An exercise may be popular and even recommended by some trainers, but that doesn't guarantee it's a good movement. In some cases, you need to eliminate what appears to be a staple exercise from your workouts because it's not effective or may increase your risk of injury.
So, what's on the chopping block? The Leg Extension.
The exercise is simple. You sit in a leg extension machine and extend your knees to straighten your legs against resistance. You know the saying, "Feel the burn?" Well, your thighs will be on fire.
The Leg Extension is found in a variety of workouts. Some athletes use it as an assistance movement on lower-body days to complement their big lifts. Gym-goers often perform it to get bigger, more defined quads. It's also commonly used in rehab settings to regain quad strength after an injury.
So what's the big issue? The exercise certainly builds bigger and stronger and quads, but the juice may not be worth the squeeze for athletes.
Stress on the Knee
During Leg Extensions, the resistance is located at the shins, just above the feet. This causes shearing forces on the knees, which means the force is experienced horizontally across the joint. Your body does not like this.
In most serious injuries, there's some form of horizontal force—whether from a collision or momentum—that causes the injury. In the case of the Leg Extension, the ACL is the primary concern.
The ACL prevents your lower leg from sliding forward relative to your thigh. The Leg Extension directs its forces on the ACL. According to a study published in the Journal of Biomechanics, peak ACL tension was experienced during the Leg Extension when compared to the Squat and Leg Press.
The weight used may be light compared to an exercise like a Squat, but that doesn't necessarily matter. A study in The American Journal of Sports Medicine found that increasing the resistance during the Squat did not increase strain on the ACL, whereas increasing the resistance during the Leg Extension did add strain to the ligament. The force is applied far from your knee, so if you're using 100 pounds, your knee will experience a much greater weight. Also, your body is designed to handle the compressive force of a Squat, so lifting hundreds of pounds is not a problem.
A study published in the Journal of Bone and Joint Surgery confirmed this. The researchers found that movements like the Squat and Deadlift cause greater compression force and muscular activation, while movements like the Leg Extension produce greater shear force and minimum muscular activation.
If you read STACK, you know that for athletic performance, we emphasize building the backside of your body rather than the 'show-me' muscles on the front of your body. For example, even if you want to impress your friends with chiseled pecs, you need to focus your time working your back muscles.
The same can be said for your quads. There's a temptation to want strong quads, because they're the most visible indicator of leg strength. You rarely—if ever—hear someone say, "Hey, look at my jacked hamstrings!"
But, overdeveloped quads and the resulting imbalance with the hamstrings causes a slew of problems.
Your quads and hamstrings work together to protect your knees. The quads relieve stress on the PCL while the hamstrings relieve stress on the ACL. If your quads are too strong, your hamstrings won't be able to do their job, and your ACL will be at risk—which is particularly problematic for female athletes.
Also, the hamstrings are absolutely critical for sprinting and changing direction. They play a role in pulling your body forward, and they also absorb momentum when you decelerate. If you have overdeveloped quads and weak hamstrings, you'll be slow and more likely to get hurt.
It's best to use exercises that are functional—meaning they help you achieve your desired goal, which should be to become a better athlete. The answer here is simple, Squat more! A study published in The American Journal of Sports Medicine found that the Squat caused greater hamstring and quad co-contraction than the Leg Extension.
Is the Leg Extension the worst exercise ever created? Absolutely not. Many people regularly perform it with no negative consequences. We could argue that some folks have overdeveloped quads and need to lay off Leg Extensions, but that's a whole different story.
But for high-performance athletes, adding potential stress and compounding an already prevalent problem is neither ideal nor recommended. Stick with primary leg movements like Squats and Deadlifts to build strong legs. If you feel the need to specifically target your quads, try moves like Front Squats or Reverse Lunges. These are better options to achieve your goal.
Zheng, N., et. al. (1998). "An analytical model of the knee for estimation of internal forces during exercise." Journal of Biomechanics, 31(10), 963-967.
Beynnon, B., et al. (1997). "The Strain Behavior of the Anterior Cruciate Ligament During Squatting and Active Flexion-Extension: A Comparison of an Open and a Closed Kinetic Chain Exercise." The American Journal of Sports Medicine, 25(6), 823-829.
Lutz, G., et. al. (1993). "Comparison of tibiofemoral joint forces during open-kinetic-chain and closed-kinetic-chain exercises." Journal of Bone and Joint Surgery, 75(5), 732-739.
Boyd, M. L., et. al. (1996). "A Comparison of Tibiofemoral Joint Forces and Electromyographic Activity During Open and Closed Kinetic Chain Exercises." The American Journal of Sports Medicine, 24(4), 518-527.
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