Are Leg Extensions Bad for the Knees?

You may use the leg extension machine to achieve definition in your upper thigh. After all, this exercise is one of the few options when it comes to isolating your quads.

The machine is likely familiar: You sit in the padded seat and hook the tops of your feet under a movable lever. Then you extend your knees against resistance and bend them back down to perform one repetition.

Although the leg extension does provide you the ability to isolate your quads, it isn't without risk, especially to your knees. If you do choose to use it, make sure you have proper form and only add the amount of weight that's appropriate for your fitness level to keep your knees safe.

The leg extension applies constant tension on the anterior cruciate ligament (ACL), so people with ligament injuries should avoid this exercise. The leg extension also increases the risk of lateral patellar deviation, meaning the knee cap can slide right or left unnaturally.

It also fails to engage the hamstring in supporting the knee, while putting a lot of force on the joint. Because of this, when you perform a leg extension with heavy weight, you drastically increase the chance of sustaining a knee injury.

Plus, the functional training benefits realized in a leg extension do not carry over into real life. Unless you want to build up your quads for aesthetic reasons or improve kicking movements, the exercise is not very practical.

So if there's risk, why would someone want to use this machine? There are a few reasons. The leg-extension exercise targets the quadriceps, and it's a relatively simple and intuitive exercise — there's no nuance in bending and extending your knee joint.

People who want to isolate their quads without activating other muscles of the legs or glutes benefit from the leg extension. Bodybuilders and people with a hamstring injury appreciate this isolation. Building your quads with the leg-extension exercise also helps improve the force of kicking movements.

Let's say you do want to include this exercise into your fitness routine, even knowing the risks. First, talk with a personal trainer, physical therapist or doctor, especially if you have knee pain or have ever suffered a knee injury.

If you feel a sharp pain at any time during this exercise, stop immediately. Always use a light weight when performing this exercise to reduce the chance of injury to your knee.

Make sure you use proper form, which involves sitting back against the seat pad and using control to extend your legs — rather than momentum to kick up the lever. Extend your legs, but don't lock them out, which can strain the knee joint.

When you do choose to use the leg extension machine, incorporate hamstring-strengthening exercises such as hamstring curls into your training program as well. When the quadriceps are stronger than the hamstrings, particularly in women, your knee loses stability and is at a greater risk of injury.