Hamstring is a group of muscles at the back of your knee or upper leg. These are the main muscles that that support your legs in walking and running. Hamstring curls particularly target your hamstring muscles to strengthen them. There are a number of variations of hamstring curls and each target specific muscles. The stability ball hamstring curls works much better in order to force you to bend the knees and extend the lower back muscles. It is advisable to primarily perform the stability ball hamstring curls to gain strength for other variations. After having sufficient expertise and strength you can increase the difficulty level through other variations as per your fitness level.