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The superior extensor retinaculum is a thickening of the fascia that extends across the anterior aspect of the leg just proximal to the lateral and medial malleoli. It attaches laterally to the fibula and medially to the tibia. The inferior extensor retinaculum is a larger, y-shaped band . It arises from the anterior aspect of the calcaneus and extends medially anterior to the ankle, where it divides into two limbs. The upper limb of the retinaculum attaches to the medial malleolus. The lower limb passes around the medial side of the foot and attaches to the plantar aponeurosis of the foot, which will be identified later.

Links and References:
Grant's: 5.77
Netter (1ed.): 488 and 490 (2ed.): 484 and 486
Rohen/Yokochi: 433 and 436