Range-of-motion measurement with therapist-joined method for robot-assisted ankle stretching

The range of motion (ROM) measurement is an important issue in a robot-assisted ankle–foot rehabilitation. This study presents a therapist-joined method to improve the ROM measurement accuracy of a robotic ankle–foot rehabilitation system by combining therapist-joined zero-torque control and proxy-based sliding mode control. The zero-torque control is applied when measuring the subject's range of motion to obtain its extreme joint angle and largest resistance torque. Moreover, a controller switch is applied to ensure that the extreme joint angle is appropriate. Comparative studies between the ROM measurement using a protractor and the proposed method are then performed, comparing the ankle resistance torque variations during stretching under different ROM-measuring results. Ten able-bodied subjects are recruited for this experiment. The results show that the proposed method is feasible in improving the measured ROM accuracy. In addition, a more accurate ROM leads to a larger improvement of the ankle stiffness.