From Elizabeth Quinn,
Your Guide to Sports Medicine.
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The Achilles tendon is the largest and most vulnerable tendon in the body. It joins the gastrocnemius (calf) and the soleus muscles of the lower leg to heel of the foot. The gastrocnemius muscle crosses the knee, the ankle, and the subtalar joints and can create stress and tension in the Achilles tendon. Tendons are strong, but not very flexible so they can only so far before they get inflammed and tear or rupture.

Causes of Achilles Tendon Rupture
The exact cause of ruptures is hard to say. It can happen suddenly, without warning, or following a tendonitis. It seems that weak calf muscles may contribute to problems. If the muscles are weak and become fatigued, they may tighten and shorten. Overuse can also be a problem by leading to muscle fatigue. The more fatigued the calf muscles are, the shorter and tighter they will become. This tightness can increase the stress on the Achilles tendon and result in a rupture.

A rupture is likely when the force on the tendon is greater than the strength of the tendon. If the foot is dorsiflexed while the lower leg moves forward and the calf muscles contract, a rupture may occur. Most ruptures happen during a forceful stretch of the tendon while the calf muscles contract.

A classic sign of an Achilles tendon rupture is the feeling of being hit in the Achilles are. There is often a "pop" sound. There may be little pain, but the person can not lift up onto his toes while weight bearing.

Treatment
A completely ruptured Achilles tendon requires surgery and up to 12 weeks in a cast. Partial tears are sometimes are treated with surgery following by a cast. Because the tendon shortens as it heals, a heel lift is used for 6 months or more after the cast comes off. Physical therapy to regain flexibility and then strength are begun as soon as the cast is off.

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