Are Leg Extensions Functional?

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What does it mean for an exercise to be functional?

Well, it’s actually a poorly defined term as different individuals will provide different definitions. However, it’s often used to discuss exercises that replicate or look similar to movements being performed on the court or field. There are many commonly prescribed exercises normally considered to be “functional” that don’t look anything like what’s done on the field or court, so looks can often be deceiving.

Let’s examine the hop tests that are commonly used to assess if someone is ready to safely return to sport after an ACL Reconstruction. These are what many people would consider functional tests.

A study by Wren et al in 2018 took patients who were on average 7 months post-ACL Reconstruction and had them perform a single leg hop test. They were said to have achieved symmetry if they were able to hop at least 90% as far on the surgical side as they were able to hop on the non-surgical side. They found that the patients who were able to achieve symmetry did not hop as far on either limb compared to healthy athletes. That means the athletes who were post ACL Reconstruction were only able to achieve symmetry by underperforming on their non-surgical side. Furthermore, Wren and colleagues reported that “symmetric patients shifted loading to the hip, increasing hip flexion angles, moments, and energy absorption.”

This means that the patients who achieved symmetry on the hop test ended up shifting the load away from their surgical knee and into that hip as a form of compensation. Other studies by Oberlander et al and Orishimo et al also showed an offloading of the surgical knee and increased loading at the hip and ankle in patients after ACLR. Offloading the affected knee might sound advantageous, but we’ll discuss why that’s not the case in a moment.

Even in a simpler task like a bodyweight squat, there are compensations that occur that we might not be able to see with the naked eye which was highlighted in a paper by Susan Sigward et al in 2018.

Of course we can all see and recognize the inter-limb compensation that occurs early after an ACL Reconstruction when an athlete will shift off of their surgical leg and onto the uninvolved side. However, between 3 to 5 months after surgery, that compensation changes from being inter-limb to intra-limb. This means that instead of a shift occurring from side to side, the shift occurs within the same leg and now, instead of the surgical knee being used and loaded normally, that load is taken up by the hip and ankle. And we can’t necessarily visualize that shift!

So why does the shift in load from the knee to hip and ankle matter if the hop and squat “look good”? Well, the compensatory strategy is a result of the quads not being strong enough to perform its primary function of active knee extension, or controlled knee flexion. This is a problem because strong quads are really important for both sport performance and reducing the risk of reinjury.

In fact, the Delaware-Oslo ACL cohort showed that quad strength is the most predictive factor for who will go on to have a 2nd ACL injury. The authors state: “of the individual components in the RTS test battery, quadriceps strength deficit prior to return to level I sport was a significant predictor of a knee reinjury, with a 3% reduced reinjury rate for every one percentage point increase in strength symmetry.” Getting strong quads after an ACL Reconstruction is vital.

If an athlete does not have strong enough quads to control knee flexion when they are landing, decelerating, and changing direction, then they end up utilizing more of a hip strategy to compensate. This means that they will land on more of an extended knee with increased hip flexion, just like the patients in the Wren et al study. The problem with this is that ACL injuries occur when the knee is in this more extended position.

In other words, we need the quads to get strong so that they can help to absorb and counteract the forces going through the knee during cutting, landing, and decelerating activities. If the quads are unable to do this job, the body will compensate by having the hips take over, and this is not an ideal movement strategy for performance, or for reinjury rates. Continued in comments...

Introduction - 0:00
What is Functional? - 0:24
Hop Tests and ACL - 0:52
Squats and ACL - 2:37
Importance Quad Strength - 3:33
Why Leg Extensions (Biomechanics) - 5:58
Summary - 7:53
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Disclaimer: The information presented is not intended as medical advice or to be a substitute for medical counseling but intended for entertainment purposes only. If you are experiencing pain, please seek the appropriate healthcare professional.

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