Recently on a ride someone asked me, "If you drank enough carbohydrate during the ride to match the amount you were using, would you fatigue?" I thought it was a pretty good question. We are taught to drink carbohydrate during a ride to prevent fatigue. There is no doubt that drinking carbohydrate will help prevent fatigue, however, it is not the whole story. Fatigue is a very complicated topic with as many problems as solutions. There is no "one size fits all" cause of fatigue. Different types of activity cause different types of fatigue. In the space provided, I could not possibly explain all the intricacies of fatigue. Therefore, I will greatly simplify the process and introduce two theories and three categories of fatigue.
There are two main theories of the causes of fatigue. The first is called "catastrophe theory". Catastrophe theory is very much what it sounds like; some process occurs that eventually causes the system to shut down. Most catastrophe theories are based on the concept that some product of metabolism builds up to a degree that ultimately causes something to fail. Catastrophe theory has spurned a ton of research trying to identify what THE fatigue-causing metabolite is. A typical catastrophe theory would go something like this: repeated muscle contraction causes potassium to leave the cell faster than it is taken up, the muscle cannot properly re-polarize thus contraction is inhibited. The depletion of muscle glycogen has been correlated with fatigue so this also represents a catastrophe theory. This particular catastrophe theory is why we supplement with carbohydrate during riding since it provides glucose to prevent glycogen depletion. Finally, everyone's favorite theory of fatigue, lactic acid buildup, is a catastrophe theory. More than once I have made the point that this theory is the biggest myth in exercise physiology. In summary, catastrophe theory identifies an item that builds up to an extent that inhibits the body's function.
The other theory of fatigue is called the central theory of fatigue. This theory is based on the concept that the brain ultimately controls everything in the body and therefore shuts down the body when things are becoming potentially dangerous. The central theory of fatigue is relatively new, but is beginning to get a lot of experimental support. Consider the following example, our body's enzymes (which drive almost all chemical reactions in the body) only function within a certain temperature range. Since we never observe conditions in which our enzymes quit working, which would certainly kill us, there must be a governor somewhere shutting the system down before we get to too high of a temperature. Another example is oxygen delivery. Our heart delivers oxygen to the periphery by pumping blood. But the heart is a muscle itself and needs oxygen to do work. Consequently, the heart's oxygen needs take priority over all other muscles because if the heart does not have enough oxygen to function properly, no other muscle in the body is going to have enough oxygen to function. In fact, "heart attack" is the descriptive word for what happens when the heart does not have oxygen. Since we do not get heart attacks from exercise, although different then blood clots, the brain must sense how much oxygen is in the body and shuts down the activity BEFORE we ever get to a state of oxygen limitation in the heart. Finally, the aforementioned glycogen depletion catastrophe theory could actually be used as another example of central regulation. When endurance studies are performed examining glycogen depletion and fatigue, glycogen concentration never goes all the way to zero, i.e. it is never truly depleted. Central fatigue advocates use this observation to support that the brain never lets the body get to a point of damaging itself, something that true glycogen depletion could do. The central theory is also called the central governor theory since the brain limits work output to prevent body damage. In summary the central theory uses a variety of input from the periphery, integrates all the information and shuts down the body before it damages itself.
No matter which theory you subscribe to, there are three main categories of fatigue: high intensity, prolonged endurance, and low frequency. High-intensity fatigue is the category of fatigue that is associated with sprinting or weightlifting. We are all familiar with this type of fatigue when doing short, high-intensity efforts. A catastrophe theorist would identify potassium or lactic acid as the cause of fatigue in this case. The second type of fatigue is prolonged endurance. This of course is the type of fatigue we feel at the end of a long ride when we run out of energy or our muscles become very sore. A catastrophe theorist would cite glycogen depletion as one cause of fatigue in this case. Finally, low-frequency fatigue is one that you are probably not as familiar with but is one that you probably suffer from. Low-frequency fatigue happens with long-term repetitive motion. Low-frequency fatigue occurs with such things as computer and mouse use. It has actually been theorized that all of us are in a chronic state of low-frequency fatigue. It is not actually clear to me at this point if the repetitive motion of pedaling can cause low-frequency fatigue. Perhaps this is an area of research for someone. In the case of low-frequency fatigue a catastrophe theorist would cite inflammatory cytokines as the cause of fatigue. In each of these theories I only gave an example of catastrophe theory, but not central. The central theory is a different construct; it does not identify singular causes, but relies on the integration of a variety of clues from the periphery. The brain takes these clues, integrates them, and regulates the system appropriately. This viewpoint of fatigue requires a very different way of thinking. For instance, since caffeine is a central nervous system (brain) stimulator, is this the real mechanism by which caffeine enhances performance?
There are two possible answers to answer the question initially proposed - if we had enough carbohydrate to perfectly match expenditure, could we keep on cycling? On one hand a catastrophe theorist might answer that eventually the muscle would start to damage itself and this would inhibit performance to a degree that you would have to stop. On the other hand a central theorist would counter that a variety of clues such as temperature, mental stability, energy levels and so on would be incorporated to tell the body to stop before permanent damage results. I personally have recently begun to subscribe to the idea that fatigue is mediated in the brain. Intuitively I like the idea that since the brain controls everything else in the body, except spinal reflexes, it must also tell us when we are doing too much. I also like the idea that since exercise rarely damages us, there must be something shutting down the system. Finally, since physiology is rarely a one trick pony, I like the idea that a variety of clues are simultaneously integrated to determine an output. However, both catastrophe and central are just theories and are still subject to continued experimentation.
Ben Miller is Senior Lecturer in Exercise Physiology. Ben did a PhD at the University of California – Berkeley and a Post-Doc at the Institute for Sports Medicine, Copenhagen before arriving in New Zealand. As a departure from his life in a closed scientific box safe from the realities of the world, he is a cyclist regularly taking his life in his own hands on the streets of Auckland and in the local club racing and criteriums. Ben's wife is much more successful at cycling having competed full-time in Europe and the US for the last 4 years.