Effects of Restricting Ankle Joint Motions on Muscle Activity: Preliminary Investigation with an Unpowered Exoskeleton

The human ankle comprises multiple joints and supports triplanar motions to allow the foot to pronate or supinate during walking. However, ankle exoskeletons are mainly designed to assist propulsion whilst inhibiting other degrees of freedom. The kinematic constraints posed by the simplified joint mechanisms may negatively affect the wearer’s performance. In that context, this paper presents a preliminary investigation on the effects of restraining ankle motions during level walking with an unpowered ankle exoskeleton having compatible joint axes. The work investigated the changes in muscle activity in the lower limbs under various constraining conditions. A healthy male subject took part in five tests involving different combinations of kinematic restrictions of the ankle. The electrical activities of key muscles were recorded using a surface electromyography measurement system. The root-mean-square feature of signals was used for comparing results. The analysis confirms that constraining non-sagittal plane motions has caused significant changes to the activities of muscles. The investigation reveals the relative importance of developing ankle mechanisms that promote higher kinematic compliance. In the future, further studies should be conducted to reaffirm the statistical significance of muscle activity across multiple test subjects and assess human comfort to derive specific design guidelines for ankle devices.