The hamstring is a muscle which extends the hip joint and flexes the knee joint. Pulled hamstring muscles are a common occurrence with sprinters. Sudden accelerations or suddenly stopping can throw a hamstring muscle out of whack.
A mild case – or Grade I - can tear the muscle fibers with overstretching. The muscle attempts to contract, while being forced in the opposite direction. This may cause bruising in the back of the thigh due to bleeding in the muscle fibers. This leads to swelling – pooling up of blood. Compressive bandages are generally used for this condition. To reduce swelling, keep the body part elevated above the heart. A spasm is a contraction of the bruised muscle, which creates further episodes of pain. Muscle relaxants may quiet the spasm.
Rest immediately. Use ice packs and compressive bandages and gently stretch and massage the hamstring muscle. If using ice, it becomes much less effective after the first 48 hours. Be sure to move the ice around the muscle as in a massage. Do not ice for more than 15 minutes at a time, or frostbite and further damage may occur. Wait about one hour between icing intervals, and use a thin towel between the ice and the skin.
Difficulty contracting and flexing the muscle means that the hamstring muscle may be ruptured – which is the most severe case of hamstring injury – Grade III – and may require surgery.