Thoughts About Fatigue

The Zombie and one of his fellow Modesto Roadmen in 1965, at the very beginning of their cycling careers, exhausted from an overly-ambitious bike ride.

What is fatigue? On the surface, this seems like a dumb question. Everyone knows what fatigue is. However, as I have been "training" over the past three to five years, as I have thought and read about this question, and as I compared my personal experiences to what I read, I have continued to wonder about the following questions:

1. Is all fatigue the same, or are there different kinds of fatigue?
2. What are the underlying physiological causes of fatigue?
3. What are the best responses to fatigue? Does it differ depending on the kind of fatigue?

In response to question 1, I have tentative concluded that there are different kinds of fatigue. One reason I suspect that is that recovery from fatigue occurs at very different rates, depending on its cause. This makes me suspect that fatigue, like pain, is a symptom that can be a signal for a range of underlying conditions. At one extreme, there is the fatigue and rapid recovery (seconds to minutes) that occurs between intervals of interval training. I believe that this fatigue is fairly well understood. Fast twitch muscle fibers are active during the interval, using either creatine phosphate or carbohydrate as fuel. There is only a few seconds of creatine phosphate available, so it is soon exhausted. When carbohydrate is used as a fuel during intervals, it is metabolized anaerobically, generating lactic acid, which contributes the the sensation of fatigue. During the rest between intervals, aerobic metabolism regenerates the creatine phosphate and metabolizes the lactic acid, which allows me to ride the next interval.

The need to regenerate the fuel for fast twitch muscle fibers is not the only kind of fatigue I experience during interval training. After half an hour or so, I cannot do any more intervals, rest period or no. In fact, it will be two or three days before I can do intervals again. Why is that? The conventional explanation is that muscles are damaged by the intervals, and that days are required to repair the damage. I confess that it doesn't make sense to me that something as natural as exercise should damage muscles, but everyone seems to believe it is so, so I guess I have to accept it as true. However, this raises a further question: does muscle damage result only from the intense effort of interval training, or does it result from easy rides as well? My experience suggests that, if not the same thing, then something similar happens on slow rides, albeit at a slower rate. After I complete a day of intervals, my legs feel tired for the rest of the day; it is difficult for me to walk up stairs, for example. If I do a long ride at slow speed, I have the same sensation. Also similar is that it takes me days to recover from a long ride. If anything, it takes me longer to recover from a long, slow ride than it does from an interval workout. In fact, I have the subjective impression that long rides are the hardest to recover from and are the biggest cause of overtraining. This difference in recovery makes me wonder if fatigue from a long ride is signaling a different underlying cause than fatigue from an interval workout.

Another well known kind of fatigue that has a similar recovery time is bonking. This results from exhaustion of the glycogen reserves in the muscles. I happen to be fairly resistant to bonking, but have bonked once or twice and for me, despite eating extensively, I did not recover until the next day. This seems to be what others experience as well. The oft given advice to eat constantly on a long bike ride is intended, I think, to spare the glycogen reserves rather than to replace them, as the speed of replacement would appear to be too slow to be useful. I think bonking is a bad thing and should be avoided, but I suspect that it does not have a lot of long term consequences, and thus I will not discuss it further.

The kind of fatigue that is the most puzzling to me is that associated with overtraining. For me, this can take weeks to months to recover from. The classic explanation given for overtraining is illustrated in the following two figures (both from the Runner Academy website):

This first figure is the classic description of training; for each time you train, you first lose fitness and then regain even more fitness. If you train again during this period of increased fitness, you will become even more fit the next time. Almost everything about this cycle will vary from person to person, but typically it takes 2 to 3 days after a workout to reach peak fitness, and that you can repeat this cycle about 10 times before your fitness stops increasing and plateaus. The next figure describes what is thought to go wrong when you overtrain:

The classic explanation is that overtraining results when a subsequent workout is attempted too soon, while fitness is still depressed. The notion is that, when you do this, each cycle is worse than the one before, not better. With all due respect to Runner Academy, it is my fervent belief that this explanation must be incorrect, or at the very least oversimplified. Here are the ways my experience differs from the above two graphs:

1. Just prior to experiencing overtraining, my progress looks like the first figure, not the second. Training works fine at first but, without changing the workouts or the time between workouts, my performance declines precipitously, not gradually as shown in the second figure.
2. If one estimates timelines on the second figure, onset of overtraining and recovery take about the same amount of time, about 10 days. For me, I go from performance increasing from workout to workout, as in the first graph, to a precipitous decline in fitness between one week and the next. However, return to baseline fitness takes at least a month.
3. If it were true that overtraining results simply from damaging muscle faster than they can be repaired, then training would simply be about damaging muscle at this fastest rate. This would lead to an optimal level of fitness that could be maintained indefinitely. What virtually every modern training book acknowledges is that this is not the case, all of periodized training is based on this not being the case, and this is very much not my experience. Rather, it is very easy to reach a level of fitness that cannot be sustained. Having reached that level of fitness and won a race or completed a brevet, you must then rest before striving for that level of fitness again.

All of the above implies that there is something other than the maximum rate of muscle repair that becomes limiting when you train for months. One explanation for this commonly given in the exercise community is adrenal fatigue; that the adrenal gland becomes exhausted and is unable to produce adequate levels of hormones. As readers of this blog know, I am a card carrying member of the medical establishment, and the medical establishment firmly rejects this hypothesis. That said, it does have the advantage of better explaining overtraining, and so even if adrenal fatigue is incorrect, then any alternative explanation must involve something similar.

Even though I have no credentials as a coach, I am going to offer a speculative explanation for overtraining. To that end, let me start with a result from animal studies. It is known that you can take half the blood from a rabbit without killing it, it will recover and be perfectly healthy afterwards. In fact, if you take half the rabbit's blood on Monday, you can do the same again on Tuesday. However, if you try it again on Wednesday, the rabbit will die. The reason is that the rabbit has "stem cells" that it can use to regenerate half its blood in a single day. However, once these stem cells are so used, it takes many weeks for them to regenerate. I wonder if, in successive cycles of super-compensation, I am using up muscle stem cells, and that once these stem cells are exhausted, I cannot recover from training until those stem cells have been replaced. As we get older, we lose the ability to replace stem cells, which would explain why training is so much harder for me today than it was 40 years ago. Of course, this is just wild speculation with no experiments whatsoever to back it up, but it is an idea I find intriguing.

Despite being rejected by the medical establishment, the adrenal fatigue hypothesis explains more than my stem cell hypothesis. To explain why, I need to describe one final piece of the puzzle; that things other than cycling affect cycling performance. If I am ill or stressed at work or worried about something, my ability to train is reduced; I overtrain sooner and respond less well to exercise. Thus, there must be some global reservoir of energy in our bodies that is shared by everything we do. We can deploy that energy for training, for work, to deal with problems at home, or to fight off an infection, but energy deployed for one purpose is no longer available for others. As a member of the medical establishment, I do not believe that the reservoir of energy is the adrenal gland, but I do have to say that some such reservoir must exist.

I remain puzzled about the underlying causes of fatigue, especially the long-term fatigue associated with overtraining. Fortunately, I think my ignorance is of little practical consequence. So long as I rest when fatigued, I think I can avoid the consequences of overtraining, even without understanding its cause. Just over a year ago, there was an interesting discussion on this blog about the "Recovery Based Training" approach developed by Iron Rider. Iron Rider has found that he can use changes in his resting heart rate to guide him as to when to train and when to rest. I have not been able to use my resting heart rate for this purpose, it does not change depending on my level of fatigue the way his does. However, I have been using other measures of fatigue; persistent muscle tiredness, being unusually grumpy, and disappointing performances during training; for the same purpose. I have been trying to pay close attention to my level of fatigue and moderate my exercise accordingly. I feel like this has been helpful, but have not been doing it long enough to be sure. I will be reporting on my experiences in future posts, so stay tuned.