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Author Topic: Why do you arms ache when you hold something in the same position?  (Read 10365 times)

Offline mcjhn

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Offline !mater

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because gravity trys to move the box, constantly against the resistance of your arm. So you have kinetic energy stored, and it trys to get to the ground, were it would transfer its power of movement int heat, movement and again kinetic energy (it would fall to the center of the earth if it could)
 

Offline Airthumbs

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I think I am correct in stating that this kind of exercise would be defined as Anaerobic, or without oxygen. Anaerobic exercise is defined as brief high intensity activity such as weightlifting.  In the case of trying to hold a box out with your arms outstretched your muscles are primarily functioning anaerobically. 

At some point you will reach what is called the Anaerobic Threshold, and is where you have the build up of Lactic acid in the blood supplying the muscles used to hold the box up against the force of gravity.

It is the role of the Lactic acid to prevent permanent muscle damage during periods of anaerobic exercise, normally between one and three minutes in duration. This forces you to stop and allow the lactic acid to be removed from your muscles in your arm and shoulders created when you are holding the box up.  The feeling of this build up of lactic acid is normally described as a burning sensation in our muscles. 

In effect the pain you feel when holding the box out is your body signalling you that it is time for a rest.   ;D

As a point of interest in one of the events at the highland games held in Scotland every year a true replica of the sword William Wallace used is held out at arms length.  Only 6 people have managed over the 60 second mark in the history of the games. http://www.thgatherin.com/index_files/Page2305.htm
« Last Edit: 16/07/2011 01:48:36 by Airthumbs »
 

Offline Bored chemist

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Muscles are not very efficient.
In order to keep exerting a force they need to be continuously supplied with energy.
Imagine lifting the weight by putting it on a bag and blowing into the bag. If you sealed the bag the weight would stay up but if there is a leak in the bag you would have to keep blowing into it to make up for the air lost through the leak.
The situation with muscles is similar (with ions leaking through cell membranes).
Also, with no movement the muscles get less blood supply than usual (normally, if the muscles are working hard they are moving. That means the waste product (lactic acid) isn't flushed away. When it builds up you get cramp.
 

Offline EvilHomer15

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The picture is wrong. Gravity affects the box, aswell as the arm that keeps the box above the ground.
 

Offline Bored chemist

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The picture is wrong. Gravity affects the box, aswell as the arm that keeps the box above the ground.
It looks right enough to me.
Gravity acts down and the force from the arm acts up so the resultant force is zero.
Another way of seeing this is that the distance moved is zero so the work done is zero.
Yet another way is to note that the arm could be replaced by a shelf. The shelf cannot do work and so the work done must be zero.

The point remains that, even though the arm does no work, it still hurts after a while.
 

Offline EvilHomer15

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Gravity acts down and the force from the arm acts up so the resultant force is zero.
Exactly.
The arm does apply energy, to keep the box up.

Quote
Another way of seeing this is that the distance moved is zero so the work done is zero.
Distance moved is, indeed, zero.
Work is done in order to make the box not move. Therefore, work is done. The amount of energy used on the task is above zero.

Quote
Yet another way is to note that the arm could be replaced by a shelf. The shelf cannot do work and so the work done must be zero.
I disagree.
The shelf is doing work in order to keep it up. It is, however, not the same kind of work a human would do.
The shelf is a platform. It can either carry the load, or it breaks down.
Humans are thinkers. We can either carry it for a given time period, or let it go. No human will carry something until it kills him.
 

Offline Bored chemist

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Gravity acts down and the force from the arm acts up so the resultant force is zero.
Exactly.
The arm does apply energy, to keep the box up.

Quote
Another way of seeing this is that the distance moved is zero so the work done is zero.
Distance moved is, indeed, zero.
Work is done in order to make the box not move. Therefore, work is done. The amount of energy used on the task is above zero.

Quote
Yet another way is to note that the arm could be replaced by a shelf. The shelf cannot do work and so the work done must be zero.
I disagree.
The shelf is doing work in order to keep it up. It is, however, not the same kind of work a human would do.
The shelf is a platform. It can either carry the load, or it breaks down.
Humans are thinkers. We can either carry it for a given time period, or let it go. No human will carry something until it kills him.
You are wrong on every count.
Try studying a bit more about physics.
Work is done when a force moves through a distance. There is no movement so there is no work.
 

Offline CZARCAR

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what about isometric exercise building muscle?
 

Offline simplified

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Yet another way is to note that the arm could be replaced by a shelf. The shelf c
You are wrong on every count.
Try studying a bit more about physics.
Work is done when a force moves through a distance. There is no movement so there is no work.
You cannot ideally hold weight on the one level.Therefor the muscles make some work. :P
 

Offline imatfaal

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Evilhomer and Simplified

"work" is a specialised term when dealing with physics. 

Hyperphysics explains as follows
"Work refers to an activity involving a force and movement in the directon of the force."
http://hyperphysics.phy-astr.gsu.edu/hbase/work.html

Wikipedia has a good explanation here http://en.wikipedia.org/wiki/Work_%28physics%29
 

Offline Bored chemist

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what about isometric exercise building muscle?
What about it?

"You cannot ideally hold weight on the one level."
Yes I can, I can use a shelf.
 

Offline burning

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It is down to how human muscles work.  Our muscles can't just set in one position unsupported and stay that way.  To hold the box "still" our muscles actually relax and contract minutely over and over.  So while the net work done on the box is zero, the net work done by the muscles is not.

A shelf does not do work over time because it is a rigid object.  When you set a heavy object on it, it will deform slightly (so long as the object isn't too heavy) but once it has reached an equilibrium state of deformation against the external force it just stays there.

I have heard that some types of animals (I believe clams were an example) have a type of muscle that can indeed set into a rigid state, so that they can hold something against an external force without expending any energy.  My understanding is that disadvantage of muscles of this sort is that they are slow moving.  I won't say that a fast moving muscle that can also set is impossible, but evolution seems to not have produced one.  I don't have any references for this, so any knowledgable biologists are welcome to tell me if I'm talking through my hat.
 

Offline JP

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It is down to how human muscles work.  Our muscles can't just set in one position unsupported and stay that way.  To hold the box "still" our muscles actually relax and contract minutely over and over.  So while the net work done on the box is zero, the net work done by the muscles is not.

Well put.  Part of the problem is that energy we expend moving or holding a box doesn't have to go into the energy of motion of the box.  In this case, your body expends a lot of energy, but the box doesn't move.

Also, as mentioned above, another part of the problem is confusion over the word "work."  In physics, work happens when a force moves an object.  It's useful to determine how much energy of motion is transferred to an object.  In this case, since the box isn't moved, no work is done (in the physics sense), even though you expend a lot of energy. 
 

Offline Geezer

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I don't think any work is being done. As BC said, it's a question of efficiency, and that's why energy is consumed and your muscles begin to ache.

Imagine you replace the muscles with an electric motor that is stalled (not moving) but is still producing a torque sufficient to hold the box elevated.

The motor is not 100% efficient, so it will consume some energy that is converted into heat while it is producing the necessary torque/force, even although it is doing no work.
 

Offline simplified

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Intensive breath makes work. Radiation of a additional heat by the body moves molecules of air. It is work also.
 

Offline JP

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Intensive breath makes work. Radiation of a additional heat by the body moves molecules of air. It is work also.

True.  In physics when you talk about work, you have to specify two things: 1) what is performing the work and 2) the receiver of the work. 

To be specific, the person in the drawing is doing no work on the box. 

But you could also say that the various cells within the person's body are doing work since they have to transport fuel and waste to and from the cells as a result of the muscle strain of holding up the box.  (Someone who knows physiology better could even tell you what parts of those cells are doing work on what chemicals.) 
 

Offline Geezer

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BTW, if your muscles (or the motor) were 100% efficient, they would not consume any energy while elevating the box and, therefore, they would not get sore.

(Please send any complaints regarding this post to JP.)
 

Offline Bored chemist

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Intensive breath makes work. Radiation of a additional heat by the body moves molecules of air. It is work also.

True.  In physics when you talk about work, you have to specify two things: 1) what is performing the work and 2) the receiver of the work. 

To be specific, the person in the drawing is doing no work on the box. 

But you could also say that the various cells within the person's body are doing work since they have to transport fuel and waste to and from the cells as a result of the muscle strain of holding up the box.  (Someone who knows physiology better could even tell you what parts of those cells are doing work on what chemicals.) 
There might have been a hint earlier.

Also, it seems that I have to complain to you that Geezer's use of the verb "elevate" is unorthodox. I think it means to move something upwards (which does need work) and he seems to think it means to hold something up (which doesn't).
 

Offline Geezer

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Also, it seems that I have to complain to you that Geezer's use of the verb "elevate" is unorthodox. I think it means to move something upwards (which does need work) and he seems to think it means to hold something up (which doesn't).


True - I should have said levitating.
 

Offline mcjhn

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thanks everyone
 

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