A closed-loop stepper motor waist-pull system for inducing protective stepping in humans.

Past approaches for inducing protective stepping to preserve standing balance in humans have either lacked the flexibility of control over the initial conditions of falling, or involved considerable mass which limits mobility of the system. This report describes the design and function of a stepper motor closed-loop waist-pull system for evoking protective stepping responses. Bench testing with applied load-motion profile combinations indicated performance degradation for force levels greater than 204 N which was well within the levels encountered in human experiments. An optical encoder feedback design allowed a positional accuracy of 0.00225 mm per step. A regression analysis of specified versus recorded velocities resulted in an acceptable fit (r2 = 0.99). The mean rise time was 63.0+/-18.0 (SD) ms and was consistent up to the load limits of the device. In human experiments repeated perturbations were consistently realized. Applied motion profiles were generally comparable at the pelvic level for subjects of different geometry, weight, and inertia despite a tendency for slight positional lag. The method allows flexible and accurate control of the initial conditions of a perturbation-induced fall to elicit steps. The system dimensions and moveability allow it to be implemented into clinical environments.