Endurance training leads to muscle atrophy? Kind of.

I'm reading a book by Steven Vogel (2001) called "Prime Mover: A Natural History of Muscle." It takes a very broad view of muscle, from how its cellular machinery was discovered, to how other animals use muscle tissue to do a lot of different things, to how human muscles are arranged to allow our own specialized movements. It's a geeky book being enjoyed by this blogger geek. Anyway, Vogel makes this statement on page 90 that I find interesting, if a little puzzling:



Steven Vogel (NASA photo)

"Long-distance runners may be skinnier, on average, than the rest of us, but for all their training, they don't look radically different. Atrophy of fast white fibers about balances hypertrophy of slow red fibers. Endurance training, sustained activity against small loads, does induce increase in muscle mass--but the mass of the heart rather than that of the [muscles of the] skeleton."

I am puzzled by a lot of Vogel's statements throughout the book, and this quote is no exception. First, it isn't clear to me what his point is about how long-distance runners might look different from the rest of us. I think what he is saying is that they have little fat on their bodies, but their muscles are no more or less developed than other people. In fact, even though their overall muscle mass isn't different, the composition of their leg muscles is different. Vogel is saying that as one trains for endurance (long-distance running instead of short-distance sprinting), the muscle fibers that are involved with sprinting (white, fast-twitch fibers) will atrophy and the muscle fibers involved with endurance running (red, slow-twitch fibers) will grow in size. But then the last sentence of the quote above contradicts the second one; Vogel says that endurance training does not lead to hypertrophy of any skeletal muscle, but instead increases the mass of the heart muscle. This is consistent with college physiology textbooks, so I assume Vogel is only talking about the heart when he is talking about red muscle fibers going through hypertrophy. He needs to be more clear.

Regardless, I hadn't thought of how my own training is affecting my muscle fiber proportions. My swimming workouts have so far concentrated on distance and not speed, so I am increasing the capabilities of slow-twitch muscle fibers. Similarly, the running I do and the cycling both lead to more efficient (if not larger) red muscle fibers. My weight-lifting workouts probably are increasing my fast-twitch fibers, but not to a great degree. Given that I do a lot of repetitions (2 sets of 15 reps) of each exercise, I'm not sure whether this is doing more for my fast-twitch fibers or my slow-twitch fibers.

Emotionally, it makes little sense that doing more and more endurance training would lead to muscle atrophy (of white fibers), but scientifically I understand how and why this would be true. And it would only be true if endurance exercises were the only kinds of exercise employed. It is a valuable point that in order to keep and grow both kinds of muscle fibers, you need to do different kinds of exercise. I'll have to think seriously about how I train to make sure both muscle types are being exercised. I don't want anything to atrophy. That would be ridiculous!