Achilles tendon rupture is an injury that affects the back of your lower leg. It most commonly occurs in people playing recreational sports. The Achilles tendon is a strong fibrous cord that connects the muscles in the back of your calf to your heel bone. If you overstretch your Achilles tendon, it can tear (rupture) completely or just partially. If your Achilles tendon ruptures, you might feel a pop or snap, followed by an immediate sharp pain in the back of your ankle and lower leg that is likely to affect your ability to walk properly.