Sinus tarsi
The sinus tarsi is formed by the apposition of the sulcus calcanei inferiorly and the sulcus tali superiorly. In an articulated foot, it contains the interosseous talocalcaneal ligament, part of the belly of the extensor digitorum brevis muscle, a fat pad, and variable neurological and vascular structures. The floor of the sinus tarsi may be identified at it's opening. To do this, begin by placing your index finger on the anterior border of the lateral malleolus. From this position, move your finger inferiorly toward the sole of the foot until it slides into a depression. This depression marks the opening of the sinus tarsi. Note that this depression may be obscured by a prominent extensor digitorum brevis muscle belly in some individuals.