Understanding stand-to-sit maneuver: implications for motor system neuroprostheses after paralysis

Standing up, standing, and walking functions can be restored to people with spinal cord injury by contracting the paralyzed hip, knee, and ankle muscles with electrical stimulation. Restoring these functions using electrical stimulation requires controlled activation to provide coordinated movements. However, the stand-to-sit (STS) maneuver involves eccentric contractions of the quadriceps to control lowering of the body to the seated position, which is difficult to achieve with stimulation alone and presents unique challenges to lower-limb neuroprostheses. In this study, we examined the biomechanics of the STS maneuver in five nondisabled individuals and five users of an implanted neuroprosthesis. Neuroprosthesis users relied heavily on their upper limbs during STS, with peak supporting forces approximately 25% body weight, and exhibited an average vertical acceleration at the impact six times higher than that of the nondisabled subjects (p < 0.001). Sitting with stimulation resulted in impact forces at initial contact with the seating surface averaging 1.4 times body weight and representing an average of twice the impact forces of the nondisabled subjects (p < 0.001). These results indicate a need for additional interventions to better control descent, minimize impact, and gently transition from standing to sitting to achieve a more natural movement and reduce the risk of injury.