The hamstring is actually three muscles (biceps femoris, semimembranosus, semitendinosus)  on the backside of your thigh.  The action of the muscles are to flex (bend) your knee, and add stability to your knee during movement.  They also play an important role is pelvic positioning.  When too much stress is induced on the muscles, or they become overstretched, you can strain the muscle fibers, or even tear them. If the injury is severe enough, you may observe bruising and swelling on the back side of the thigh.  If the strain is minor, rest, ice and mild stretching when appropriate should help. However, it is important to properly rehabilitate the muscle(s) with physical therapy to prevent recurrent injury, and to return to your prior level of function. 

Associated Symptoms:

• Posterior thigh pain
• Ischial bone (sit bone) tenderness to touch
• Bruising of posterior thigh
• Swelling of posterior thigh
• Pain with walking/running
• Tightness of posterior thigh