You've been playing tennis or squash, or riding a bicycle, running, swimming, or rowing for an extended period of time without any trouble, when suddenly your muscles begin to feel tired and you're not sure how much longer you can continue. Without a doubt, low muscle glycogen is the culprit behind your sudden fatigue. Your muscles are running so low on carbohydrate fuel that they are no longer able to function normally.
Your muscles aren't machines, after all. Although your car engine can race along full-tilt until the last drop of petrol is exhausted, your muscles begin slowing down well before the glycogen is gone. Perhaps muscles work this way for purposes of self-preservation; even the most compulsive, highly driven athletes can never destroy their muscle cells by totally wiping out their energy stores, because fatigue stops them first.
But even when muscle glycogen dips to low levels, there's still plenty of carbohydrate available to the muscles. The trouble is that it's not actually inside the muscle cells - it's floating by in the bloodstream in the form of blood glucose, or 'blood sugar'. So, as muscles grow tired, shouldn't they simply step up their intake of the fuel wafting along in the nearby blood? Wouldn't pulling extra glucose into the muscles help you avoid fatigue and keep you exercising for longer periods of time?
Well, yes - except it hasn't been clear that muscles are actually 'smart' enough to do that. Many scientists have speculated that the extraction of glucose from the blood is fairly constant during exercise, even when glycogen levels get low and muscles should know enough to pull in glucose in augmented amounts.
To find out whether muscles can actually increase their intake of blood glucose in a time of need, scientists at the University of Limburg in the Netherlands asked six subjects to deplete muscle glycogen in one leg by engaging in prolonged, one-legged exercises. As a result, each subject ended up with one leg with normal glycogen levels and one glycogen-poor leg.
The following morning, each subject exercised one leg for 90 minutes at 60 per cent of maximal workload and did the same with the other leg two hours later (the order of leg exercise was random; some subjects used the glycogen-poor leg first, while others started with the normal limb).
The normal leg had more than twice as much glycogen as the impoverished leg at the beginning of exercise and still had 73-per cent more glycogen after 90 minutes. As you might expect, blood-glucose levels were the same in each leg, and when the subjects were at rest the glycogen-depleted leg muscles were extracting over three times as much glucose from the blood as the glycogen-rich muscles (the glycogen-poor fibres were obviously attempting to take in glucose in order to build back their glycogen concentrations).
However, once exercise started the story was different. The rate of glucose uptake from the blood did increase during exercise, but it increased by about the same amount for glycogen-poor and glycogen-rich leg muscles. As a result, after 10, 30,60, and 90 minutes of exercise, both glycogen-poor and glycogen-rich muscles were taking in about the same amount of glucose. The glycogen-famished muscles weren't 'smart'.
So if your muscles take up about the same amount of glucose during exercise, whether they're glycogen-depleted or not, why should you bother to consume a carbohydrate-containing sports drink during exertions which last for an hour or longer, as sports nutritionists are always advising? Obviously, using a sports drink during exercise is no substitute for carbo-loading prior to your exertions, but the drink does ensure that your blood-sugar levels won't drop too low as you train or compete. If blood glucose falls too far, your muscles can get into 'double trouble' - no glycogen fuel in the 'tank' and no glucose fuel from the 'pump' (the blood).
('Use of Glucose during Prolonged Exercise in Muscle with a Normal and Low Glycogen Content, ' Medicine and Science in Sports and Exercise, vol. 26(5), Supplement, # 1144, 1994)
