When you perform a search on YouTube for exercises, you will often come across videos like „Don’t do THIS exercise“ or similar click-baiting and fear-mongering titles that feed on patients’ insecurities about exercising at the gym. One of the exercises that have received lots of criticism is the leg extension machine. In this blog article, we will explain why Leg Extensions are actually one of the best exercises you can do!
Excessive joint forces?
So why does the leg extension machine receive so much criticism? The main arguments that are made are that open-chain exercises like leg extension are not functional and that this exercise puts too much „unnatural“ stress on the patellofemoral joint or on the ACL from 30 to 0 degrees of knee flexion.
So how come the stress in low degrees of knee flexion is so high? As you can see in the image below, the external moment arm L increases with decreasing knee flexion angle in a seated leg extension machine with the mass applied at the ankle. This results in an increasing external flexion moment that the quadriceps muscle needs to oppose. The higher the quadriceps demands, the higher the compression forces of the patellofemoral joint.
In a seated leg extension machine with a cable pulley btw, the moment arm stays constant by the way as it is perpendicular to the lower leg.
But how high are these forces on the patellofemoral joint compared to other, „more functional“ closed-kinetic chain exercises like the squat? A study by Powers et al. (2014) has shown that patellofemoral stress is actually higher in the squat when compared to the seated leg extension in the range of 90-45° of knee flexion. So if we wanted to avoid high stress on the patellofemoral joint, we should have our patients perform leg extensions from 90 to 45° of knee flexion and mini squats.
Patellofemoral stress is actually higher in the squat when compared to the seated leg extension in the range of 90-45° of knee flexion
Now if we look at the maximal force on the joint throughout the whole range of motion, this study shows that the squat at 90° of knee flexion causes significantly higher joint stress than the seated leg extension at full extension.
The next question that arises, in this case, is: Are high compression forces on the joint necessarily a bad thing? Sure, when you have a patient with a sensitive patellofemoral joint, you will have to make sure not to overload the knee to further increase sensitivity. In a progressive exercise program, however, you want joint forces to gradually increase in order to drive adaption of the bone, cartilage, and nervous system to higher forces.
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Not functional?
The next argument that is regularly made is that the seated leg extension exercise, similar to other open-chain exercises is not functional. Functional in these cases is often defined as the degree to which an exercise replicates the movements that are performed in sports. Now a lot of sports like football, basketball, or tennis require the ability to decelerate forward momentum and to accelerate in the opposite direction, called cutting.
Forces in the leg extension machine are almost identical to cutting
A single-leg squat for example looks like this task from the outside but does not recreate the demands of the task. The reason being is that the ground reaction forces during cutting are almost perpendicular to the lower leg, while the ground reaction force in a single-leg squat is perpendicular to the ground.
If we look at the forces in the leg extension machine, however, we can see that the forces are almost identical to cutting. For this reason, leg extensions are a great way to prepare athletes for cutting movements, especially in early rehab when the task of cutting itself is still too demanding for the athlete.
Quadriceps Isolation
The third reason why leg extensions are a great exercise is that you can minimize compensation away from the quadriceps. In close-kinetic chain exercises like squats, patients with knee pain often develop strategies to unload their injured knee and shift load to their hips or onto the other leg. While strong hips are great, a weak quadriceps muscle is a risk factor for re-injury after ACL reconstruction. In the leg extension machine – when performed as a single leg variant – patients are forced to put load through their quadriceps. For the same reason, Jill Cook argues that you cannot rehab patellar tendinopathy if your practice does not have a seated leg extension machine – and who is to argue with the queen of tendons?
Alright, we really hope you enjoyed another one of these myth-busting blogs. As always, thanks a lot for reading!
Kai
