Traumatic damage to the groin or hamstrings.

Pain in the groin is often the result of a groin (adductor muscle) pull or strain. This is similar to any other type of pulled or strained muscle, but it occurs when the muscles of the inner thigh are stretched beyond their limits. This can result in small muscle tears that cause pain and swelling. The groin muscles include the adductor magnus, adductor longus, adductor brevis, pectineus, and gracilis. The most common cause of groin pain is a muscle strain that causes localized pain in the inner thigh, or groin. A severe tear can cause a sudden, acute pain and may be accompanied by swelling and bruising. They are painful to the touch, and pain increases with resistance movements, and stretching of the inner thigh and hamstrings. These injuries often occur during a sudden change of direction while running and quick starts and stops. These injuries are especially common in field or court sports.

Hamstring injuries are common among runners. The hamstring muscles run down the back of the leg from the pelvis to the lower leg bones, and an injury can range from minor strains to total rupture of the muscle. The three muscles that make up the hamstrings are the biceps femoris, semitendinosus and semimembranosus. A hamstring injury is recognized by a sudden, sharp pain in the back of the thigh that may stop you mid-stride. After such an injury, the knee may not extend more than 30 to 40 degrees short of straight without intense pain. Sprains and strains are caused by excessive stretching (tearing) of muscle fibers soft tissues. Hamstring strains are classified as 1st (mild), 2nd(moderate), or 3rd (severe) degree strains depending on the severity.