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| This graph, taken from my Garmin cycle computer, recorded on the Rice Track, probably is recording something other than classic decoupling, except perhaps at the very end. Note the relatively constant reduction of speed at a constant heart rate over the entire time of the hour and a half ride. This is something I almost always observe, but it probably reflects something other than decoupling because it happens too soon; classic decoupling normally appears after a couple of hours of riding. The fall off in speed seems to accelerate at the very end of the ride, which might be decoupling. I do not know what the relatively constant fall off in speed vs heart rate is, but one guess is that it is due to heat. This ride, like many of my rides, starts early in the morning to avoid the heat, and the temperature rises significantly over the course of the ride. |
A frustration I have reading the training literature is that it seems that nobody puts all the facts together in logical order in one place. I feel like I pick up a fact here, a fact there, and it is only months later that I can start putting all the pieces together and make sense of it. Such is the case with decoupling. A brief description of decoupling is as follows: My heart rate is normally determined by the intensity at which I exercise. If I ride my bicycle on a flat course (e.g. the Rice Track) with no wind, the faster I ride, the higher my heart rate is. If I ride at a steady speed, my heart rate stays constant, at least for a while. However, after some period of time at that steady pace, my heart rate will start to increase. This phenomenon is called decoupling. How quickly decoupling occurs is used as a measure of endurance. The longer I can ride before my heart rate increases, the more endurance I have. But, despite having read a number of well respected training manuals, it is only now, after years of research, I feel like I am beginning to get a sense of what causes it.
There a lot of indicators I can use while training to let me know if I am training at my intended level of intensity. I can monitor how I feel; if my legs feel sore or if I am breathing heavily, for example. This is sometimes referred to as Relative Perceived Exertion (RPE). I can monitor my speed using my Garmin cycle computer. Of course, the meaning of that speed is influenced by many factors: whether I are going uphill or downhill, the intensity and direction of the wind, and which bicycle I ride to name but a few. I could remove many of these variables by using the output of a power meter instead of speed to measure how much effort I am expending, but power meters cost more than I am willing to spend (in the ballpark of $1,000). And finally, I can monitor my heart rate.
Speed and heart rate are two different ways to measure the intensity of a ride. Both are affected by issues other than the intensity of the ride, so can be misleading. However, in both cases, if I stay aware of these other factors, I can allow for them and avoid being misled. Because heart rate and speed reflect somewhat different things, more information can be gathered by comparing them than by looking at either one by itself. In fact, I have used comparison of speed and heart rate many times on this blog. That is, after all, what a MAF test is. After warming up, I ride on the Rice Track which is flat and contains no traffic or stops, keeping my heart rate between 130 to 140 beats per minute, and measure my average speed over 45 minutes. If I am off the bike for two months or so, I return to a low level of fitness and that speed may be as low as 12 miles per hour. If I am at my peak fitness, that speed can be over 16 miles per hour. Another way I have used this comparison is to note not just the average speed, but how that average changes over time; is the average speed for the second half of a ride significantly slower than for the first, for example? If they are almost the same, then heart rate and speed are coupled. If they are significantly different, then heart rate and speed are decoupled. What would cause these two factors to become decoupled over the course of a ride?
Answering this question is difficult. It would seem that muscle fatigue must be involved, but much less is known about muscle fatigue than I would have expected. As recently as 2010, the our whole understanding of what limits aerobic exercise was turned upside down (European Journal of Applied Physiology (2010) Volume 109 Pages 763-770; explanation here.) I hope to return to this topic later, but for now, I suggest we consider some aspects of muscle fatigue to be an observation without explanation, it happens and we don't know why. However, one part of muscle fatigue is well understood, and that one is important; glycogen depletion. To to explain how this contributes to decoupling, I have to talk about two1 different kinds of muscle fibers, fast twitch and slow twitch,and the fuels that they can use.
The names fast twitch and slow twitch refer to technical aspects of the electrophysiology of these different kinds of muscle fibers; it is only coincidence that fast twitch muscle fibers are used when I ride fast, and slow twitch when I ride slow. Fast twitch muscle fibers are very strong but tire quickly. They are used in a sprint, for example. Slow twitch muscle fibers are relatively weak, but can keep going for hours. They are used in a long bike ride. Both fast twitch and slow twitch muscle fibers can use glucose as a fuel. This glucose can either come from glycogen stored in the muscle fiber itself or from the blood, which mostly comes from food I eat. However, the glucose from glycogen can be provided much more quickly than glucose from blood; no matter how much I eat, I will have to slow down when my muscle glycogen is gone. In addition to glucose, slow twitch muscle fibers can use fat as fuel. (Fast twitch muscle fibers cannot use fat, they can only use glucose.) Again, this is a slower process and turns out not to be all that directly relevant to decoupling, but does have an indirect effect, discussed at the end of this post. The final, key point is that glycogen cannot be shared between muscle fibers. When fast twitch fibers run out of glycogen, they cannot borrow from slow twitch fibers, and vice versa.
It turns out that the rate of availability of different kinds of fuels is a major contributor to decoupling. If I am riding at a relatively slow pace, it will primarily be my slow twitch muscle fibers pushing the pedals. However, after a couple of hours, the glycogen in my slow twitch muscle fibers will be used up. These fibers can continue working using blood glucose or fat, but not at the same rate; fast twitch muscle fibers will have to make up the difference. The problem is, fast twitch muscle fibers are stronger but less efficient than slow twitch muscle fibers; my heart will have to beat faster to accomplish the same amount of work, resulting in decoupling.
I had always assumed that there are mechanisms for fatigue of slow twitch muscle fibers other than glycogen exhaustion, but truth be told, I do not know if there are or are not. However, if there are, they should play an almost identical role in decoupling as glycogen exhaustion. Once slow twitch muscles loose their ability to function at the same level, fast twitch fibers will be called upon to pick up the slack and their lower efficiency will result in decoupling.
The above explains "classic" decoupling, where after some period of riding, heart beat rises with no increase in effort. There are other situations where heartbeat is higher than expected for a given level of exercise. Three conditions, besides fatigue, that can cause this are emotional upset, dehydration, and heat. Heat turns out to be very important, as is the correct response to the decoupling created by heat. If one is competing in a race and attempts to maintain constant speed under conditions of high heat, one will "run out of steam" by the end of the race. Rather, one should maintain constant heart rate and allow speed to drop.
If "classic" decoupling is a measure of endurance, and endurance is something I am trying to train, what are the changes in my physiology that occur during training to improve my endurance, to delay decoupling? I'm think it is highly likely that these effects are not yet completely understood, but here are two that I think are fairly well established. First, the amount of glycogen stored in my slow twitch muscle fibers will increase. This will allow me to ride for a longer period of time before that glycogen is exhausted and decoupling begins. Second, my slow twitch muscle fibers will become "better" at using fat as a fuel. Fat will never be able to completely substitute for glycogen, but the more efficiently it can be used, the more it will slow down the use of glycogen, making it last longer.
So who cares? I do, but not because I expect any of this to improve my fitness. I care because I am curious. I like to understand why my body behaves the way it does when I ride and when I train. That said, I firmly believe that our current understanding of normal muscle physiology is woefully lacking, and that as a result, we are far from being able to design a training routine based on theoretical considerations. That is why I am such a fan of coaches like Joe Friel. I think the years of experience that the best coaches have provide a much better basis for designing a training program than the kinds of theoretical understandings discussed in this post. Finally, I think for the kind of cycling I want to do and the goals that I have, training is not all that complicated; "just ride" is probably pretty good advice. Still, trying to figure this stuff out, while not all that useful, I find to be a great deal of fun.
1) There are more than two kinds of muscle fibers, there are at least three, but my simplified explanation is adequate for this post.
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