Biomechanics
Gait, forces, and tissue mechanics
The gait cycle is divided into stance and swing, with double support during walking. Know the three rockers: heel rocker, ankle rocker, and forefoot rocker. At heel strike, the ankle begins to plantarflex and is controlled eccentrically by dorsiflexors, especially tibialis anterior. During midstance, the tibia advances over the foot while the plantarflexors control forward tibial motion. At push-off, the plantarflexors generate a large ankle moment. Subtalar joint motion couples foot pronation and supination with tibial rotation. The windlass mechanism tightens the plantar fascia during toe extension and elevates the medial longitudinal arch.
Mechanics questions use levers, moments, stress, strain, and viscoelasticity. A moment equals force times perpendicular distance from the axis. A longer lever arm increases torque for the same force, which is why foot position changes demand on the Achilles tendon and plantar fascia.
Traps and worked rule
A frequent trap is labeling the first rocker as push-off. The first rocker is heel strike to foot flat and is controlled by dorsiflexors. The third rocker is forefoot contact to toe-off. Another trap is assuming the plantar fascia is passive only; the windlass mechanism makes toe extension mechanically important. Worked example: if a ground reaction force of 700 N acts 0.05 m from the ankle joint center, the plantarflexion moment is 35 N·m. Use consistent units and perpendicular distance.