The hamstring consists of three muscles that run down the back of the leg from the pelvis to the lower leg bones — the biceps femoris, semitendinosus, and semimembranosus. Hamstring injuries can range from minor strains to a total rupture of the muscle. Hamstring pulls are caused by excessive stretching of these muscles resulting in a sudden, sharp pain in the back of the thigh. Following such an injury the knee may not be able to extend beyond 30-40 degrees without pain.
Treatment: Treat the injury following the R.I.C.E. regimen. Anti-inflammatories can be used to reduce pain and inflammation. Once swelling and pain subsides, a stretching program can be initiated. The injured muscle should ultimately be rehabilitated through a strengthening program to prevent re-injury.
Pulled hamstrings can best be avoided via proper stretching/warm ups, as well as post-workout stretching. In the case of a severe injury, seek the opinion of a trained medical professional.