Alignment of anatomical flexion axis when using one-degree-of-freedom ankle stretching exerciser

Most of the existing ankle stretching machines have one degree of freedom, whereas the human ankle has three degrees of freedom of motion. We observed the behavior of the anatomical flexion axis when the ankle was stretched in the dorsiflexion direction by using an exerciser with one degree of freedom for flexion. For this purpose, under several levels of heights and external/internal rotation angles, we monitored the difference in the angle between the anatomical axis of the ankle and the flexion axis of the exerciser. When the foot was placed on the exerciser in a slightly internally rotated state, the inversion angle of the ankle's anatomical axis was nearly aligned to that of the exerciser's flexion axis in the dorsiflexed posture. In contrast, when the foot was placed in a slightly externally rotated state, the ankle's anatomical axis and exerciser's flexion axis nearly aligned in terms of the internal/external rotation.