Good point, Martin. Let me add to my initial statement.
Studies have shown that even the most high intensity short duration exercise sessions (i.e. Wingate, which is only 30 seconds) greatly depletes intramuscular glycogen stores.
Creatine-ATP is typically the dominant energy pathway for only 10-30 seconds. Even during these seconds, glycogen is still used. But after this, glycolysis becomes the dominant pathway.
When you analyze a typical bodybuilding routine--higher reps, shorter rest, greater volume--glycogen depletion becomes a serious concern.
If you are interested in more information, read our post workout nutrition articles, particularly this one, http://www.abcbodybuilding.com/scientific.php
Then, do a search in the journal of strength and conditioning research, and you will find bundles of references which discuss this topic.
For more on fuel utilization, you will also want to read these articles, http://www.abcbodybuilding.com/cardioindex.php
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I had great results from this personally but I tend to train for excessive periods (2-3 hours of lifting) so I need that second wind the carbs give me.
This is just what works for me and I know it doesn't work for everybody.
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If you are training for that long, it would definitely have a greater impact on your performance relative to say, a 1 hour session.