Biomechanical Effects of a Passive Hip Structure in a Knee Exoskeleton for People with Spinal Cord Injury: A Comparative Case Study

The inability to stand and walk is one of the major consequences of spinal cord injury (SCI). Wearable lower limb exoskeletons are emerging as a promising solution to restore mobility after SCI. The ABLE Exoskeleton is a powered-knee exoskeleton intended for patients with low SCI, who preserve motor function at the hip. This paper presents a comparative study to investigate the biomechanical effects of adding a hip structure that connects two individual powered-knee exoskeletons, and constrains hip abduction-adduction and internal-external rotation. We carried out a series of tests with an adult man with a motor complete SCI (injury level T11) walking with two independent knee-exoskeletons one in each leg (ABLE Knee) and with the knee-exoskeletons connected to the hip structure (ABLE Hip). The participant was asked to walk with both configurations while we recorded body motion, actuator current, and foot contact pressure. We observed significantly larger deviations in terms of hip abduction-adduction, internal-external rotation and pelvic obliquity with the ABLE Knee compared to the ABLE Hip. In addition, walking with the ABLE Hip significantly increased the range of motion of knee and hip flexion-extension. However, these improvements in joint kinematics did not translate to significant differences in step length or walking speed, when comparing the two configurations. In conclusion, these results suggest that adding a hip structure that limits undesirable hip rotations can improve walking performance for people with SCI.