Gym Exercise Physics Quesion

[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.

[ajb]

In your rope example, yes, the tension at any point in the rope (assuming it is frictionless, weightless, does not stretch, and the weight is a spherical cow) will be equal to the force exerted by gravity on the weight at the bottom.

However, the way that forces are transmitted through the body is a lot more complicated than in the rope example, and changing arm posture will absolutely change the way that force is transmitted through the shoulder.  To get any more specific than that, you'd need to take a detailed look at the anatomy involved and how the mechanical arrangement changes between the two postures and through the motion of the exercise.  The shoulder is, like the hip, a sort of complicated joint since it has multiple degrees of freedom, and in order to perform its full range of motion the angles of multiple muscle groups relative to the joint and each other is quite variable.  The joint itself also has limitations imposed by the way the bones and ligaments connect, and this can also affect the way that muscle force is transmitted between positions where the joint is free and where it locks up.  Finally, the additional tension in the muscles around the joint will tend to support and stabilize the joint itself, and it's possible that "it takes weight off your shoulders and transfers it to..." could refer to the fact that bending the elbow causes the surrounding muscles to bear some of the static load that would otherwise be born by the shoulder joint.  This is a big part of proper weightlifting technique (and part of why lifting weights is a better overall exercise compared to using machines), where in addition to the muscles that are doing the work of the lift you engage a lot of additional muscle mass to support the weight rather than letting it all rest on your joints.

You might look into some videos on weight training, there is a lot of good information on how to correctly position your body for various lifts and some of them get into the anatomical mechanics, which are ultimately what determine the best way to lift.  I know Mark Rippetoe (a pretty well respected weightlifting trainer who's written a few books on the topic) has some videos where he talks about shoulder anatomy

[Rick Law]

First, bending your elbow:

An easier way to think of your bend-elbow is "it is acting like your car's shock absorber."  Your arm muscle is acting as a spring.  The give and take that your muscle has adjusted the angle of your elbow thus reduced the force needed to accelerate upward against gravity.

A bit deeper in details, Force=Mass*Acceleration

That angle at your elbow allows the entire system to modulate the acceleration.  Relax (or stretch) that muscle lengthens the linear distance between shoulder and hand.  The lengthening lowers the height of the weight (you are holding) as the arm and shoulder go up.  Thus that bend elbow translates to your weight and the shoulder are not lock to moving at the same pace - you can reduce the pace of the weight's upward movement.  The natural force alone will stretch your muscle even if you do not intentionally relax the muscle.  Net net is a reflex and/or controlled reduction in acceleration upward.  Mass is the same, the reduction in acceleration upward reduces the force.  Exact same way as a car's shock absorber reduces the force the wheel joints must take when the car hits a bump moving your car upward or downward.  It modulated the acceleration vertically.

That said, you are adding same potential energy to the system - the system being your center of mass (you, plus the metal you are holding).  You are however adding it slower.

Second, rope with a kink in it:

Your rope will always form a plumb line - with or without a kink.  Think about the surveying equipment surveyors use.  They have a weight hanging down from a tripod with a scope on the tripod.  The thin line that hangs the bulb of metal with a point-tip is the plumb line.

The "kinky rope" in this case can be considered as a bad overly thick "line".  The point the rope hangs from (point 1), and the center of mass of rope plus metal bulb (point 2), and the center of mass of earth (point 3) will be a straight line.  That is your plumb line.  However many kinks and however big these kinks are you put into it, that three points will be a straight line.

Even if you go all the way and use something as rigid as a piece of metal bend in to a boomerang shape as the kink rope.  The point of hung (point 1), the center of mass of boomerang plus the bulb (point 2), and the center of mass of earth (point 3) will again form a perfect straight line.

I ignored the rotation of the earth.  So the plumb line is not a mathematically perfect plumb line but typically close enough.

Now, if the boomerang is not free-swinging - say screwed into a wood plank hanging from the ceiling.  You can screw it in hard such that it wont form a plumb line - the screw holding the boomerang must press it against the wood plank.  It must be screwed in hard enough so the static friction force against rotating the join is enough to hold against the torque needed to hold that thing in an angle against plumb.

Naturally, the boomerang must be structurally strong enough to take the tension/compression force caused by that torque - same as your bad thick rope needs to be rigid enough to take that torque.  If it isn't, that torque will pull the stiffness-bends in your rope straight.  The stiffness in the rope in this case serves as spring of the shock-absorber.

Hope this helped.

EDIT:  Minor changes in wording to make it a bit easier to understand

[bostonman]

You might look into some videos on weight training, there is a lot of good information on how to correctly position your body for various lifts and some of them get into the anatomical mechanics, which are ultimately what determine the best way to lift.  I know Mark Rippetoe (a pretty well respected weightlifting trainer who's written a few books on the topic) has some videos where he talks about shoulder anatomy

I have done weight lifting for many years, however, I never thought of the angles of forces. Not to dive into weightlifting as I wanted this to be more about physics, but I was always taught to never straighten joints because it can damage them. So I got into the habit of (or at least trying) not to lock my arms/legs, and, when doing certain exercises, to keep a very small bend (and not moving it) on a joint (such as "shrugs").

Someone came up to me and told me I'm doing them wrong. As he was describing the reason such as my minor bend, I was analyzing the direction of the force. In all cases, the weight is dangling directly below the shoulder.

Having not considered the bio mechanics in all this, I began questioning a more drastic arm bend such as 90 degrees to make things easier to interpret. Thinking about this now after reading all the insight, it seems a transfer of the mass would take place. Instead of 50kg being seen by the shoulder, it would now focus on preventing a rotation since the weight wants to be directly below it thus not focusing 100% on the muscle (the "trap"). At the same time, it still seems like the shoulder would be the only source of holding the weight against gravity thus still having the 50kg mass plus holding it from rotating.

[IanB]

You can overanalyze this.

If you are holding a 25 kg weight off the ground, the downward force exerted by that weight is always 25 kg (force). It doesn't matter how you bend or contort your arms, the downward force at your shoulder remains exactly 25 kg. Now, if the weight is not hanging directly below your shoulder there can be other forces too, such as bending moments, and maybe you don't want these due to unwanted stress on muscles and ligaments.

But the fact remains that a 25 kg weight exerts a 25 kg downward force due to gravity, and no matter how it is suspended, this force appears in a downward direction at the suspension points.

[Rick Law]

You can overanalyze this.

If you are holding a 25 kg weight off the ground, the downward force exerted by that weight is always 25 kg (force). It doesn't matter how you bend or contort your arms, the downward force at your shoulder remains exactly 25 kg.  Now, if the weight is not hanging directly below your shoulder there can be other forces too, such as bending moments, and maybe you don't want these due to unwanted stress on muscles and ligaments.
...

You are right about the downward force, but when one lift something, it is not just the downward force one must fight against.  Different positions of the weight will put different forces on different parts of your spine, legs, arm and shoulder.  The combined force you need to apply to hold it up differs significantly.

You can validate this by holding something moderately heavy (say a jug of milk) with your elbow closed and the jug of milk close enough to touch your body while at shoulder height.  Now, try to extend your elbow while keeping the jug shoulder high until your arm is horizontal, you can feel the weight seem to be increasing as the jug moves further from your body.

Force there is actually easy to visualize.  With your arm fully extended, the weight at the end of your arm will try to rotate your arm downward until the jug of milk is closest to the ground.  Your shoulder is the center of rotation, torque is Downward_Force*Radius.  Radius is the length of your arm (approx).  Your shoulder and arm has to pull to hold up against that torque.  The farther away the weight is from your body, the bigger the rotational radius and the larger the torque you have to fight.  The faster you lift, the bigger the acceleration upward so that is more force.  If the weight moves horizontally to any extend, there would be added force as well.

Keeping the weight close to the body alone is not the answer because that may require a position that puts more force on a part of the body that can't take it.  Some parts of the body (such as spin) can't take much, so you could easily injure it and if so that could be the last time you try to lift anything.  So do be careful.  Make sure every force exerted to your body is below your limitation for that part of your body.

[thm_w]

I have done weight lifting for many years, however, I never thought of the angles of forces. Not to dive into weightlifting as I wanted this to be more about physics, but I was always taught to never straighten joints because it can damage them. So I got into the habit of (or at least trying) not to lock my arms/legs, and, when doing certain exercises, to keep a very small bend (and not moving it) on a joint (such as "shrugs").

Don't believe random advice people give you in the gym. Do your own research to verify.
You want to exercise full range of motion, at least some of the time, to improve strength in those end ROM positions (eg lockout).

Of course in this case its unlikely to really matter, as you are just shifting the load slightly away from tricept onto something else (as others have explained). And you probably have other arm exercises you are doing anyway.

[EEVblog]

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.

The pioneers of body building like Arnold have told stories of how they discovered these things. They simply did thousands of reps of each of the different angles of motion and they found out which muscles were being worked by the pain and spasm etc in the effected areas.

[EEVblog]

You can overanalyze this.

If you are holding a 25 kg weight off the ground, the downward force exerted by that weight is always 25 kg (force). It doesn't matter how you bend or contort your arms, the downward force at your shoulder remains exactly 25 kg. Now, if the weight is not hanging directly below your shoulder there can be other forces too, such as bending moments, and maybe you don't want these due to unwanted stress on muscles and ligaments.

But the fact remains that a 25 kg weight exerts a 25 kg downward force due to gravity, and no matter how it is suspended, this force appears in a downward direction at the suspension points.

But when you bend your elbow, even if the weight is still directly below your shoulder, you engage other muscles and have also rotated the shoulder a bit. It makes a difference.

[eugene]

So a simple free body diagram might not be incorrect per se, but the actual load path of the forces through the shoulder joint depends on which muscles are being used, if any.

[bostonman]

Don't believe random advice people give you in the gym. Do your own research to verify.
You want to exercise full range of motion, at least some of the time, to improve strength in those end ROM positions (eg lockout).

I agree, but this wasn't necessarily random advice, however, it's also not exactly about lifting weights.

Technically it seems holding 25kg in each hand hanging vertical would exert the same force on each shoulder regardless if elbows are bent or not, but common sense, years of going to the gym, and this thread, it makes sense that it causes other muscles to change the direction of forces.

Now if the arm was replaced with two pieces of metal, connected by a hinge for an elbow, and a ball/socket for the shoulder, things should be different. If the hinge could be tightened so it's being bent at a 45 degree bend, the ball/socket would rotate so the weight is still perpendicular to the ground, however, now force would need to be calculated on the hinge and the ball/socket. Unless I'm thinking about this wrong, the same force would be on both the hinge and ball/socket.

In this example, it would be 25kg * 9.8 (rounding up to 10) 250N on the hinge (i.e. elbow) but at 125N on the bottom piece of metal and 125N on the upper piece of metal, and then another 250N on the shoulder; but it wouldn't have any angles because it's just rotating on a ball and the downward force would be the full 250N.

I think in the case of a human, the back muscles and would pull on the arm muscles thus relieving some of the force on the shoulders. Same with the elbow, the bicept/tricept would pull on the forearm thus relieving stress on the elbow.

Maybe I'm reiterating everything said here, but believe this is the most common and simple explanation.

[EEVblog]

Technically it seems holding 25kg in each hand hanging vertical would exert the same force on each shoulder regardless if elbows are bent or not, but common sense, years of going to the gym, and this thread, it makes sense that it causes other muscles to change the direction of forces.

The shoulder is an incredibly complicated piece of biomachinery.

[bostonman]

You're correct and I believe it's one of the most involved joint - I'm just unfamiliar with the many names and know the basic names.

BTW, great picture.

[jpanhalt]

If your question is related only to the amount of weight/force, then as others have said, 25 kg doesn't change.  However, if you are concerned about forces on the shoulder, particularly the rotator cuff, then the situation is more complex.  Relative to the scapula only and ignoring the acromion/clavicle, some muscles in the shoulder originate on the scapula and insert on the head of the humerus (1).  Other muscles (e.g., biceps) originate on the scapula and insert in the upper forearm (2).

Dead weight on a fully relaxed arm and shoulder puts stress on the rotator cuff.  When appropriate muscles are flexed, that force on the rotator cuff is shared with the scapula.

Anyone who has had serious rotator cuff injury can describe for you the ease with which that shoulder may feel slightly dislocated and is uncomfortable.  Flexing the muscles puts it back where it belongs.

1)  https://www.ortho.wustl.edu/content/Patient-Care/3127/Services/Shoulder-Elbow/Overview/Shoulder-Arthroscopy-Information/The-anatomy-of-the-shoulder.aspx#:~:text=The%20rotator%20cuff%20connects%20the,by%20pulling%20on%20the%20tendons.

2)  https://en.wikipedia.org/wiki/Biceps#:~:text=The%20biceps%20or%20biceps%20brachii,attached%20to%20the%20upper%20forearm.

[Bud]

Since my early childhood, I have been practicing many different sports. One of them was running. Growing up, I ended up practicing only this one. Now, I'm keen on waking up every morning and running for at least half an hour. Also, I started understanding that if you want to lose weight or get fit, you should take some products that help you get the best results.

And in your next post can we expect you giving us links to weight loss products you sell?

[bostonman]

Hahahah

This wasn’t about shoulder injuries, it was about forces.