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It is a widespread proposition on multiple fitness websites that joints are better able to withstand compressive forces than shear forces.

For example: Precision Nutrition and Boot Camp Military Fitness.

But I can't find any evidence that this is true.

From first-hand experience I do believe it to be true. Take the leg extension, a commonly used isolation (single-joint) exercise that involves extension of a weight. All I've gotten from the leg extensions were joint pains.

For those who have exercised using these machines vs a squat/push-up, I think we can agree the latter is less harmful on the joints.

So where is the scientific evidence for this claim?

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  • (This is an unusual question, because it seems you already accept the claim.) It would be good to link to a sample of the "multiple fitness websites", so we can see the claims. At the moment, it isn't clear whether the claim is the joints are stronger or less likely to be injured or you are more likely to "see results" (?).
    – Oddthinking
    Mar 1, 2015 at 22:26
  • @Oddthinking A simple google search for the claim "joints...forces" turns it up google.com/….
    – Jossie
    Mar 1, 2015 at 22:31
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    Here is an example site, that makes the argument that shear forces are more likely to injure the knee than compression.
    – Oddthinking
    Mar 1, 2015 at 22:33
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    (Ah, we overlapped with comments.) Yes, a Google search turns up a lot of claims, but select some explicitly which make the claim you want investigated. You talk about "results" which is muddying the waters if the question is about injury.
    – Oddthinking
    Mar 1, 2015 at 22:34
  • I mean the actual claim is obviously true: two surfaces touching will move less if the force is compressive than when it is shearing - BUT: the concrete forces on "the knee joint" (already a massive oversimplification) during various exercises (including the forces imparted by the then load-bearing sinews that run alongside) where the angle of the bones changes (instead of an isometric exercise) - massively complex
    – bukwyrm
    Jun 30, 2023 at 13:37

1 Answer 1

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Seems both squats and leg extensions can be safe, if done right (including in the case of leg extensions the right equipment)

It is not as if some force z upwards is only resultant in an exercise-dependent split into compressive force y and shear force x, with x+y=z - so the question whether joints are 'better' able to withstand compressive forces vs shear forces falls short of the mark.

As i have no background in bodybuilding, here what i think you mean by the exercise of 'leg extentions': enter image description here here what i think you mean by 'squats' enter image description here and here what i think you mean by 'push ups' enter image description here I am not sure whether you included the push-ups in your Q to allude to similar exercises being available for non-knee regions, or whether you meant the load on the knee during push up (depending on how you model it, there is a clear shear component in that case) - I'll ignore the push-ups

There is a study (review of 150+ studies) regarding the loads during squats (though they look at squats with additional load on the shoulders), and whether deep squats or lesser squats are safer. They conclude deep squats are safer (if exercising with an expert) than shallower squats, and that (slow) deep squats are safe overall).

Calculated shear forces in the knee, even with absurd loadings (250kg of additional weight, didn't even know that was possible) remain manageable, though the authors note that calculated compressiive forces in the spinal column go beyond the theoretical limit, so the authors question the calculation...

Interestingly, the results for shear forces can be better predicted, because force pulling tendons apart is quite easily measureable in the lab, while the force (actually, pressure, which is even more confounding, because now even with a known force you need to know the area of contact precisely to know the pressure) necessary to compressively destroy cartilage (death of chondrocytes) is very non trivially found. The authors report values that are more than a factor of five from one another, the lower bound lying in the region that is surpassed in the hip while simply getting out of a chair...

The shear force in leg extension training can be made zero with the right equipment (pad needs to move to specific locations during exercise), according to this.

There even are papers that recount how the different muscles impact the tibiofemoral shear force (TFSF), which largely determines the tensile force on anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) - they conclude that with the right muscle activations during the exercise, leg extensions can be safe.

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