The leg extension machine is one of the most recognizable in any gym. Even trainees who rank as beginners know what the exercise is and how to perform it. The popularity of this machine dates back many decades.
Many gym veterans can recall how doctors and other healthcare providers maligned squats and recommended leg extensions. Squats were supposed to be "bad" for the knee and leg extensions were supposed to be "good" for the knee. This led to leg extension machines being used in rehabilitation for knee injuries. Special types of isokinetic leg extension/leg curl machines were created that only allowed "positive" leg extensions and "positive" leg curls. Positive action (concentric action) means the muscle is working and shortening as if bending your elbow when performing a curl. "Negatives" cannot be performed on this equipment. Negatives are muscle action while the muscle is lengthening as in lowering your arm during a curl. However, this is not the problem. The problem with the leg extension is twofold:
- The leg extension places maximum force on the back of the patella ("knee cap"), the thinnest portion of the cartilage. This can lead to cartilage damage. When you squat, the thickest part of the cartilage is loaded during the squat's maximum force. Biomechanists call the squat "physiologic flexion" because it is a natural movement. Many of the doctors who decided squats were bad and leg extensions were good allowed their own personal bias to get in the way of clinical judgments, and they did not communicate with other fields such as biomechanics to learn more about these motions.
- The leg extension causes the lower leg bone (tibia) to move forward on the thigh bone (femur). This stretches the anterior cruciate ligament (ACL). The squat and leg press cause the tibia to move backwards on the femur. This is due to the co-contraction of the hamstrings along with the quadriceps. The hamstring muscles pull the tibia backward on the femur, which reduces the stretch on the ACL. This is a normal occurrence. When the isokinetic machines were used in rehab, and health care providers became more aware of this problem, "anti-shear" devices were placed on these machines to protect the ACL. Who knows how many ACL reconstructive surgeries were complicated by this equipment and exercise.
The leg extension became popular in the gyms. Some trainees performed very heavy leg extensions and fitness magazines were filled with champions performing max-effort leg extensions. However, once the patellar cartilage is worn down enough, all leg training becomes difficult. The squat was traditionally given up due to knee pain or back pain. And while power lifters and Olympic weightlifters had longer squat careers, this may be due, in part, to the fact that they don't perform leg extensions.
So, does the leg extension belong in your workout? The answer is, "not really," with an exception. If you perform heavy leg extension, especially if the machine allows you to have even more range of motion by positioning your feet under the seat, you'll place a great deal of stress on your patella. This could accelerate arthritic changes in your knee. However, one set of ultra-light leg extensions can actually serve as a local warm-up for the knee before squats and/or leg presses, and is not enough force to compress the patellar cartilage to any significance.
Many trainees with knee pain believe that if they add the leg extension, they will feel better. Your knee may feel better while it's warmed up, but may also ache more later in the day. If the load repeats each week and month, the aches and pains may progress. The very exercise you chose to help your knee may contribute to its ongoing pain.
So, keep the leg extension to a minimum amount of weight and one or two sets at most. This may help your knee warm up before a heavy leg workout. Train smart, then train hard.
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