The hamstring muscles (or under-quad as my daughter calls them) are a group of muscles in the back of the leg that flex the knee and extend the hip.  The hamstrings include the biceps femoris, semitendinosis and semimebranosis.  If your pelvis flexes forward it can cause pinching (impingement) in the front of the hip.  Hamstring strength/tension is necessary to resist this pelvic tilting and therefore hamstrings are a very important and under-rated set of pelvic stabilizer muscles.  Below are examples of exercises that can help improve hamstring strength.