How to Do the Leg Extension Exercise

Leg extensions work the quadriceps, but take care to avoid knee injury

The leg extension exercise is popular for the front thigh muscles: the quadriceps, including the rectus femoris and vastus muscles.

The equipment version of the leg extension uses a lever machine with a padded front bar that is pushed upward as the legs are extended.

There is some debate regarding leg extensions among fitness authorities. Critics say that because it traps the knee joint into one path this can lead to injury. Even so, the exercise used in knee/thigh rehabilitation, utilizing light weights and monitored repetitions. If you are concerned about leg extensions as an exercise, an alternative exercise you can perform is the lunge, such as the front lunge.

If you need more general guidance on weight training, please refer to the beginner's guide.

Leg Extensions: Setup and Movement

The starting position.
The starting position. Arthur Tilley/Getty Images
  1. Sit on the padded seat of the machine (see image).
  2. Hook the feet under the padded bar. Adjust the bar so that it rests comfortably at the lower end of the leg, above the feet.
  3. Adjust the machine so that your legs are at a 90-degree angle. Don't perform the exercise with less than a 90-degree angle, because this places the knees beyond the toes, which stresses the knee and can result in injury.

    Note: If the machine in your gym does not have leg length adjustment, don't use it; a poorly adjusted position can apply force in improper places in the legs and knees, which can lead to injury.
  4. Choose a weight that enables you to extend the legs straight with effort but not too much strain. Initially, select a light weight from the stack. Do not use this exercise to test your maximum leg strength, as this stresses the knee and can cause injury.
  5. Grip the bars firmly at each side. Hold your head steady. Brace, and breathe out on exertion and in on recovery.
  6. Extend your legs, pushing with your quadriceps to lift the weighted bar. Pause at the top of the extension.
  7. Slowly lower the weight back to the starting position.
  8. Repeat the exercise, doing 2 to 3 sets of 8 to 12 repetitions each.

Leg Extensions: Points to Note

A leg extension.
A leg extension. Oxford/Getty Images
  1. Make sure your back is pushed back firmly against the backrest, particularly at the base of the spine.
  2. Keep the head and neck still on the leg extension. Don't wobble forward, back or to the sides. Use the hand grips to keep your body stable.
  3. Don't use very heavy weights for this exercise. Keep the load light to moderate because the knees will be engaged during the extension.
  4. Adjust the leg bar length appropriately so that the padded bar is not too far up the shins. As mentioned, the bad should rest against the lower leg, just above the feet.
  5. Don't perform this exercise if your knees hurt during the process.