Achilles tendinitis is an injury of the Achilles tendon, which is a band of connective tissue that joins the calf muscles on the lower leg to the heel bone, according to Mayo Clinic. Intense strain and overuse of the tendon cause the condition.
Achilles tendinitis is common among runners in training and middle-aged people who play sports like basketball or tennis, notes Mayo Clinic. The condition begins with mild pain centralized over the heel or down the back of the leg that becomes progressively worse. Often, people afflicted with Achilles tendinitis feel more pronounced pain after sprinting, climbing stairs or running long distances. The condition also causes stiffness and tenderness, particularly in the morning.