A 25-year-old male is running and trips over a curb, sustaining a twisting injury to his ankle. He notes obvious deformity to his ankle. He is taken to the nearest trauma center by ambulance, where the injury is noted to be closed and his neurovascular status intact. Radiographs reveal a trimalleolar ankle fracture/dislocation. After closed reduction and splinting in the emergency department, he is scheduled for open reduction and internal fixation the following day. A CT scan of the ankle is obtained and based on these findings, you plan to place the patient prone and perform dual posterolateral and medial approaches to address all of the fracture fragments. Reduction and stable fixation of the posterior malleolus is important to restore the function of which structure seen in Figures A/B?