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I'm curious what the exact cause is of dramatic heat generation that comes along with muscle activity. Can anybody explain this in understandable language?

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    $\begingroup$ does "conservation of energy" count as a valid answer? $\endgroup$ – John Dvorak Dec 23 '14 at 15:58
  • $\begingroup$ @JanDvorak Well, I'm looking for an answer that is understandable with a shallow knowledge of biology/bio-chemistry/bio-physics. An answer consisting out of only this phrase, assumes background knowledge of those scientific areas. It's basically like: "Dad, why does the ball fall down when I throw it? Well, E=MC2, son." $\endgroup$ – Alph.Dev Dec 23 '14 at 16:08
  • $\begingroup$ How about the wikipedia article on motor proteins? $\endgroup$ – John Dvorak Dec 23 '14 at 16:20
  • $\begingroup$ ... or, rather, the article about muscle contraction? $\endgroup$ – John Dvorak Dec 23 '14 at 16:22
  • $\begingroup$ Short answer: energy transport always has heat loss. $\endgroup$ – inf3rno Jun 24 '15 at 16:30
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First, it might be helpful to talk briefly about what heat is. It is a form of energy - molecular energy. Basically, heat is the amount of jiggliness of the molecules of a substance or object. When we look at a hot object, it doesn't apear to be moving any more than a cold object, but on a scale much smaller than we can see, it's molecules are moving faster than in a colder object. Read this for more information if you want.

There are at least two things going on in muscle contraction (as well as in other metabolic processes) which produce heat. The first is the chemical reaction that powers muscle contraction. You probably don't want to get into great detail, but basically there is a molecule which your cells use to store power, which we abbreviate as ATP. When our muscles use that power, an exothermic chemical reaction occurs that "burns" ATP, breaking it into two pieces (ADP and phosphate). That reaction releases energy. Some of that energy is used in the actual movement of the muscle. But some of it just "jiggles" the nearby molecules. Again that jiggling is the very definition of heat.

The second source of heat is probably much smaller: friction. In short, just about any time anything moves, nearby molecules get jiggled some more. Imagine trying to walk through a room packed full of harps. Everywhere you move, some of those strings get plucked and vibrate. It's a little bit like that. When your muscle fibers tighten and flex, they rub past each other and jiggle each others atoms. All of that jiggliness is heat, and the jiggling continues until it can transfer to some other medium, as when, sweat evaporates from your skin.

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  • $\begingroup$ Aside of an extremely clear explanation on the source of heat, +1 for explaining what heat is and +1 for explaining what friction is. Not that I totally didn't know that, but the way you explained it makes me realize that the heat we feel with our skin receptors is tiny waves, caused by molecules jiggling at a higher rate. Also I get that if you manually push the molecules to jiggle faster, they start producing waves (heat). And that's all there is to conversion of kinetic energy into thermal energy (friction). Your answer explains heat production, heat transfer and my microwave oven. Thanks! $\endgroup$ – Alph.Dev Dec 23 '14 at 19:14
  • $\begingroup$ @Mark. "When your muscle fibers tighten and flex, they rub past each other and jiggle each others atoms." What a fantastic comment. We should have gotten you in my physiology class. $\endgroup$ – Lanka Jun 5 '15 at 18:18
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This is oversimplified, but hopefully clear.

When we eat food, metabolic processes break down larger, multi-carbon molecules (like glucose) into smaller molecules. This process is called catabolism, and it releases energy. Here's a simple example of a catabolic reaction:

AB -> A + B + energy

For more on this principle, look at the section on Le Châtelier's Principle on this page from Washington University.

Our body can use the energy released to drive more chemical reactions. Specifically, our cells make a molecule called ATP, which is used all over the cell when energy is needed. However, our cells can't convert 100% of the energy from catabolic processes into chemical energy, some of it is lost as heat (or entropy). When we use ATP, a high energy phosphate group is hydrolyzed (chemically split) from the ATP molecule (ASPU, look at the arrow on the right for energy release):

atp hydrolysis

This releases energy, some of which (during exercise) is used to make muscles contract, but much of it is lost as heat. We heat up when we exercise because the muscles are being used more than at rest, which means more ATP is being used to contract the muscles, which means more heat is being released by the ATP hydrolysis reaction.

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    $\begingroup$ Also to be noted that exercise causes increased blood circulation. That also generates heat. $\endgroup$ – WYSIWYG Dec 23 '14 at 17:52
  • $\begingroup$ Well explained. Please check if I summarize this correctly: the heat is generated as a side effect of splitting molecules to generate energy needed for muscle contraction. $\endgroup$ – Alph.Dev Dec 23 '14 at 18:48
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    $\begingroup$ @WYSIWYG How does blood circulation generate heat? Doesn't it only spread the heat that is generated elsewhere? $\endgroup$ – Alph.Dev Dec 23 '14 at 18:49
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    $\begingroup$ @WYSIWYG Yeah, I wouldn't really say high blood circulation causes heat at all. The primary correlation between heat and circulation is really just that the body uses circulation to regulate core temperature. But in fact, when the body has a fever, it increases temperature by decreasing circulation, leaving the extremities cold, and triggering shivers (muscle movement) in order to produce the required heat. You can read more about that here if you want. $\endgroup$ – Mark Bailey Dec 25 '14 at 6:26
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    $\begingroup$ @WYSIWYG Well, the friction of blood flow generates some small amount of heat, but assuming blood volume throughput stays the same (which I would expect to correlate approximately to heart rate) I would think that again, decreasing circulation (tightening the blood vessels) would increase friction as the same blood volume is pressing through a smaller, tighter space. Increased heart rate during exercise, on the other hand, would raise temperature a bit. Higher blood flow increases friction a bit, and more significantly - the heart muscle activity generates more heat. $\endgroup$ – Mark Bailey Dec 26 '14 at 16:51

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