The structure indicated is the fibularis (peroneus) brevis muscle.
The fibularis brevis muscle is one of the muscles of the lateral compartment of the leg. The leg consists of three muscular compartments: anterior, posterior and lateral. The lateral compartment consists of two muscles:
- Fibularis (peroneus) longus
- Fibularis (peroneus) brevis
The lateral muscles are responsible for eversion of the foot and are innervated by the superficial fibular (peroneal) nerve, which is a branch of the common fibular nerve. The common fibular nerve is a branch of the sciatic nerve and divides into the superficial fibular nerve and the deep fibular nerve. The deep fibular nerve innervates the muscles of the anterior compartment of the leg.
Origin: Lower two thirds of lateral shaft of fibula
Insertion: lateral tubercle at base of 5th metatarsal
Innervation: Superficial fibular nerve
Action: Eversion of foot
Learn more about the muscles of the leg in this tutorial.