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There are many biomechanical analyzes of the squat on the web, but there are lots of things are not clear for me.

Let's consider for instance a high-bar back squat. An analysis is provided by this website:

enter image description here

It says that:

In this squat you can see the moment arms around the hip and knee – they measure from the joint axis to the line of force. In this bar position the moment arm around the hip is slightly longer than that around the knee. This means the hip extensors (gluteal muscles) will be doing slightly more work than the knee extensors (quadriceps) in terms of the force they’ll need to generate to overcome the load.

and

The trunk, knee and hip stabilisers will be worked significantly, especially as more load is added.

My questions are:

  1. To be properly balanced (and not to fall backwards or forwards), shouldn't the two torques (referred to the midfoot) be equal and opposite? Why isn't this fact well highlighted by all the biomechanical analyzes (I've seen) on the web? Further examples 1,2.

I've sketched the depicted scenario in this way:

enter image description here

Since the two moment arms are not necessarily equal (it does depend on the back inclination), they can be equal and opposite only if the weight acting on the knee joint is different from that on the hip joint. How can we ensure we are balanced?

  1. Is it important (if question 1 is correct) only that the two torques are equal and opposite, or also that they are not too high? These analyzes stress the fact that the knee torque may be greater (bad) or lower (good) accordingly to the squat variation we choose (front, low bar, high bar). But it seems a non-sense to me, as we want to gradually increase the load as much as possible whilst mantaining proper form. And since the torque is: Weight x Moment arm, it will increase whatever variation we choose.

  2. Which are the "Knees stabilizers" the website refers to and which is their role?

External Forces acting in squat at the bottom:

enter image description here

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To be properly balanced (and not to fall backwards or forwards), shouldn't the two torques (referred to the midfoot) be equal and opposite?

No. In order for a body to be balanced, the sum of all external torques, including both pure torques and torques resulting from coupled forces, must be zero.

In the case of a human squatting, there are no external pure torques applied to the body, and there any only three forces acting, those being gravity acting on the lifter's body, gravity acting on the bar, and the ground reaction force of the floor pushing up on the lifter's feet. These three forces can be couples into two moments - the moment between the lifter's weight force and a part of the ground reaction force, with a moment arm equal to the horizontal distance from the lifter's centre of mass to their midfoot, and a moment between the barbell's weight force and the other part of the ground reaction force, with a moment arm equal to the horizontal distance from the bar to the lifter's midfoot. These two must cancel each other out in order for the lifter to remain in balance, which is normally achieved by the lifter's centre of mass being behind the midfoot, and the barbell being slightly in front of the midfoot.

Both the hip and knee torques are internal torques, and so they already balance themselves out when considering the body as a whole, and do not affect whether the whole body will rotate. The reason that they balance themselves is that the hip torque actually consists of (when viewed as in the above diagram) an anticlockwise torque applied to the torso by the thighs, and an equal clockwise torque applied to the thighs by the torso, while the knee torque consists of a clockwise torque applies to the thighs by the shins, and an anticlockwise torque applied to the shins by the thighs.

Is it important [...] that the two torques [...] are not too high? These analyzes stress the fact that the knee torque may be greater (bad) or lower (good) accordingly to the squat variation we choose (front, low bar, high bar). But it seems a non-sense to me, as we want to gradually increase the load as much as possible whilst mantaining proper form.

Yes, your intuition is correct here. It is a common mistake in biomechanical analysis of lifts to try to minimise knee torque, for fear of causing knee injury. This is where the myth of squatting with your knees moving beyond your toes being harmful came from. But creating stress on the knee is actually the entire purpose of leg strengthening exercises, and is the mechanism through which growth of the quadriceps is stimulated.

The consequence of attempting a lift where knee torque demands are greater than what your body can provide is not injury, it is merely that the lift will be failed.

Which are the "Knees stabilizers" the website refers to and which is their role?

As a rule of thumb, I would suggest that any time anyone talks about "stabiliser muscles", or worse still, the ubiquitously used "small stabilising muscles", without being able to actually name those muscles, that person probably doesn't know what they're talking about, and should be ignored.

There are two definitions of "stabiliser muscles": They are either muscles which contract isometrically to maintain posture or fixate a joint (such as the front deltoid during a bicep curl, or the trunk musculature during a squat), or the muscles responsible for keeping the load in balance in a free-weight lift.

I'll use the latter definition, because including any isometrically active muscle as a stabiliser means that the back is a stabiliser for the deadlift, or that the rectus abdominis is a stabiliser for the plank, rather than them being the primary targets of those exercises.

In the squat, the only challenge to balance is the weight tipping too far forward or back, and it is actually the prime movers of the exercise (the calf, quadricep, and gluteals) that are responsible for stability.

Some exercises do have distinct stabiliser muscles though, for example the bench press primarily works the pectoralis and triceps muscles, but uses the latissimus dorsi and deltoid muscles for stability, where the lats activate to bring the bar back if it shifts up over the face, or the deltoids will activate if the bar shifts down too low on the chest.

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  • Thank you very much. A last couple of questions.
    – Kinka-Byo
    Aug 1 at 8:26
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    1) Neither torque is due to gravity. Gravity can be thought of as acting on the trunk, and only creates a downward force, not a moment. The pair of torques occur because, at the hip, the glutes pull on the pelvis in one direction, and pull on the femur in the other direction. So if the trunk (including pelvis) is held rigid, the result of this is an anticlockwise torque applied to the torso, and an equal clockwise torque applied to the thighs. Aug 1 at 10:54
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    2) In a bodyweight squat, the person's centre of mass must stay above their mid-foot. This means that the downward force of gravity is inline with the upwards ground reaction force, so no moment is created between them. Aug 1 at 10:55
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    For your final question, an external torque would be like someone grabbing your hips and twisting them with their hands. Gravity is an external force, but it acts downwards on your centre of mass, and while the hips and knee torques occur as a result of it, they aren't directly caused by it. The hip and knee torques are just the body's internal attempts to prevent the whole body from collapsing under gravity. Aug 1 at 11:30
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    Yes, that additional image is exactly right. Strictly speaking, both bodyweight and ground reaction forces are distributed forces, in that bodyweight force is spread over the whole body, and ground reaction force is spread over the whole sole of the foot (maybe more so in the heel and ball than the arch). However for this type of analysis these forces are usually simplified to a single force acting at the centre of the distributed force, i.e. through the centre of mass for bodyweight, or though the mid-foot for ground reaction. Aug 2 at 1:32

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