Lateral subtendinous bursa of gastrocnemius muscle

The gastrocnemius muscle (plural gastrocnemii) is a superficial two-headed muscle. It is located superficial to the soleus in the posterior (back) compartment of the leg. It runs from its two heads just above the knee to the heel, extending across a total of three joints (knee, ankle and subtalar joints). The muscle is named via Latin, from Greek γαστήρ (gaster) 'belly' or 'stomach' and κνήμη (knḗmē) 'leg', meaning 'stomach of the leg' (referring to the bulging shape of the calf). (Wikipedia)