The Achilles tendon is the largest tendon in your body. It connects the upper calf muscles to the back of the heel bone. Achilles tendinosis is a condition in which the Achilles tendon degenerates and becomes inflamed. Sometimes, it may also be called Achilles tendinitis. If you have Achilles tendinosis, your tendon can swell and become painful. This condition is common in athletes, runners, and people who have calf tightness. Achilles tendinosis may occur in the middle of the tendon (known as midsubstance Achilles tendinosis) or at the point where the tendon connects to the heel bone (known as insertional Achilles tendinosis).