When we are exercising in zone 2, most of the work is being done by our type 1, or “slow-twitch,” muscle fibers. These are extremely dense with mitochondria and thus well-suited for slow-paced, efficient endurance work. We can go for a long time without feeling fatigued. If we pick up the pace, we begin to recruit more type 2 (“fast-twitch”) muscle fibers, which are less efficient but more forceful. They also generate more lactate in the process, because of the way they create ATP. Lactate itself is not bad; trained athletes are able to recycle it as a type of fuel. The problem is that lactate
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