The relationship between exercise intensity and fatigue has continued to fascinate me and I am constantly prowling the Internet for clues to help me understand what, for me, continues to be a conundrum. Of the many ideas out on the Internet, which ones should I believe? One approach is to purchase a textbook on muscle physiology and exercise to get a solid foundation in the basics, but textbooks are expensive and up until now I have not been willing to do that. Finally I realized that the cost of a textbook could be justified not in terms of any impact it would have on my cycling but rather in satisfying my curiosity, so, after reading reviews on various texts, decided to purchase this one. What have I learned from it?
The first thing I learned is that much of that I thought I knew, I didn't. Coaches proclaim with great certainty facts about how muscles, exercise, training, and fatigue work, facts this book questions. It's not that what these coaches say is necessarily wrong, it is rather that it isn't really known if it is right or wrong, the data just isn't there. In contrast, when this book states a fact, the evidence for that fact along with the reservations with which this evidence should be taken are clearly stated in detail, and thus those facts can be trusted to whatever level of confidence the evidence justifies. This book also tends to be thorough, there are very few of those missing links in the chain of logic that I find so frustrating in other sources. I am not really criticising coaches, they have a job to do and when science fails to provide them with the information they need they have little choice but to make their best guess based on their experience. To the extent I have a criticism at all it is that these guesses are sometimes presented as facts rather than working hypotheses, and this book has helped me tell the difference. On the other hand, though I do not regret for a second purchasing this book, I don't think it is perfect, there are occasional gaps in what it covers that I find disappointing. (I will point these out later in the post.)
So much for the things I unlearned, what did I learn from this book? This book has 200 pages (relatively short) divided into 15 chapters. It is an introductory textbook so covers some very basic, theoretical material that has almost no impact on my exercise and training but provides the foundation for understanding what comes later. My plan was to read this book from front to back, word for word, but so far I have been unable to do that. When I start reading it I cannot stop myself from skipping ahead to the parts that interest me most, using it more as a reference than a textbook. Nonetheless, by this point I have read most of it. The first two chapters cover the basic structure and cell biology of skeletal muscle. I remember learning much of this material during my biochemistry classes oh so many years ago but of course have forgotten much of what I learned, some of what I learned is obsolete, and so reviewing this material was helpful. Chapters 3, 4, and 5 cover how the nervous system controls the muscles. This is the beginning of material that starts overlapping with what coaches talk about. Chapter 6 describes the kinds of "fuel" (energy sources) muscles use, when and why they use these different sources, and the implications of that. This largely confirms what coaches say, though I found it helpful to have a complete discussion together all in one place. Chapters 7 and 8 covers in great detail experimental evidence about the effects of heat on muscle function and other related topics. I confess I have not fully taken advantage of these chapters. Chapters 9 through 12 were the candy that repeatedly distracted me away from whatever chapter I was attempting to read, four chapters on various aspects of fatigue and muscle pain. Those four chapters, along with Chapter 14 on muscle growth and ageing were my favorites in the book. I expected to enjoy Chapter 13 on Energy Efficiency, but it turned out it was too theoretical and removed from real world exercise. The book finishes with a chapter on muscle diseases, far from my primary of interest and a candidate for the chapter I may never read.
So now to the specifics. This book confirms the widespread belief in the training community that human skeletal muscles are made up of three kinds of muscle fibers named, in modern terminology, I, IIa and IIx. (In older terminology, IIx was named IIb.) Type I is low power high endurance, IIx is high power low endurance, and IIa is in between. A concept central to the training literature (confirmed by this book) is that as ride intensity (e.g. speed) increases, use moves from type I to type IIa to type IIx muscle fibers. Another concept not as commonly discussed is that as ride volume (e.g. miles) increases, a low intensity ride where one is using predominantly type I fibers will start using IIa and then IIx fibers as type I fibers start exhausting their glycogen reserves. The following figure from the book confirms both these ideas:
The upper panel varies time at a constant intensity of 31% VO2max, a very low level of effort, a level that a cyclist would typically use for a recovery ride. An interesting point in the graph is that at this intensity, pretty much only type I muscle fibers are used until they have largely exhausted their glycogen reserves which occurs after two to three hours. The book does not suggest any significance to this fact (being very stingy with speculation) but I wonder if recruitment of type II muscle fibers might have something to do with decoupling, the increase in heart rate that occurs at about this point in a long ride in the absence of any increase in effort. The lower panel keeps the time of exercise constant at 120 minutes but varies the intensity. So far, I have found this lower panel less useful.
Imagine I come home from a ride with my glycogen reserves depleted, what happens then? One of the most common pieces of advice from the exercise community is to eat carbs after a ride to replenish glycogen. How long does that take? This next figure from the book addresses that with results that I found striking:
What about this did I find striking? First, that it takes a full three days to reach maximum glycogen. Second, that the supercompensation is so immediate and so large. After a single bout of exercise, glycogen reserves are increased two and a half fold!
In the past on this blog I have cautioned against overinterpreting results from a single study, and yet the last two figures are just that. Why am I taking them so seriously? It is a matter of trust. This book comes highly recommended from a variety of credible sources and so I assume that, although the figures are drawn from a single experiment, the results they show are representative; that the authors of the book made sure there is enough other evidence in the scientific literature in support of these particular experiments so that they can be trusted.
Examples of other things I learned from this book are:
- Lactate transport out of the muscle and into the blood where it can be carried away is what limits the rate of anaerobic glycolysis in muscle, not the rate at which glycolysis occurs. In evidence of that, blood lactate remains elevated for 10 minutes after the end of exercise.
- The muscle pain that occurs during high intensity exercise correlates with potassium levels in the fluids surrounding the muscles, not those of lactate or pH.
Limitations of This Book
I complained at the top of the post that there were limitations in what this book covers. I'd like to moderate that complaint a bit, it is possible that what I am seeing as a limitation is a strength, that the information that I see as missing is not missing because the authors failed to put it in the book but because it doesn't really exist. Similarly, there may be "facts" that seem plausible, seem like reasonable extensions of what is known, which most experts would bet on, but which have not been experimentally verified with a sufficient degree of rigor to satisfy the authors of this book. When I would hit one of these gaps, I would run off to Wikipedia or search the Internet trying to fill it, and I often could. But could I trust these sources? As I rule, I tend to trust Wikipedia, but I'm sure it has its own strengths and weaknesses. Although I am sure that in some cases my alternative sources are wrong, I am pretty sure that there are other cases where the information I find elsewhere could and should have been in the book. So, with all those reservations, here are some examples of gaps:
- The two kinds of fuel used by muscle to generate ATP and thus muscle power are fat (lipids) and carbohydrate (glucose/glycogen.) The book discusses the importance of the kinetics of anaerobic glycolysis, oxidation of glucose, and oxidation of lipids to the endurance athlete and thus the importance of utilizing lipids to spare glycogen but does not discuss how long the lipid stored in muscle lasts, the kinetics of utilization of muscle lipids vs lipids from other parts of the body, nor the impact of glucose from foods eaten while exercise, the information needed to build a complete picture out of their kinetic argument.
- I was delighted to read about Satellite Cells in Chapter 10 of this book. These seemed like they might be the muscle stem cells in which I am so interested. It turns out they are, but I had to go to Wikipedia to find that out.
I think I am working myself up to a series of posts on exercise and fatigue, and if I am, this is the first of those posts. The way this topic is organizing itself is that I am presenting it in multiple posts and getting the foundational material out of the way first so that the climactic post will be shorter and more readable. If I pull this off, this post will be the first of those foundational posts. In any case, I expect to refer back to this book often.
