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let's consider this table which shows agonist - antagonist muscles pairs in our body.

enter image description here

Let's focus on a specific exercise, for instance squat. When you are rising up from the floor, your legs are gradually extending.

I'd say (but let's correct me if I am wrong), that this leg extension, by simplifying, happens thanks to:

  1. Quadriceps contraction

  2. Hamstrings extension

So, what is the role of glutes in that movement?

In squats, when you raise up from the floor, you squeeze your glutes to do that. But why should we do that, if the leg extension movement is due to hamstrings and quads? Traduzione vocale

in fact, I was thinking,it is perfectly possible to contract the glutes even with the legs flexed (for example by kneeling on the floor). So, what is the role of glutes?

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We need to clarify some terms. "Extension" means movement of a joint which increases the angle between two body parts. So elbow extension is the movement that increases the angle between the humerus and radius/ulna, i.e. straightening the elbow. The opposite of extension is "flexion", which is a movement of a joint which decreases the angle between two body parts.

For muscles, we use the term "contraction" to describe when the muscle is producing force, and "relaxation" for when it is not producing force. Movement of the muscles is also described as "lengthening" and "shortening".

So, if you mean to ask what's happening with your glutes when you extend your knees, then the answer is probably nothing, since the glutes are hip extensors, not knee extensors, and knee extension only requires the quadriceps to contract and shorten (while the hamstrings relax and lengthen).

However if you want to know what happens in a squat, then the glutes are the primary muscle responsible for extending the hips, the movement where your thighs move so that they're in-line with your torso. The upward portion of a squat involves extension of the knees and hips together, and the glutes are responsible for the hip movement but not the knee movement.

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  • Is this the reason why, for instance, leg extension is an exercise for quads, instead of glutes? While squat is both knees and hips extension, leg extension is only the first one and so doesn't target glutes primarily. – Kinka-Byo Feb 11 at 9:20
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    @Kinka-Byo yes, that is exactly correct. – David Scarlett Feb 11 at 11:05
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An efficient squatter mainly uses the glutes to extend the hips, not the hamstrings.

As can be seen from this image the hamstrings crosses both the hip and knee joint: enter image description here

They therefore have two functions: extending the hips and flexing the knee. If you use the hamstrings to extend the hips in the squat you also get a flexing torque at the knee. This means that the quads have to work even harder.(1)

As can be seen from this image the gluteus maximus crosses the hip joint and also extends the hip (as well as externally rotates the femur): enter image description here

In the squat we can vary the ankle, knee and back angle. In order to stay balanced the bar must stay over midfoot. So we have 1 equation and 3 variables, which mean 2 degrees of freedom. So we have a choice to make; do we want to minimize or maximize the moment around the knee (or somewhere in between). The first leads to a hip dominant squat:

enter image description here

The second leads to a knee dominant squat:

enter image description here

The moment around the knee is proportional to the horizontal distance from the bar to the knee and the moment around the hip is proportional to the horizontal distance from the hip to the bar.

In the knee dominant squat the quads are clearly the limiting factor and we want to avoid using the hamstrings.

In the hip dominant squat the quads are less of a limiting factor and the hamstrings can be used to a larger extent.

I think it may be a problem for some people that their glutes are very weak, in which case they will tend to also use the hamstrings even when this is inefficient.

If this is the case one should focus on taking a wide stance and hitting full depth (2). The wide stance causes an inward rotational force on the femur from the bar which the gluteus maximus have to counter. In the bottom position of the squat the knee is flexed which results in less tension in the hamstrings. It is therefore difficult for the hamstrings to contribute to hip extension and the glutes will contribute more.

(1) Hamstrings–The Most Overrated Muscle Group for the Squat

(2) Squats Don’t Work Your Glutes (Because You’re Squatting Wrong)

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