Popliteus muscle the ankle. The gastrocnemius muscle acts on both joints. It originates from the lateral surface of the lateral condyle of femur by a rounded tendon, the fibers pass downward and medially and give insertion to the posterior surface of tibia, above the soleal line. The muscle arises within the capsule of knee joint and its tendon separates the lateral meniscus from the lateral ligament of the joint. Nerve supply is via the tibial nerve from spinal roots L5 and S1. There is sometimes an additional head from the sesamoid bone in the lateral (outer) head of the gastrocnemius muscle. Rarely
Anatomical terms of muscle need antagonistic pairs located in opposite sides of a joint or bone, including abductor-adductor pairs and flexor-extensor pairs. These consist of an extensor muscle, which "opens" the joint (by increasing the angle between the two bones) and a flexor muscle, which does the opposite by decreasing the angle between two bones. However muscles don't always work this way - sometimes agonists and antagonists contract at the same time to produce force, as per Lombard's paradox. Also, sometimes during a joint action controlled by an agonist muscle (see above definition of agonist), the antagonist will be slightly activated, naturally. This occurs