So essentially there are two ways to get to these type 2x fibers. One is by doing interval training, and one is by doing long-distance running at a moderate, at least moderate, pace. Now, if you’re doing it like these very long distance runners who run somewhere between 8 and 9 minutes a mile pace forever, then they probably never get to those fibers because they’re able to do their exercise utilizing fat as a fuel to a large extent. When you exercise at a higher level, i.e., 7 minutes a mile and faster, then you just can’t do that on fat. Glycogen has to be used, and eventually it becomes depleted in the fibers that are being used. And when that happens, others get called in to take over. -Peter Snell
MY COMMENTS: Find a way to recruit ALL of your muscle fibers- forcing them to deplete of glycogen. This is one reason why hard interval sessions are important in preparing for marathons.