This image shows two of the major muscles found on the frog shank. The large gastrocnemius (1), the calf muscle that is located on the medial surface of the shank, originates on the femur, inserts on the Achilles tendon and flexes the shank and foot. The peroneus (2), which is located lateral to the gastrocnemius, also originates on the femur but inserts on the distal end of the tibiofibula. The peroneus extends the shank and foot.