Effect of the relative rotation axis position of the stretching machine and ankle

At the onset of a stroke, paralysis of the lower leg typically causes a foot deformity called the foot drop. Stretching is an effective technique for physical therapy. An automated stretching machine provides the option to incorporate long-term stretching at home. We have remodeled a commercial foot exerciser to function as a stretching machine for ankle dorsiflexion [1]. It is believed that the rotation axis of such machines should be aligned with the biomechanical axis of the human body, which is the design principle our prototype is based on. However, there have been no studies that have investigated the best position of the axis required to achieve effective stretching. In this study, we evaluated several positions of the machine's rotation axis in the sagittal plane with respect to the stretching effect and physical burden (safety). The force applied on the foot and ankle dorsiflexion angle were measured during the stretching experiments. We computed the work involved in ankle dorsiflexion and the force not contributing to the dorsiflexion movement, which served as indicators for the stretching effect and potential physical burden, respectively. It was found that the best position of the machine's rotation axis can be above the ankle.