Final answer:
Supination at the subtalar joint produces a rigid lever used in gait as it involves inversion and plantar flexion, stabilizing the foot for propulsion during the push-off phase.
Step-by-step explanation:
The motion at the subtalar joint that produces a rigid lever used in gait is supination. Supination involves the complex movements of inversion and plantar flexion. During supination, the foot inverts, angling the bottom of the foot towards the midline, and the plantar flexion lifts the heel and points the toes downward.
In the context of gait, this position of the foot creates a stable base for propulsion. In contrast, pronation, which includes eversion and dorsiflexion, prepares the foot for shock absorption when striking the ground. Therefore, supination contributes to the transformation of the foot into a rigid lever for efficient force transmission during the push-off phase of walking or running.