The gastrocnemius (also gastrocnemius muscle, latin: musculus gastrocnemius) is a superficial muscle of the posterior group of the lower leg muscles.
The gastrocnemius originates with two heads: the medial and the lateral head. The medial head of the gastrocnemius arises from the medial condyle, while the lateral head originates from the lateral condyle of the femur. The gastrocnemius inserts onto the calcaneal tuberosity of the calcaneus, as its both heads join in the midline of the lower leg in a tendon (Achilles tendon).
The main function of the gastrocnemius is flexion of the knee. Contraction of the medial head of the gastrocnemius rotates the leg medially (internal rotation of the leg), while contraction of the lateral head rotates the leg laterally (external rotation of the leg).