A muscle strain is a common injury, particularly among people who participate in sports. The thigh has three sets of strong muscles: the hamstring muscles in the back of the thigh, the quadriceps muscles in the front, and the adductor muscles on the inside. The quadriceps muscles and hamstring muscles work together to straighten (extend) and bend (flex) the leg. The adductor muscles pull the legs together. The hamstring and quadriceps muscle groups are particularly at risk for muscle strains because they cross both the hip and knee joints. They are also used for high-speed activities, commonly used while playing sports. Muscle strains usually happen when a muscle is stretched beyond its limit, tearing the muscle fibers. They frequently occur near the point where the muscle joins the tough, fibrous connective tissue of the tendon. A similar injury occurs if there is a direct blow to the muscle. Muscle strains in the thigh can be quite painful. Once a muscle strain occurs, the muscle is vulnerable to re-injury. It is important to let the muscle heal properly and to follow preventive guidelines from your doctor. A person who experiences a muscle strain in the thigh will frequently describe a popping or snapping sensation as the muscle tears. Pain is sudden and may be severe. The area around the injury may be tender to the touch, with visible bruising if blood vessels are also broken.