3.2 Selection of the cylinder
We chose a cylinder that satisfies the required stroke and thrust from the geometrical relation between the cylinder and footrest. The required specification is the stroke necessary to rotate 20 deg in the dorsiflexion direction from the axis of rotation of the footrest and thrust that gives a force of 40 N, which is the average value of a female hand pressing force on the sole of the foot [23]. We investigated and compared calculated values and actual measurements, which is the relation between the ankle joint angle, the thrust and the force applied to the sole of the foot. Parameters used for calculations are presented in Figure 4: N signifies the foot reaction force; F represents the cylinder thrust; R denotes the distance from the axis of rotation of the footrest to the measurement position; r stands for the distance from the axis of rotation of the footrest to the attachment part of the cylinder rod and footrest from the rotation axis of the footrest; p expresses the distance from the cylinder mounting part to the rotation axis; q is the cylinder length; and θ is the angle between q and r. Here, R, r and p are invariable even when the footrest rotates. They are known parameters. Calculation based on these equations revealed the necessity of strokes of 43.6 mm in the pushing direction and 98.2 mm in the pull direction. Results show that a stroke of 43.6 mm in the pushing direction and 98.2 mm in the pulling direction are necessary because a range of movement of 20 deg in the dorsiflexion direction and 45 deg in the dorsiflexion direction is necessary.