Exoskeletal System for Neuromuscular Rehabilitation

NASA's Jet Propulsion Laboratory, Pasadena, California

A system of sensors and actuators designed to fit on the lower extremities of human patients and astronauts like an exoskeleton is under development to serve diverse purposes in neuromuscular research and rehabilitation. This system will be operated under both Earth and microgravity conditions. A product of integrated research efforts in several fields of science and engineering, the design of the system incorporates advances in microsensors, robotics, and mathematical modeling of the dynamics of walking. The design of the system has been guided by research findings that show that the spinal cord (even when cut off from the brain by injury) is capable of relearning the ability to walk.

The system could be used by an astronaut exercising in space under the conditions of microgravity to help maintain normal locomotion skills, muscle mass, and bone calcium levels. On Earth, the same system could be used for the rehabilitation of stroke or spinal-cord-injured patients in an effort to restore part or all of their ability to walk.

At present, one way to perform such rehabilitation is to suspend the patient in a harness over a treadmill so that their legs bear only part of their entire weight while therapists manipulate the patient's legs to assist in stepping on the running treadmill. As the patient gains more ability to step, the amount of assistance needed is decreased. The disadvantages of this approach are several: a limited number of therapists can assist only a small number of patients, the assisted movements are only approximations of normal stepping motions, and there is no way to quantify changes in the amount of assistance needed. Thus, there is a need for something that can accurately measure limb motions, apply and measure controlled forces and torques, and be used to manipulate an entire leg in normal kinematic patterns and speeds. Such a device could be used to study more subjects with greater thoroughness and precision, and could make it possible to rehabilitate more patients with higher levels of success than can now be attained with human assistance alone. Such a robotic device could also be used to preserve normal locomotion skills for astronauts during long-term microgravity conditions.

The present exoskeletal system is a prototype of such a robotic device. Encouraging results have been obtained in preliminary tests performed on humans in the Neurological Rehabilitation and Research Unit of the University of California at Los Angeles. When the system is fully developed, continuous analysis and control of the force and torque actuators needed for normal walking motions will be generated from a combination of dynamical motion modeling and sensory feedback from the exoskeleton.

Thus, the development of the exoskeletal system involves the parallel development of sensors capable of measuring six degrees of freedom and computational resources capable of instantaneous analysis and control. This approach uses the exoskeleton to control and monitor the movements of each segment of the lower limb during locomotion and a mathematical model that accounts for such details as the masses and kinetics of limbs and the moment arms of individual muscles of the knee, hip, and ankle. Variables that can be investigated by use of the model include the percent of body weight loading, the frequency of stepping, the speed of walking, and changes in muscle output that would occur in hypertrophy or atrophy.

This work was done by James Weiss, Antal Bejczy, Bruno Jau, and Gerald Lilienthal of Caltech for NASA's Jet Propulsion Laboratory. NPO-20370