Leg Extension & Shearing at the Knee

Open- and Closed-Chain Exercise

Leg extensions fall into the open-chain exercise category, meaning your feet move freely while the rest of your body remains stable. In contrast, closed chain exercises, such as the squat, keep a fixed foot position as the rest of your body moves. Open-chain exercises isolate muscles, while closed-chain exercises encourage cooperation between teams of muscle groups. In real life, teamwork encourages stability. Anatomy imitates life. Closed-chain leg exercises push the head of your thigh bone together with the head of your shinbone. These compression forces enhance knee stability.

Shear Terror

While the closed-chain exercises promote teamwork, open-chain exercises wage war between your knee bone and your shinbone. Exercises such as the leg extension shift your shinbone forward and your thigh bone backward, explains athletic coach Brian Mackenzie. The resulting shearing forces destabilize your knees, making them more susceptible to injury. Some physical therapists might use open-chain exercises during the earliest stages of rehab, but they generally prefer closed-chain alternatives, reports the "Harvard Health Newsletter."

The Quad Dominatrix

As if shearing forces were not scary enough, the leg extension contributes to the muscular imbalances between your hamstrings and quads. This double whammy leaves your anterior cruciate ligament, or ACL, vulnerable to injury. Many women initiate movements from their knees, as opposed to their hips. This is called “quad dominance," because knee movement makes your quads the first muscles to fire. Falling is bad news for the quad dominatrix. If your hamstrings and glutes lack the strength to absorb the impact, your knee ligaments remain unprotected.

It's Not Functional

Sitting across the dinner table from your husband and extending your legs to play "footsies" simulates the movements of the leg extension machine. Unless you wanted to kick him in the shins, this movement doesn't require much strength. Playing footsies is one of the only real-life examples of seated leg extensions. Professional body-builder Charles Poliquin once joked that the only reason gyms keep the leg extension machine around is because even "motor morons" can perform them. Harsh words, but he's got a point. Although they require a certain amount of skill, compound exercises such as squats and lunges use multiple muscle groups, have a superior transfer of training to other activities, and do a better job than the leg extension at protecting your knee joints.