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This is my summary of an interesting paper that I reviewed for my nutrition research column for Muscular Development magazine.
Tom Fahey M60

Glycogen: Train Low/Compete High?
Glycogen is stored carbohydrates found in high levels in the liver and muscles. Muscle biopsy experiments in the 1960s and 70s showed that glycogen depletion was associated with fatigue in endurance events such as the marathon. Scientists viewed glycogen as stored energy. Recent studies showed that glycogen levels are important signals to the cells that cause increases in fitness with training. Exercising with low glycogen stores causes metabolic and cell stress and also triggers the release of fight or flight hormones such as adrenaline. In turn, this activates signaling chemicals to increase mitochondrial mass, power output capacity, and improve endurance performance. Mitochondrial mass is highly related to endurance capacity. Increasing it is a primary goal of endurance training. The activity of the signaling chemicals is lower when glycogen levels are high. Australian researchers suggested that endurance athletes would benefit from training in a glycogen-depleted state during the off-season— a time when they do base conditioning. This practice is not a good idea during peak season because glycogen depletion reduces training intensity. Manipulating muscle and liver glycogen levels might be a promising technique for maximizing cellular responses to endurance training.
(European Journal Sports Science 8: 97-106, 2008)

Interesting, but how do you manipulate glycogen levels?...just go on a low carb diet for a period of time?

Very interesting indeed. What a great liaison between science and sport you are! Thanks.