Leg extensions…. good burn, right? Crank up that weight, kick it out, easy to do with one leg or two. You get a chance to admire the separation on your thighs. Veins budging from that awesome pump…. and you knees getting rekt.
Let’s get this out of the way. If you’re doing light leg extensions as a warm up to get ready for squats or dead lifts or some other compound exercise, you are on the right track and will probably never have a problem. And by light I mean a weight you can do for 15 reps without much effort. Just enough to get the blood flowing and the synovial fluid moving around. If that’s what you’re doing, have at it!
But if you’re one of those people who likes to see how many 45s you can load on each side and still crank out reps, read on….
There is ample evidence showing leg extension, like leg curls, can cause some unfortunate problems. So let’s take a look, mmmmkay?
Damage to your Knees
During normal leg motions, the quads don’t act in isolation, they contract with and are balanced by the hamstrings. This bilateral tension stabilizes the knee, since force from both the the front (quads) and back (hamstrings) work together. This tension provides both medial and lateral support. The leg extension doesn’t engage the hamstrings, resulting in tension only from the front, which places an unnatural load on supporting tendons and can, over time, result in increased joint instability.
Leg extensions place the load at the shins, usually just above the feet. The location of resistance here, working against the fulcrum of a stationary knee, causes shearing forces on the knee. A sheering force is a force applied horizontally across the joint. While your body is built to handle compression stress (such as squats), it doesn’t do very well when it comes to shearing forces. Most serious injuries result from some form of horizontal force.
The leg extension is particularly hard on the anterior cruciate ligament (ACL) because of the above mentioned shearing force. The purpose of the ACL is to prevent your lower leg from sliding forward relative to your thigh. According to a study published in the Journal of Biomechanics, peak ACL tension was experienced during leg extension when compared to the squats and leg press.
A study in The American Journal of Sports Medicine found that increasing resistance while doing leg extensions increases strain on the ACL. Since the resistance is located away from the knee, the lower leg acts as a lever, resulting in your knee experiencing greater strain.
Another study, this on by the Journal of Bone and Joint Surgery confirmed the squat and dead lift cause greater compression force and muscular activation, while the leg extension produces greater shear force and lower muscular activation.
The quadriceps and hamstrings work together to stabilize the knee. The quads reduce stress on the PCL while the hamstrings reduce stress on the ACL. Overdeveloped quads can result in an imbalance in strength with the hamstrings and can cause problems. If your quads are too strong compared to the hamstrings, the hamstrings will be at a disadvantage and the ACL will be at risk.
When most people think of legs, they think of the front of the leg, the quadriceps. But it’s the hamstrings that are critical when it comes to sprinting and quickly changing direction. The hamstrings play a critical role in pulling the body forward, as well as absorbing momentum during deceleration. If the quads are overdeveloped compared to the hamstrings, there’s a greater risk of injury.
- Increase patellofemoral joint reaction force, knee movement, and joint stress in the most commonly used range of motion
- Reduce hamstrings activation and can result in muscle imbalance
- Don’t engage hip adductors and abductors
- Shearing force on the ACL
- Cause higher and potentially damaging patellar ligament, quadriceps tendon, and patellofemoral and tibiofemoral forces
- Increase lateral patellar deviation
The leg extension, like many isolation exercises, is not a natural movement. Doing them puts unnatural stress on the knee and can result in muscle imbalance that increases the risk of injury. Use them to warm up, but stay away from using them as a muscle building exercise.
If you are looking to pack mass on your legs, consider squats, front squats, lunges, and leg press instead.