In this post we take a look at the evidence to answer the question, is the leg extension machine bad for your knees?
The leg extension machine is probably one of the most controversial pieces of equipment in the gym. You will read plenty of articles by trainers and physios advising you not to use it.
Critics of the leg extension machine argue it’s bad for your knees because:
1) it loads the quadriceps in a non functional way.
2) it creates a shear force at the knee which is potentially damaging to the Anterior Cruciate Ligament (ACL).
Let’s look at each of these points in turn.
Got weak quads? The leg extension will improve their function
If you have weak quadriceps, or if you want to improve the strength of your quadriceps, the leg extension will do precisely that.
The question is, does that strength transfer to everyday activities and sports performance?
It’s critics contend that strength gained on this machine has no transfer to any task that doesn’t look exactly like a leg extension.
It’s an assertion with no scientific basis whatsoever. Even the most basic understanding of motor learning would indicate that your central nervous system (CNS) is able to incorporate strength as it sees fit.
It’s rather like saying strength gained in a seated bicep curl won’t transfer to the standing version. Or that a sit up has no transfer to any other position than lying down.
The same people will eulogise a squat however. An exercise which many people with weak quadriceps may find ineffective as it’s multi joint nature facilitates compensation.
The leg extension also does something the squat can’t, it challenges the last 30 degrees or so of knee extension.
If you’ve ever squatted you’ll know that once you’re out of the bottom position, the exercise gets progressively easier.
There is virtually no challenge to your quadriceps as you straighten your knees at the top of the movement. This is precisely where the leg extension continues to offer resistance however.
Strength transfers directly to the joint positions that are challenged. Why not ensure you’re challenging all of your available motion?
What about shear and the ACL?
It’s true that the leg extension machine introduces a shear force to the knee. So does the squat however. The only difference is the point in the range where that shear force is greatest.
In the squat it occurs at the bottom of the motion (knee flexed), in the leg extension it occurs at the top (knee extended).
For the record the exercise that has the potential to create the greatest shear force at the knee is the split squat. On the trailing leg. I don’t hear anybody saying we shouldn’t perform a split squat due to shear however.
Additionally shear is not necessarily a bad thing. In this case it’s actually produced by your quadriceps pulling on the tibia via the patella tendon and is therefore under your control.
The only time your ACL would be unduly stressed is if your knee flexors were unable to resist the force of your quadriceps. This is extremely unlikely.
Even if that were the case, the loads required to actually injure the ACL are very difficult to achieve with these machines. The failure point for the average ACL is approximately 1745N. Typical loads on the ACL during a leg extension exercise are around 300N.
How do most ACL injuries occur?
I’m not aware of a single person injuring their ACL on a leg extension machine. I do however know lots of people who have injured their ACL as a result of being unable to control force around their knee.
Typically most non contact ACL injuries occur when the foot is on the ground and the individual is attempting to decelerate their body from a jump or from running.
The knee is commonly in a small amount of flexion and there’s usually a certain degree of knee valgus (think knock kneed) and either internal or external rotation of the lower leg.
None of these potential scenarios are possible with remotely sensible leg extension use.
So how did we get to this situation?
The fitness industry seems to have taken specific research from ACL reconstruction rehabilitation and applied it to healthy populations.
The particular paper most often cited is that by Henning from 1985. This study was investigating when leg extension exercises should be used with ACL reconstruction patients. Not if. Their conclusion was 12 months after surgery.
This has since been questioned with researchers now suggesting it’s safe to use the leg extension machine for ACL reconstruction patients anywhere between 4 and 12 weeks post surgery.
In fact some studies now suggest full recovery of quadricep strength can’t be achieved unless they’re isolated with a leg extension machine.
It’s critical to understand the tools that you are working with to maximise your results. There are advantages and disadvantages to every exercise be it free weight, fixed weight or otherwise.
The leg extension provides a challenge to the quadriceps that is difficult to replicate in any other way.
Because of that the leg extension machine is actually good for the health of your knees when used appropriately.