Robot-assisted passive exercise for ankle hypertonia in individuals with chronic spinal cord injury

Hypertonia is one of the major causes of movement disorders in individuals with central nervous system lesions. Robot-assisted therapy can minimize the immobilization side-effect such as severe hypertonia by providing repetitive motor practice with minimal labor assistance. The purpose of this study was to design a robotassisted passive exercise device and establish optimal parameter settings for reducing both reflexive and non-reflexive muscle tone. A custom-made device with a built-in force transducer was developed to provide ankle cyclic passive exercise at various speeds and measure the total resistance during stretching. Ten individuals with spinal cord injury received 8min of low-speed passive exercise (20cycles/min), 8min of high-speed (50cycles/min) passive exercise, and repeated contractions elicited by electrical stimulation (ES), in randomized order over 3 consecutive weeks. Maximum ES-elicited isometric torque, M waves, H reflexes, and total resistance during stretching were measured before and after each of the treatments. The results show that the H reflex was reduced after passive exercise at both speeds but not after repeated ES-elicited contractions. The reduction of the total resistance during stretching was speed-dependent. Significant fatigue was induced only after repeated ES-elicited contractions. This study successfully developed a robot-assisted passive exercise device, evaluated parameter settings, and provided evidence of the reduction of both reflexive excitability and total muscle tone in hypertonia muscles.