The Achilles tendon attaches the calf muscles–the gastrocnemius and the soleus–to the heal (calcaneus), and is responsible for the pushing off motion during ambulatory activities.

The Achilles tendonitis is a common condition found amongst runners. The condition is increasingly referred to as Achilles tendinopathy rather than Achilles tendonitis due to the fact that the condition involves degenerative tissue rather inflammation of the tendon. The condition can be acute or chronic; onset of Achilles tendon problems can come on suddenly or develop over time.

Further Reading:

American Academy of Orthopaedic Surgeons: Achilles Tendonitis