Flexor Accessorius (Quadratus Plantae) (Musculus flexor accessorius)

Anatomy

A flat rectangular muscle of the second layer of the sole with two heads: a medial head from the medial calcaneal surface and a lateral head from the lateral calcaneal border. Both converge on the tendons of flexor digitorum longus where they split. Its pull is aligned with the toes, so it straightens the oblique, medially-directed pull of the long flexor before the toes grip the ground.

Origin & insertion

From the plantar surface and lateral border of the calcaneus, into the tendons of flexor digitorum longus.

Action

Assists flexion of the lateral four toes and corrects the oblique line of pull of flexor digitorum longus.

Innervation

Lateral plantar nerve (S1–S2).

Clinical notes

It is the test case of the second layer in claw-toe surgery: flexor-to-extensor tendon transfers must respect its insertion into the long-flexor tendons, and a muscle-sparing release here preserves the straight pull the toes need for terminal stance.

Related structures

• Calcaneus (heel bone) (right)
• Flexor digitorum longus

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