Whether you have a gym membership or a home gym, you likely have access to a leg curl/extension machine or stations. These pieces of equipment come in different shapes and sizes. For example, some require you to do the exercises from a seated position, others require you to lie down, and others require you to stand. Whatever type of machine you’re using, the basic principles remain the same. Leg curls and extensions are single-joint exercises that produce movement only at the knee. Because of this, they strengthen the muscles that attach to the knee. Before beginning a new exercise program, please see your health-care provider.
In leg curls, you either sit or lie on the bench, or you stand facing the cables with your legs straight. After securing your ankles under the pad or attaching the cuff to your ankle, you bend your knee to bring your heel toward your bottom. You then control the weight as you return slowly to the starting position.
Leg curls target your hamstrings. This set of three large muscles on the back of your thighs can both bend your knees and extend your hips. However, in the leg curls, you’re only targeting the ability of the hamstrings to bend the knees. If you keep your ankles flexed throughout the exercise, your gastrocnemius, one of your main calf muscles, will help your hamstrings bend your knees.
For leg extensions, you either sit on a bench or stand with your back to the station. You bend your knee to place your ankles under the pad or to attach the cuff to your ankle. You then straighten, or extend, your knee and return slowly to the starting position. Throughout this exercise, you keep your thigh stationary, extending your lower leg until it comes in line with your thigh.
Leg extensions target your quadriceps. This large muscle group located on the front of your thigh contains four muscles. Three of the muscles -- known collectively as the vasti -- are only capable of extending the knee joint. The fourth muscle -- the rectus femoris -- can extend the knee and flex, or bend, the hip. In this exercise, you only train the ability of these muscles to extend the knee.