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Gym Exercise Physics Quesion
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bostonman:
This may sound silly, but I have a physics force/weight question regarding shrugs.

If anyone is unfamiliar with shrugs, you basically hold weight in your hands with your arms hanging down (parallel to your body), not bending elbows, and lift your shoulders to lift the weight.

Say I hold 25kg in each hand with my arms and "shrug" my shoulders (or even hold the weight and don't move), each shoulder would see 245N (25kg * 9.8m/s^2).

Now my question is: what if I bend and hold my elbow slightly - not bending each time I "shrug" - keeping it bent slightly the entire time I'm doing the exercise? Someone told me it takes weight off your shoulders and transfers it to your forearms, bicepts, tricepts, etc..

My argument is: the 25kg is still hanging from your hand, and (due to pulling the shoulders back slightly) the weight is still in line with the shoulder, then some force would be exerted in each associated muscle holding the weight at a slight angle, but, overall, the shoulder is still supporting all the weight.

T3sl4co1l:
The point is probably more that you aren't going to actually keep your arms rigid, you'll think about moving the weights so you'll do a more natural arm movement as well.  We're rather bad at perceiving things like this, so it's easier to just keep them straight so you know you aren't cheating.

Alternately, do the whole arm retraction and extension plus the shrug, to get different parts of the shoulder working?  Might want a smaller weight for that, give or take experience.  Though I suppose that's about the same as stiff arm swings, which might be more efficient or something.

Tim
Domagoj T:
Isn't the point of it to use muscles to carry the weight, instead of joints?
M0HZH:
Biomechanics is not as simple. Muscles have multiple types of fibers that behave differently, the muscle insertions are not singular points, muscle fibers achieve different tension forces at different lengths, there is even a neural impact on how much strength you can achieve. With every change in angle or travel, the muscles will work a bit differently and therefore the exercise will be a bit different.

In bodybuilding there is a quest for the best "isolating" exercises, which are the exercises that will work ideally a single muscle group with minimal effort from others. While these are great for the purpose, sometimes they put additional strain on ligaments or joints. If you're just looking to stay fit, I wouldn't bother with these and just go for the safer form.
bostonman:
Well, this is more about static forces and no so much about joint damage or muscle fiber.

I'm looking at it as hanging a weight on a rope. The anchor on the ceiling is going to see the whole 25kg, and the entire rope will see the 25kg.

Now put a kink in the rope (let's say you feed the rope through a 45 degree PVC elbow). The elbow will cause a slight bend, and the weight will move so it dangles vertically underneath the anchor (due to gravity), but the anchor will still have 25kg to support (ignoring the weight of the PVC elbow).

Now I can (hopefully) calculate the force the 45 degree elbow has, but the same weight is still seen by the anchor. Same goes if I put a very long 45 degree elbow. Say my rope is 3' long, and the elbow is 1' on each side (so it occupies 2' of rope). The weight will move so it's vertically underneath the anchor, it will have a very large 45 degree bend in it, and, ignoring the weight of the PVC elbow, the anchor will still see 25kg.

Am I correct?

I believe (without digging into this), if the rope was a straight piece of steel, it would see the 25kg (ignoring the weight of the steal), however, if I used a 45 degree piece of steel, the anchor would see a much greater force because now it's holding a 25kg weight, but also a force on the x-axis; thus a net force that has an angle.
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