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Are Leg Extensions Bad for the Knees?

author image Rick Rockwell
Rick Rockwell is a self-employed personal trainer and experienced freelance writer. His articles have been published throughout the Internet. He has more than eight years of experience as a certified personal trainer, group fitness instructor and lifestyle coach. His company, Rockwell Fitness, is dedicated to educating and empowering others to live healthy lifestyles.
Are Leg Extensions Bad for the Knees?
Are Leg Extensions Bad for the Knees? Photo Credit: Ozimician/iStock/GettyImages

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

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The machine is likely familiar. You sit in the padded seat and hook the tops of your feet under the movable lever. Extend your knees against resistance and then 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. Use proper form and only the amount of load that's appropriate for your fitness level to keep your knees safe.

Benefits of the Leg Extension Machine

The leg-extension exercise targets the quadriceps and is a good machine on which to start training the muscles at the front of the thigh. The exercise is simple and intuitive -- there's no nuance in 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.

Drawbacks of the Leg Extension

The leg extension applies constant tension on the anterior cruciate ligament, 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 shearing 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.

Proceed With Caution

Do not add leg-extension exercises to your fitness routine without first speaking with your doctor, physical therapist or personal trainer, 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.

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