A Muscle-Powered Exoskeleton for Weight-Bearing Exercise After a Spinal Cord Injury

A dynamic simulation shows the feasibility of powering a standing and stepping exoskeleton for exercise after a spinal cord injury using only voluntary upper body movement and surface electrical stimulation of the paralyzed quadriceps muscles. An energy storage system eliminates the need for motors and enables the stimulation electrodes to be located only in easy-to-reach areas. The dynamic simulation demonstrates that 15Nm of torque supplied by electrical stimulation of the quadriceps for a maximum duration of 285ms per step is sufficient to complete a stepping motion, where the stimulation time can be decreased by reducing the step length. The remaining power supplied to the exoskeleton comes from voluntary upper body input using the support of parallel bars, a walker, or arm crutches. A quasistatic dynamic analysis indicates that 245N of upper body force is needed from the user to upright their torso and supplement the quadriceps muscle stimulation. A system of springs and brakes at the exoskeleton joints traps excess energy generated by the upper body and stimulated quadriceps muscles and releases it to complete the stepping motion. The simulation demonstrates the exoskeleton can operate between step lengths of 275mm and 450mm and achieve stepping speeds between O.18m/s and O.23m/s, with up to 45mm of toe clearance when pelvic tilting is permitted. These simulation results demonstrate the potential of using energy storage to enable a fully muscle-powered exoskeleton for exercise after a spinal cord injury.