Photo by Meghan Holmes on Unsplash
Lock your knees out. Especially when you lift!
But have you heard the advice that you shouldn’t lock your knees out because it’s bad for you? Ever wonder why that is?
This myth has origins in stories of soldiers standing at attention for hours on end only to end up fainting as they had their knees locked out. It was postulated that this was due to compression of the popliteal artery (a branch from the important femoral artery).
Secondarily, this also centers around old beliefs about arthritis and how excess wear and tear might cause a breakdown. Instead, researchers now are finding it may not be a disease of breakdown but rather uncontrolled local inflammation in the knee joint. (1)
Screw-Home Mechanism
The screw home mechanism is an integral part of ensuring good knee stability. For most healthy individuals this is something you don’t think much about. However, for anyone with prior operations or lingering anterior knee issues, this is an important concept to understand around general knee function.
The medial condyle is 1/2 in. longer than the lateral condyle. This allows the femur to screw home when fully extended and this stabilizes the knee joint. This is an essential aspect of being able to walk and support yourself during the swing phase (Standing on one leg while the other one swings through).
The movement of internal rotation of the femur on the tibia associated with the later stages of extension of the knee constitutes a locking mechanism, which is an asset when the knee is subjected to strain.
In general, it works like this:
Closed chain (like squatting) the femur moves on a stable tibia (shin bone). (2)
The Femur rotates externally to unlock the tibia.
The femur rotates internally to lock on the tibia.
Open chain (like a leg extension) where the tibia moves on a fixed femur. (2)
Tibia internally rotates to unlock
Tibia externally rotates to lock
I Don’t Have Knee Problems…Why is This Important?
Most knee problems occur at the terminal extension with a lack of VMO function. Ah, for some, I’m sure you recognize the VMO, this is why so many coaches prescribe terminal knee extensions. Why isolate one motion when you can easily train this whenever you hit leg day!?
As you flex your knee, the PCL relaxes and the ACL is tightened. The reverse is true, as you extend your knee the ACL tightens and contributes to stability while the PCL relaxes(3). Seems important to ensure these structures are exposed to stressors so they can improve stabilization (albeit minimally).
If you lack terminal knee extension, you will likely rely on a few strategies.
Think of an elderly person who shuffles their feet. They can’t lock their knee out, therefore, decreasing the amount of time they stand on one leg during the gait cycle. So they shorten that time by shuffling their feet instead.
You can’t lock your knee out, you either walk with more flexion and increase stress on other structures or you walk with some limp. A limp also increases the reliance on other structures, namely your hips to help keep this show running.
This is why you may not need to squat “ass to grass” but you do need to expose your knee to deep flexion to end-range extension. Pain-free deep flexion tells our brains via our receptors in the deep knee capsule stuff to maintain these structures. This in turn supports our ability to access such positions.
The easy take-home is to do leg presses or lunges maybe. It doesn’t have to be heavy, it just needs to be deep.