Achilles tendinitis is a common condition that causes pain along the back of the leg near the heel.  The Achilles tendon is the largest tendon in the body. It connects your calf muscles to your heel bone and is used when you walk, run, and jump.  Although the Achilles tendon can withstand great stresses from running and jumping, it is also prone to tendinitis, a condition associated with overuse and degeneration.

Symptoms of an Achilles tendon injury are:

• Pain along the back of your foot and above your heel, especially when stretching your ankle or standing on your toes; with tendinitis, pain may be mild and worsen gradually. If you rupture the tendon, pain can be abrupt and severe;
• Tenderness;
• Swelling;
• Stiffness;
• Hearing a snapping or popping noise during the injury; and
• Difficulty flexing your foot or pointing your toes (in complete tears of the tendon).

Tendinitis from arthritis is more common in middle-aged and older adults. A bone spur or growth may form in the back of the heel bone. This may irritate the Achilles tendon and cause pain and swelling. Flat feet will put more tension on the tendon.

Learn more at Mayo Clinic and Wikipedia.