Quote:
Originally Posted by Hexcsx
During the first 15 to 20 minutes of aerobic activity, glycogen or sugar within the muscles is used for energy. Fat metabolism for energy doesn't occur until about 15 to 20 minutes after beginning aerobic activity. Therefore after 20 minutes the intensity with which you are running enables your body to oxidise more fat for energy and thus you will need to up the intensity to get back into the aerobic glycolysis training zone. This explains why your heart rate could drop at this time. RUN FASTER!
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This is absolutely wrong. The fuel you burn is related to intensity, not time. Time to fatigue is related to intensity, but the fuel you burn doesn't change as the time you can continue increases, just as the time to fatigue changes, only with intensity and availability.
In the first minute or so (maybe shorter) you will burn mostly intramuscular glycogen, ATP and CP at an accelerated rate in order to generate the chemical stimulus to use and mobilize other fuels. The fuel selection at any given intensity is regulated by a complex series of neuro-hormonal chemicals, substrate availability and allosteric modification that is basically dependent on intensity.
The harder you go, the more you rely on anaerobic glycolysis and produce lactate, the easier you go the more you use aerobic glycolysis and fat metabolism through beta oxidation.
If you are at rest, such as sitting here reading this on the computer you are burning primarily fat for energy in your muscles. It's not like you switch one energy source off and another one on, it's a continuum, you are always burning fat and using aerobic and anaerobic glycolysis what changes is the percent of energy coming from each metabolic pathway.
