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The triceps surae are a group of muscles on the back of the lower leg, better known as the calf, that share a similar function and therefore may be referred to as a single muscle. Collectively, these include not three muscles, as the name suggests, but two: the gastrocnemius muscle and the soleus muscle. Since the gastrocnemius features two separate heads, however, the triceps surae can be viewed as having three sections. These muscles, which unite at the heel to become the Achilles tendon, are responsible for plantarflexion of the ankle, which is the action of pointing of the foot downward.
Visible just under the skin in the lower leg, the gastrocnemius is the large two-headed muscle of the triceps surae. It finds its origins behind the knee on either side of the distal or lower femur bone, on the two rounded bony eminences on the bottom of the thigh bone known as the medial and lateral condyles. From here, the two heads run down either side of the calf, tapering and converging roughly halfway down to form a powerful tendon known as the Achilles, or calcaneal, tendon that affixes to the back of the heel bone in the foot.
As the gastrocnemius is the most powerful plantarflexor of the ankle, it is involved in all movements of locomotion, from walking to running to jumping. When it contracts, its fibers pull upward on the heel bone, shortening the back of the leg and hinging the foot downward as seen when a jumper pushes off the ground. This muscle also plays a major role in standing, keeping the body from falling forward.
Beneath the gastrocnemius in the calf is the soleus, the third muscle of the triceps surae. Slightly smaller than the gastrocnemius, it originates just below the knee on the posterior upper fibula bone in the lower leg. Following the same path as the larger muscle above it, it tapers and inserts its fibers into the Achilles tendon somewhat lower in the calf than that muscle.
Like the gastrocnemius, the soleus acts to plantarflex the ankle during walking, running, and jumping movements, and it also works to maintain standing posture. The main distinguishing feature between these muscles of the triceps surae is that while the gastrocnemius is more heavily involved in plantarflexion when the knee is extended or straightened, the soleus is more active when the knee is flexed. As the soleus does not cross the knee joint, it does not relax like the gastrocnemius when the knee is bent. Therefore, it performs much of the action of plantarflexion during movements like sprinting and other motions in which the knee is bent while the foot is pointed.