|Derivation||Gastroc, belly; neme, leg - the belly of the leg.|
|Origin||The medial head attaches to the femur just above the medial condyle; the lateral head does the same above the lateral condyle.|
|Insertion||Via the Achilles tendon to the bone of the heel, the calcaneus (cal-KAY-nee-us).|
|Action||Straightens the joint of the ankle, either pointing the foot, or lifting a standing figure onto the ball of the foot.|
Gastrocnemius is familiar as the great calf muscle. Its two heads attach to either side of the femur in order to pull symmetrically on the heel.
The medial head is larger and lies lower on the limb than the lateral head. (Compare with the biceps muscle, for which this is also true.) On a muscular gastrocnemeus, the furrow between the heads can be quite noticiable on the lower end, and one or two more breaks may be found on the heads running with the muscle fibers.
The medial head extends further forward on the leg than the lateral head. A view from this side shows that the medial head and the tibia comprise most of the upper calf. The lateral head is a flatter structure that sits well back on the leg.
The Achilles tendon forms a long, inverted cone shape under the muscle heads that attaches at the heel.
Above the lateral head lies plantaris (plan-TAIR-riss), a muscle that assists the action of gastrocnemeus. It can be seen lending fullness to the back of the knee below the fold.