant amounts of hand-support forces are commonly required to move from a sitting to standing position. The goal of this project was to determine initial seated postures that reduce vertical hand-support forces while keeping anterior/posterior hand-support forces below levels that would cause walker slipping or tipping. A multi-segment biomechanical model was further developed and expanded to test multiple combinations of initial postures. The muscles that were analyzed included the vastus lateralis and semimembranosus. Varying the initial knee and hip angles created alternative seated postures. For vastus lateralis stimulation, the lowest vertical hand-support forces (63-66% of body weight) were predicted at the lower (70-74 degrees) and upper (110 degrees) ranges of initial knee flexion. With combined vastus lateralis and semimembranosus stimulation, the lowest predicted vertical hand-support forces were 2-10% of body weight at initial knee flexion angles between 70-82 degrees. Initial hip flexion angles above 110 degrees were required to prevent walker slipping and tipping in these cases. The development of hip extensor torque with semimembranosus stimulation was critical in reducing the vertical hand-support forces. One implication is that when training with parallel bars for NMES-assisted standing, hand-support forces should be monitored to avoid conditions that would tip a walker. In future models, optimizing the timing sequence for stimulating muscles may produce smoother coordination of joint rotations and further reduce the vertical hand-support forces.