Selection of an optimal muscle set for a 16-channel standing neuroprosthesis using a human musculoskeletal model

This study proposes an optimal set of lower-limb muscles to be stimulated electrically with a 16-channel neuroprosthesis that will allow persons with paraplegia caused by spinal cord injury to stand and shift postures smoothly, thus minimizing muscle fatigue and facilitating performance of activities of daily living. We used a three-dimensional (3-D) 15 degree-of-freedom musculoskeletal model of the human lower limbs to assess different muscle combinations that would maintain specific standing postures while minimizing the overall metabolic energy consumed. We initially selected the postures by discretizing the joint-angle space over the ranges of the knee, hip, and ankle angles and then refined the postures by relating the lower-limb joint angles to the center of mass (COM) of the musculoskeletal model to generate smooth transitions between desired postures. We found a set of four 3-D second-order polynomials adequate for obtaining the best fit between the joint angles and the COM components. The results showed that adding the gluteus medius and the adductor magnus to balance nonsagittal movements at the hip and adding several different combinations of ankle muscles should allow users to shift postures over 75% of the forward-backward range that nondisabled individuals use during typical activities. The simplest complete ankle-muscle set only requires the soleus and the tibialis anterior, and the medial and lateral gastrocnemii could be added for additional plantar flexion. Alternatively, if the ankle is consistently being inverted, the peroneus muscles could be added.