Update:
The leg extension, along with hack and sissy squats, are the best exercises to develop the rectus femoris, the V-shaped two-headed muscle on the upper middle portion of the front thigh. The single-joint action at the knee allows this muscle to actively lengthen and shorten, leading to more intense contractions impossible through compound exercises like the squat or leg press. Also, by adjusting the seat so it’s far behind you while leaning back, you can deeply stretch this muscle for a better distal growth stimulus by increasing passive tension.
The spread of scientific studies that condemn the leg extension machine has made it far less popular as an exercise. It still remains a staple in bodybuilding and with most of the general public that rely on machines to stay safe. Ironically, machines will harm them in the long run.
Some that like to think of themselves as better informed will argue their case by failing to address the issue at hand. They may commit a straw man fallacy. They attack the functional training philosophy rampant in fitness today, which advocates compound free weight exercises. This movement has a lot of flaws unrelated to the leg extension debate though, so it serves as an easy yet wrong target.
Many will also appeal to moderation. They state we should use many exercises, allowing leg extensions for the sake of variety. This is another logical fallacy.
The evidence is stacked firmly against leg extensions. They bring all the disadvantages of isolation and machines, and share issues with leg curls, but have their own unique problems as well.
Reasons
Success does not consist in ever making mistakes but in never making the same one a second time.
– George Bernard Shaw
- It can allow active insufficiency.
A good leg extension machine, as perfect a misnomer as any others, requires a dip in the seat where it rests against the hips. This flexes the hip to prevent overshortening of the rectus femoris muscle of the quadriceps.
This overshortening occurs because this muscle attaches to both the hip and the knee. If it shortens at both joints it allows active insufficiency. The contraction sites bunch up and allow fewer connections. This may also lead to cramping. The rectus femoris acts strongly only when the knee extends along with the hip, as occurs on a squat.
Even with a machine that compensates for this, you tend not to get enough hip flexion. This weakens the strengthening effect since it forms much less tension throughout the range of motion. Tension is the main stimulus for more size and strength.
- The long moment arm harms the knees.
The long distance between the weight placed at your ankles and the knees maximizes the shearing forces.
- It draws the patella back into the femur.
This maximizes shearing forces on the kneecaps.
- It places force on a locked knee.
During a compound movement, the lockout can give the muscles a short reprieve as the joints bare the load in a strong position. The leg extension loads the muscles here, which function in a shortened, weak state. The knee is not prepared to deal with force from this angle.
- It stretches the ACL.
During the leg extension, the tibia slides forward onto the femur to stretch the anterior cruciate ligament (ACL). This stretches the ACL, which weakens it, predisposing you to an injury.
When the hamstrings contract on a multi-joint hip and knee movement, such as on the squat, this fails to happen since the tibia slides backward onto the femur instead.
Orthopedic surgeons often say to eliminate leg extensions after ACL reconstruction. I would extend this advice further. Avoid the exercise, healthy ACL or not.
- The VMO likely does not work harder.
Some early evidence suggested that the vastus medialis muscle had a greater activation throughout the degrees near the end of a leg extension (20°- 30°). If you flex your quads near full extension of the knee, the feeling seems to confirm this theory. This would seem give a reason to use the exercise.
The anatomy fails to support this though. More recent evidence suggests that all four muscles of the quads stay about equally active throughout the range of motion with a heavy enough load. Even if the vastus medialis does contract preferentially near full extension, it contracts at a short and weaker length, since it bunches up.
When performing an exercise with any heavy weight, all of the muscle fibers will come into play based on motor unit recruitment. Only with light weights, which would have a negligible effect on muscle building, could you emphasize certain muscles. Even in the case of a slight activation difference, you may actually imbalance the knee, while still bringing the other consequences.
- It harms the quadriceps-to-hamstrings strength ratio.
A very uneven ratio may predispose you to injuries such as muscle tears. Many aim to balance the quads-to-hamstrings ratio with the leg extension. As an unnatural exercise though, this makes little sense as a solution. It instead imbalances the lower body through isolation.
- It harms knee stability.
During normal activities for the lower body, the quads contract with the hamstrings. This stabilizes the knee, since forces from the front and back work to hold it in place. The ligaments and tendons provide medial and lateral support though the muscles assist with this as well.
The leg extension prevents force from the back, destabilizing the knee.
- Knee extension acts as an unloaded function of the quads.
It is not natural to load the knee while the foot travels through space with open-chain movement.
Try to think of a normal free weight movement that would involve knee extension with resistance. You would have to attach something to your leg. If you performed this while standing, you would have to try to kick awkwardly while raising the hip.
Loading this motion would have served no role in the movements needed throughout our evolution. The body would have used the quads for knee extension at the same time as hip extension. This takes place when running, jumping, and so on.
Pure knee extension only occurs to alter the position of the leg.
Avoid Leg Extensions
Those that defend the leg extension justify it in many ways.
They may use it to add volume without the demand of a compound exercise to avoid burning out the central nervous syste,. Avoiding CNS fatigue has more to do with avoiding overtraining versus your exercise selection.
Those that admit the shortcomings of the leg extension may try to mitigate them by using more reps. They may strive for more lactic acid and go after the pump. Perhaps they use it as a warm-up to get the blood flowing throughout the thighs and knees. Keep in mind though that an injury can develop over time as opposed to just suddenly.
They also may try to avoid the lockout or reduce the range of motion. This still carries most of the disadvantages described.
Some advocate that they play a role in rehab. This suggests that an unnatural exercise can help us to function naturally again. Why not just perform good movements at a lesser level until the weak muscle catches up?
None of these reasons can lift the leg extension out of a deep hole. Science shows us that the safety of an exercise is not relative. A bad exercise, such as an overhead press, will harm everyone, just more or less so.
Perform hip extension with knee extension to strengthen the quads. Avoid overemphasizing knee extension through front squats and other pseudo-compound exercises though. Pick a lower body movement that balances knee extension and ankle plantarflexion with hip extension. This occurs during a squat with the right bar placement and good form. You should feel as if no muscle or joint takes over the exercise.
Ignore leg extensions and focus on squats.