A leg extension, also known as a knee extension, is an isolation exercise focused on the quadriceps, which are made up of four specific muscles on the front of your thigh—the rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius. This exercise requires a leg extension machine that includes a seat and a padded bar that presses against your legs, promoting strength and flexion. (Variations may involve ankle weights or a resistance band.) Leg extension exercises are useful as a warm-up for other leg exercises like lunges, leg presses, and barbell squats. Leg extensions and leg curls, also called hamstring curls, both work muscle groups in your lower body, but they focus on completely different ligaments and muscles in your legs. While leg extensions activate the quadriceps muscles on the fronts of your legs, hamstring curls target the muscles on the backs of your legs. Each of the two exercises requires a different starting position and a different range of motion. When using a leg extension machine, you sit and extend your legs to push a padded bar away from your body. When using a leg curl machine, you lie flat on your stomach and use your legs to pull a padded bar in toward your body. For the leg extension, begin by using a weight that you can control for 2–3 sets of 10–12 repetitions. Choose a weight that allows you to maintain good technique throughout all sets and repetitions. 1. Sit on the leg extension machine with your knees in line with the axis of rotation of the machine. Place your hands on the handle bars. 2. Your sitting posture should be tall with a neutral head and neck position. Your chin should remain tucked throughout the movement, as if you were holding an egg under your chin. Keep your back against the back pad of the leg extension machine, engage your core, and pull your glutes toward the seat. 3. Place your lower legs in contact with the padded bar. The bar should rest just above your ankles. Your knees should be pointed forward and your ankles should be in line with your knees. Pull your toes slightly toward your knees. All repetitions should begin from this position. 4. Initiate the upward movement by squeezing your quadriceps and straightening your legs. 5. Lift your legs until your knees are straight without locking your knees. 6. At the top of the movement, squeeze your quadriceps for a second, then slowly return to the starting position. 7. Bend your knees until your ankles are underneath your knees. If your seat is adjusted correctly and you’re in an optimal position, the weights should not contact the weight stack during the downward movement If you have any pre-existing problems with your knee joints, consult a physical therapist or certified personal trainer before performing leg extensions to minimize your risk of injury. Proper exercise technique is essential to ensure the safety and effectiveness of an exercise program, but you may need to modify each exercise to attain optimal results based on your individual needs. Always select a weight that allows you to have full control of your body throughout the movement. When performing any exercise, pay close attention to your body, and stop immediately if you note pain or discomfort. In order to see continual progress and build body strength, incorporate proper warm-ups, rest, and nutrition into your exercise program. Your results will ultimately be based on your ability to adequately recover from your workouts. Rest for 24 to 48 hours before training the same muscle groups to allow sufficient recovery. Throw on some athleisure, fire up a [MasterClass Annual Membership](https://www.masterclass.com), and get ready to sweat it out with exclusive instructional videos from Nike Master Trainer and *GQ* fitness specialist Joe Holder. Want to improve your cardiovascular endurance? Give Joe’s HIIT workout a go. Trying to get a little swole? He’s got a strength training workout for that. From fitness tips to nutrition hacks, Joe will have you feeling healthier in no time. The leg extension is one of the most common leg workouts to promote muscle growth in your quads.