What motion during the gait cycle unlocks the knee by causing the femur to rotate laterally?

• A. Dorsiflexion of the ankle
• B. Plantarflexion of the foot
• C. Internal rotation of the tibia
• D. Pronation of the subtalar joint

Answer: (D)

Pronation at the subtalar joint is the key mover. As the foot unfolds through stance, the subtalar joint pronates, which drives internal rotation of the tibia. With the knee in a weight-bearing, closed-chain position, this tibial rotation is accompanied by a lateral (external) rotation of the femur. That lateral rotation of the femur unlocks the knee, allowing it to move from a rigid, extended position into flexion during the transition to swing. Dorsiflexion or plantarflexion mainly affect the ankle and don’t trigger this knee unlocking mechanism, and while tibial internal rotation occurs with pronation, the specific driver for the knee’s lateral femoral rotation in gait is subtalar joint pronation.