Is the leg extension machine bad for your knees?

By January 12, 2018Equipment, Rehabilitation, Training

The leg extension machine is one of the most useful pieces of equipment in any gym. Unfortunately plenty of people still ask the question, is the leg extension bad for your knees?

In this post we take a look at the evidence to give you a definitive answer.

Is the leg extension machine bad for your knees?

Critics of the leg extension argue it’s bad for your knees because:

1) it loads the quadriceps in a non functional way.

2) it creates a sheer force at the knee which is potentially damaging to the ACL ligament.

Let’s deal with 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 standing. 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 gets difficult.

Strength transfers directly to the joint positions that are challenged. Why not ensure you are 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. The only thing 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 the 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.

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. Something an exercise like the leg extension and indeed the seated leg curl would assist with.

There may be times when a leg extension exercise would not be appropriate. Such as in the early stages of an ACL rehab programme. But those decisions should be made with facts not on hearsay and ‘gym science.’


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.

Get our guide explaining exactly how to use lower body resistance training machines to recover from injury and improve your performance.