Knee Injuries & The Leg Extension

August 3, 2023
The best way to resolve muscle and joint pain, city of London, St.Pauls private personal training

Is the leg extension a good or bad choice if you have a knee injury or experience knee pain?

Why you should be using the leg extension machine.

The fitness industry must be one of the few industries in the world where if you follow the consensus, you may end up achieving the opposite of what you set out to do. The demonisation of strength training machines is a perfect example of this. And of course true to form, the most useful machines are the ones that receive the most criticism.

Top of the ‘machines you should never use’ list is always the leg extension machine. You will read things like: It’s not functional. It places a shearing force across the knee. It might injure your ACL. Never use it if you have knee issues.

Let’s address the issue with each of these statements in turn.

1. It’s not functional.

This argument goes something like this; because you are sat down and extending your knee against a resistance placed at the front of your lower leg, strength gained from this exercise will only be useful in precisely that scenario. This is nonsense. Have you ever used a strength training programme and noticed activities like running, walking, or digging the garden were easier to perform after a while? Did you use a specific digging the garden exercise in the gym? Probably not right? You likely just trained your muscular system and noticed that gardening became a little easier as a result. Strength gains transfer to the joint angles that are challenged. Simple as that. What the activity looks like when those joint angles are being challenged is irrelevant. Your nervous system is intelligent enough to incorporate strength gains as it sees fit.

2. It creates a shearing force at the knee.

This is true. But so do many other exercises like the squat and the split squat. Shear is not necessarily bad either. Like any force, it’s only an issue if it’s not controlled by your muscular system and the ligaments that keep the joint safe. Our knee is an extremely stable joint with four strong ligaments and powerful muscles that attach in and around it. If shear is a reason not to use the leg extension, then you should also avoid squatting, lunging, and doing things like kicking footballs. Basically, just stay in bed.

3. It might injure your ACL.

The idea the leg extension machine is dangerous for your anterior cruciate ligament (ACL) seems to have come from this 1985 research paper. The paper concludes it isn’t safe for ACL reconstruction patients to use the machine until 12 months post-surgery. Even that conclusion has since been poo-pooed, with more recent papers suggesting the leg extension machine is safe to use between 5 and 12 weeks post ACL surgery. Some studies even suggest full recovery of quadricep strength is not possible without isolated quadricep exercise following ACL surgery. Which brings us to the final point…

4. Never use a leg extension machine if you have knee problems.

This is like saying never take a headache pill if you have a headache. If you have knee issues the leg extension is probably the single most useful tool in the gym.
As the research above suggests, isolated quadricep work might be the only way to strengthen these muscles if you are experiencing knee problems. Your nervous system will simply programme movement around the problematic joint in any other scenario. Sure you’ll need to set the machine up correctly and be cautious with the resistance you select. We would argue you should apply the same caution with every other exercise you use.


Used appropriately the leg extension machine is safe. It’s unique because it can be used to regain lost quadricep function. This is of particular importance if you have either suffered from knee issues in the past, or would like to avoid them in the future. If you have a knee injury be sure to get a personalised assessment to determine the right exercise and rehab path for you, to determine the suitability and correct application of the exercise for you as an individual.

Paul Argent – Strength House Author