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Carbohydrates are stored in the muscles as glycogen. It is a major energy source for weight lifting. I want to know if a muscle can only use the glycogen stored in itself or can it borrow glycogen from other muscles.

I'll give you a practical example. Let's say I do many sets of bicep curls followed by many sets of tricep extensions. Will my tricep strength suffer because the bicep curls not only used glycogen stored in the biceps, but also the triceps? Or will my tricep strength remain unaffected because the bicep curls cannot be fueled by the glycogen stored in the triceps?

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Technically speaking, the energy used by the muscles is not glycogen, but the phosphate bonds in adenosine triphosphate (ATP). When energy is needed, one of the phosphate bonds is broken, resulting in an energy release and the creation of the subsequent adenosine diphosphate (ADP) and waste materials. This is one of the main reasons that creatine phosphate works, is that it provides cells with a ready supply of extra phosphate to recreate the ATP from ADP. Glucose is part of this cycle in that it is broken down into pyruvate to feed into the Kreb's cycle. IIRC, one molecule of glucose can provide somewhere around 24 net molecules of ATP. It is the glycogen that is broken apart to provide the glucose.

Grohlier is somewhat correct, in that when muscles start to run out of stored glycogen they start relying on blood glucose and release of stored glycogen from the liver, and this can be slower than the immediate availability of muscle glycogen. However, for short burst activity such as weightlifting, the amount of time you spend in the recovery phase between sets is enough to replenish muscle glycogen from circulating blood glucose. It can take anywhere from 60-120 minutes of continuous exercise to completely deplete muscle/blood/liver stores of glycogen (This is commonly known as "bonking" or hitting the wall, another term that is vastly overused, usually incorrectly.) This time can be lengthened if necessary by eating/feeding/drinking. It's also extended by your body adapting to exercise, as your body will start to adapt by storing more glycogen in the muscles.

Recursive is correct, once the glycogen is in the muscle, it can only be used by that muscle specifically. So if you do deplete the biceps glycogen (as per your example), the triceps would not be affected much and could still do work. However, it would be impacted somewhat, as your biceps also play a part in triceps movement, and vice versa. You can't ever really work a muscle in "isolation" outside of a lab.

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I very much appreciate this detailed scientific answer. –  Dave Liepmann Dec 12 '12 at 20:40
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Glycogen can be an enormous molecule: uic.edu/classes/phar/phar332/Clinical_Cases/… lists a molecular weight of 400kDa, which is somewhere around 2200 glucose molecules. According to Wikipedia, one molecule of glucose yields that order of ATP production. en.wikipedia.org/wiki/… –  RecursivelyIronic Dec 13 '12 at 2:11
    
@RecursivelyIronic Edited - I didn't completely proofread. You are correct, it's glucose not glycogen. –  JohnP Dec 13 '12 at 3:49
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Only the muscle cell storing the glycogen can use it for energy. From Wikipedia (my emphasis):

Muscle cell glycogen appears to function as an immediate reserve source of available glucose for muscle cells. Other cells that contain small amounts use it locally as well. Muscle cells lack the enzyme glucose-6-phosphatase, which is required to pass glucose into the blood, so the glycogen they store is destined for internal use and is not shared with other cells. (This is in contrast to liver cells, which, on demand, readily do break down their stored glycogen into glucose and send it through the blood stream as fuel for the brain or muscles).

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The Priority Principle is touted as an advanced bodybuilding technique for weak-point training. This principle says one should train a weak body part first in a workout because you will have the most energy in the beginning. This seems to contradict what Wikipedia says about glycogen usage. According to Wikipedia, working out my biceps first would not degrade my triceps exercise later. I'm just throwing this out for discussion. –  JoJo Dec 12 '12 at 20:22
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mmm...that's a bit of a misnomer. Weider promoted the Priority Principle as a concept of working your weak spots first, so that if you do get tired overall, the work doesn't suffer on your weak points. Also, if you have a weak portion of a muscle (Such as the rear head of the deltoids) you can work that specific area while the entire muscle is still fresh. At least as I understand it anyway... –  JohnP Dec 12 '12 at 20:32
    
It would contradict what Wikipedia says about energy usage if muscle glycogen stores were the only component of the system you're trying to exercise. Obviously that's false. There's other sources of energy released into the blood from the liver and fat cells. Additionally, tiredness is partially determined in the brain, and occurs before muscles become totally exhausted. If your brain or nerves become tired from working other muscles, you'll work later muscles less hard. –  RecursivelyIronic Dec 12 '12 at 20:49
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That's why you shouldn't necessarily take glycogen too seriously if your goal is training success: it's only one component of a poorly understood, very complex system. I suspect that the physiology isn't well enough understood to answer the question of which muscles to work first definitively, but I'm sure empirical studies have been done which do answer the question definitively. –  RecursivelyIronic Dec 12 '12 at 20:52
    
@RecursivelyIronic - Unfortunately, even the empirical data doesn't explain it fully. Much of what we understand about routine based items in exercise is still a mystery. One program may work great for one guy, but be rubbish for the next. Some people can go bollocks out for weeks, while the same schedule destroys the guy next to you. Same with sleep, recovery, nutrition, etc. It's all a game of playing with things until you find what "clicks" for you. –  JohnP Dec 12 '12 at 21:49
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It is all balanced by osmosis. Once your muscle stores of glycogen start being depleted, your body converts blood sugar (glucose) into glycogen in the muscle being depleted. So your muscles will still get glycogen, just not from other muscles.

As an aside, your body is not very good at this.

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Can you expand on what the body is not very good at? –  JohnP Dec 12 '12 at 2:57
    
Rapidly converting glucose to glycogen. –  Grohlier Dec 12 '12 at 12:08
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