Excessive pronation from midstance to terminal stance (heel off) is most likely due to weakness of which muscle?

• A. Extensor digitorum
• B. Tibialis posterior
• C. Extensor hallucis longus
• D. Fibularis longus

Answer: (B)

Excessive pronation in the late stance portion of gait is best explained by weakness of a muscle that normally inverts the foot and supports the medial arch. Tibialis posterior provides the primary inversion and arch stabilization during midstance and toward heel-off, helping to keep the subtalar joint from everting and the medial longitudinal arch from flattening. When this muscle is weak, that stabilizing effect is lost, the arch collapses more, and the foot tends to roll inward excessively as you move from midstance into heel-off.

The other muscles listed don’t play the same central role in controlling this part of pronation. Extensor digitorum longus and extensor hallucis longus mainly dorsiflex the foot and extend the toes, not arch support or inversion. Fibularis longus does evert the foot and supports the arches, but its weakness is not the primary driver of excessive pronation during late stance, so tibialis posterior weakness best explains the pattern.