In the biomechanics of the subtalar joint, which motion couples inversion of the foot with which movement of the tibial shaft?

• A Foot inversion couples with internal (medial) rotation of the tibia
• B Foot inversion couples with external (lateral) rotation of the tibia ✓
• C Foot eversion couples with internal rotation of the tibia
• D Foot dorsiflexion couples with eversion through the subtalar joint

Explanation

The subtalar (talocalcaneal) joint is a mechanical coupler between foot pronation/supination and tibial rotation. Foot supination (which includes inversion, plantarflexion, and adduction) couples with external rotation of the tibia. Conversely, pronation (eversion, dorsiflexion, abduction) couples with internal rotation of the tibia. This coupling is why abnormal foot pronation can contribute to medial knee and hip stress by causing excessive tibial internal rotation. Option A has the coupling reversed.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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