The leg curl is an isolation exercise that targets the hamstring muscles. The exercise involves flexing the lower leg against resistance towards the buttocks. Leg curls, since you are using less muscle mass, the exercises are easier to do. They won’t affect your whole body the way squats will. It is most commonly performed on a machine; force you to bend your knees against resistance. The muscles that make up your butt — gluteus maximus, gluteus minimus and gluteus medius — are involved in controlling stability and movement at the hip joint. It primarily works on your hamstrings, which is a collection of three muscles at the back of your thighs. When using either the seated or lying leg curl machine, the muscles on the back of the thigh will contract to bend your knee.