In part 2 I will dissect the squat pattern itself and explain how anthropometric, or body dimension, differences affect squat form and performance. Not everybody is built to squat the same way and trying to fit a person into a preconceived form can have poor results if their body is not built for that pattern and style.
For those who are have skipped part 1 you can read it Here.
Part 2: Key Points
- The goal of a squat is not to maintain a vertical torso, although a vertical torso can be achieved with special loading and movement strategies
- Variation in body proportions alter an individual’s optimal squat pattern
- Short femurs and long tibias are good squatting proportions, long femurs and short tibias are disadvantaged proportions
- Disadvantaged proportions can often be overcome with specific changes in form
It’s pertinent to acknowledge the general physics and kinematics of the squat to better understand how body proportions change the way load is handled by the body. To start we will consider how movements relate to the line of gravity. The line of gravity extends through the center of mass to the ground. Let’s approximate this line by using the center of pressure of the foot, just anterior to the malleoli. Also keep in mind that the center of mass is the point at which an objects entire mass is equally distributed around. For an unloaded human standing in anatomical position the center of mass is said to be just anterior to the body of the S2 vertabra. With increased load held on the shoulders the center of mass progressively moves towards that load.
So during a squat, external and internal “moments” will occur. A moment is the turning effect of a force about an axis. The magnitude of the moment depends on the magnitude of force and distance from the axis. Think of a wrench. The longer the handle and the more force you apply to the handle the greater the turning force exerted and the easier it is to turn the bolt. Same concept in the body. An external moment is the result of the force of gravity and the internal moment is the body’s reaction by using muscular force.
So as you descend into a squat the hip moves backwards and the knee travels forwards relative to the line of gravity. The line of gravity is the force in the moment equation and the center of each joint is the axis. Remember we are going to assume that the line of gravity stays around the standing center pressure of the foot, although this certainly changes a bit while squatting. So the further back the hip moves the more gravity will exert a flexion force about the hips. The further forward the knee moves forward the greater the flexion force exerted about the knee. As such, the further backward the hip the greater the demand on the hip extensors and the further forward the knee the greater the demand on the knee extensors as both are fighting against the overall external flexion moment. Pretty straightforward.
We also must realize that the forward/backward position of the hip and knee is intimately linked because both are connected via the femur. The same overall external flexion moment has to be combated regardless of the hip/knee position, the relative contributions from hip extensors and knee extensors are what change with variation in form. Another thing to note is, all other things the same, when the hip move further backward an increase in forward lean of the trunk must occur to maintain balance. This forward lean also increases the external lumbar flexion moment. It may seem like it would make for the primary goal to be to keep the torso as upright as possible, but this is not the case. Torso inclination comes as a consequence of load acceptance and patterning in the ankle, knee, and hip rather than the torso dictating the entire lower extremity’s kinematics.
In order to keep the spine vertical it is necessary to have a much more anterior knee position as this helps balance a load which is located more posteriorly with an upright spine. Try this yourself. Focus on keeping your spine as vertical as possible, feet straight ahead (for now), and squat. Most will notice tension builds in their calves as the necessary amount of anterior rotation of the tibia requires is restricted by limitation in the range of dorsiflexion. If you take dorsiflexion our of the picture and squat on your toes, or with something under your heels, it is much easier to achieve an upright spine as you can now more easily achieve the forward knee position.
There are really two negative things that can happen when a squatter’s goal is to keep the torso upright. If the ankle lacks the necessary dorsiflexion then pressure will go onto the toes (not ideal.) If the trunk compensates for preservation of even foot contact then the hip and knee will shift backward necessitating a forward trunk lean to quickly increase at the bottom of a squat. We do not want to be accelerating into peak anterior trunk lean in the bottom of a squat, because in the bottom of a squat we have to suddenly stop this motion and reverse back upwards. With increasing anterior trunk lean we increase the lumbar flexion external moment and increase the stabilization requirements of the lumbar spine. It is best to not be accelerating into forward trunk lean in the bottom of the position, but rather have this amount of lean already set and controlled once we reach the bottom so that we may stop and reverse back upwards more safely. So for these reasons, it is most appropriate to determine the stability and mobility of the ankle, knee, and hip before consideration of the vertical nature of the torso. The degree of torso incline is a byproduct not the primary goal.
Body proportion considerations in the squat:
As I’ve discussed, the further the hip and the knee are away from the line of gravity (approximated by the center of pressure through the foot) the greater the external moment on these joints and thus the more force is required from the corresponding hip extensors and knee extensors. Well what happens if BOTH the hip and the knee are far away from the line of gravity, which happens when an individual has a very long femur. Well this is a double whammy. Because not only does a long femur increase the required force from hip and knee extensors, but it also drastically increases the forward trunk lean and dorsiflexion required. Let me draw you a picture…
This illustrates that because the left stick figure’s femur is so long that it necessitates both the hip to be further backward and knee further forward. All other things the same, the more forward the knee, the more dorsiflexion is required. The further backward the hip the more forward inclination required. Furthermore, you will find many individuals have difficulty stabilizing their spine at greater degrees of hip flexion, so as the stick figure on the left (above) descends he is more likely to have resulting lumbar flexion. A long femur is not good for squatting.
A long tibia, on the other hand, is great for squatting. With a long tibia, for every degree of dorsiflexion the knee is allowed to move forward more anteriorly. Another way to phrase it, is that for any given knee position a person with a long tibia will require less dorsiflexion.
The notion of a long tibia has important and relevant implications especially when we consider the use of weightlifting shoes with elevated heels. What the elevation of the heel does, either with a special shoe or by placing the heel on a plate, is to effectively lengthen the tibia and to decrease decrease dorsiflexion requirements by placing the foot into a more plantar flexed starting position. As we discussed above this situation can allow for a more vertical torso.
Another reason that elevated-heel squatting allows for a more vertical torso is that it slightly shifts our center of mass forward. Here is another thing to try. Take whatever squat stance is comfortable and extend your arms straight out in front of you as far as you can. Now trying to keep the back vertical squat down. Notice how it feels. Now switch and do the squat with the arms either right at the side or just behind your pockets. You’ll notice it is likely more difficult to maintain the upright torso with the arms at the side. This is because when you have your arms in front of you, you have shifted your center of mass slightly forward. This helps to offset the posterior displacement in the center of mass as the hip flexes. When you have your arms at your side and you go to squat the center of mass again moves posteriorly with hip flexion but because you lack the counterbalance of the arms you must lean your torso forward to maintain your balance. The same counterbalance situation exists when you load a squat from the front as opposed to the back.
These conditions are PART of how an olympic weightlifter is able to achieve incredible depth while maintaining an upright back (see the photo below.) And to note, the torso position in these olympic weightlifters is achieved out of necessity mostly because it would not be possible, or safe, to catch a weight on the front of the shoulder, as done in a clean, without being upright–this is a topic for another blog so I’ll keep that point short.
One last proportional consideration of the squat is that of torso length. A short torso will require a greater forward lean to keep the center of mass balanced over the line of gravity down through the foot. This only implies consideration of the load placement during a squat, but this typically comes down to personal preference.
I will now bring to your attention that all of these kinematics have been described within the sagittal plane exclusively. However, we can incorporate the frontal plane into the squat which can effectively shorten a long femur’s sagittal plane length. If you are to picture one of my stick figure squatters above and take the femur section and envision externally rotating/ abduction the femur bringing the knee section towards you, you will notice that with the sagittal plane dimension of the femur is now shorter. This is how we can circumvent a disadvantaged squatting proportion. With a the femur now shorter in the sagittal plane a long femur will not require as forward of a knee position nor as backward of a hip position, which decreases not only the external flexion moments but also decreases the requirements of dorsiflexion and forward trunk lean, respectively.
So now you are wondering what are these measurements/ ratios of femur to tibia length that are ideal for squatting. I do not have those, and I do not believe the exist. The reason I have brought up the discussion is to point out that we should not be mashing our clients/ patients into what we think a good squat should look like if we haven’t considered that their dimensions may not fit this mold. But there are ways of determining if an individual’s body dimensions should guide us to changing their squat form or if we are dealing with a mobility or stability impairment we can treat instead.
How you ask? See Part 3