This exercise is used to develop the hamstrings and isolate the back of your legs.
Begin by adjusting the leg curl machine so that your upper thigh is pressed against the thigh pad and the ankle pad is at the height of your ankle. Lean forward into the machine so that your leg is fully stretched out. Now squeeze your hamstring muscles and curl your leg up the same way you would do if you were lying down. Curl your leg up until it either hits your upper thigh or your upper thigh begins to pull off of the pad. Hold the contracted position for a second and then slowly lower the weight back to the starting position. Repeat this movement until you have completed your set.
It is important to stretch your hamstrings on a regular basis. Hamstring injuries are common because people neglect them in both their stretching and strength training routines.