A heel tendon is any of the tendons of those muscles situated in the posterior compartment of the lower leg that attach to the calcaneus or heel bone. Found on the back of the leg in the calf, these include the tendons of the gastrocnemius, soleus, and plantaris muscles. Tendons are strong bands of fibrous connective tissue that link the end of a muscle to a specific bone or bones. These particular tendons are located at the lower end of these leg muscles on the back of the ankle. Together, the three form one powerful heel tendon known as the calcaneal or Achilles tendon.
Of the three tendons, the most substantial is that of the gastrocnemius, the large two-headed muscle of the calf. The gastrocnemius is a powerful plantarflexor, meaning that it flexes the foot downward at the ankle joint. As the muscle belly is located in the calf, this action would not be possible if not for its heel tendon, which crosses the ankle and attaches to the calcaneus. Contractions of the gastrocnemius, as in pointing the toe, shorten the muscle and pull upward on the heel bone, which hinges the foot downward.
Beneath the gastrocnemius in the calf is the soleus, a slightly smaller muscle whose heel tendon runs just deep to that of the gastrocnemius. These two muscles are considered by some to be one large muscle known as the triceps surae. What differentiates the two, however, is their function. Both are responsible for plantarflexion of the ankle, however, the soleus is more active when the knee is flexed than when it is extended.
The third muscle with a common tendon in the Achilles attaching to the heel is the plantaris. A relatively minor muscle of the leg — it is in fact considered a vestigial structure — its tendon is commonly grafted for use elsewhere in the body. With a muscle body originating on the bottom of the femur in the thigh and ending just below the knee, the plantaris tendon is very long, running from the top of the calf between the gastrocnemius and soleus to insert via the Achilles tendon on the calcaneus. As such, it is involved in both ankle plantarflexion and knee flexion, though in a minor capacity.
Though the Achilles is the toughest tendon in the human body, the tendency toward tightness in the muscles of the calf leaves it susceptible to injury. The damage to this heel tendon most often cited is Achilles tendinitis and Achilles tendon rupture, and those most vulnerable are individuals who perform explosive jumping and sprinting movements. Tendinitis is brought on by frequent, repetitive stretching of the tendon that eventually wears it down, leading to painful inflammation. Tendon rupture, on the other hand, is caused a sudden trauma, resulting in a partial or complete tear of the tissue.