An Achilles tendon rupture is an injury that affects the back of the lower leg. It most common among people who play sports. The Achilles tendon connects the calf muscle to the heel. It is the tendon that is responsible for making it possible to walk, jump, run, and point your toes. The Achilles tendon is the largest and strongest tendon in the body. If it is not used often, it can become weak. An Achilles tendon can tear or rupture if it is overstretched. The tear can be a partial tear or a full tear.