The Achilles tendon includes three muscles at the back of the thigh: the semitendinosus, semimembranosus, and biceps femoris. They work for knee flexion, leg retraction, and hip extension. "Muscle pull" means that the muscle is stretched and torn. There are three grades of severity based on the number of muscle fibers injured. Grade 1 means there is a slight stretch and injury, Grade 2 means there is a partial muscle tear, and Grade 3 Severe means there is a muscle tear and rupture. may need surgery Achilles tendon strain is common in activities that involve running, jumping, and kicking, and can also be caused by over-stretching (such as yoga) and other activities that require a quick start and stop.