Cycling, while generally pleasurable, is ultimately all about pain and suffering at the sharp end of competition. Fatigue and exhaustion is something we have all felt at some time or other. What's less apparent is what causes fatigue on the bike and what we can do to avoid it.
Let's take a look at the science behind exhaustion.
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At some point during every ride, no matter how easy the intensity, you will end up getting tired and not capable of sustaining that same workload. This is true whether it's a time trial up l'Alpe d'Huez, a sprint down the Champs Elysee or a leisurely Sunday afternoon ride.
It may come as a surprise, but even after 100 years of exercise physiology research, the physiological reasons why humans fatigue during exercise remains a complete mystery to scientists. We certainly have a number of clues and strong theories, but no single model of fatigue seems to be able to accommodate all physiological mechanisms or real-life situations.
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Why do we care about fatigue or what causes it? Most cyclists are masochists, and nothing is really more important than understanding and ultimately prolonging the onset of fatigue. At its core, every race or ride at some point comes down to how much pain and suffering you can put up with. If we understand the contributing causes of fatigue better, we then determine and optimize the training adaptations needed to extend our limits and improve our performance.
Fatigue can be viewed as the first signpost that the body cannot handle a particular workload. Once lactate begins to build up in your muscles and you start to feel the burn in your legs, your workload has become to high for your muscles. In contrast to this point of fatigue, the point of termination of exercise is better defined as the point of exhaustion—the moment when you can no longer sustain that workload and are forced to slow down or stop.
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We will go with these rough definitions of fatigue and exhaustion as they relate to our interest in cycling:
Fatigue: Sensations of tiredness and associated decrements in muscular performance and function. This can generally be thought of as a safety mechanism to prevent harm within your body.
Exhaustion: The point of voluntary termination of exercise at a particular workload.
By examining each of these potential mechanisms of fatigue in isolation, we'll try to get a better handle on just how important they are in affecting the onset of fatigue, which might also help us to understand how we might extend our own limits.
1. Cardiovascular: This is the reason why blood manipulation by both legal and illegal means can be so effective for endurance cycling. The more red blood cells and hemoglobin coursing throughout your body, the more oxygen and nutrients you can deliver to your muscles. In turn, you can maintain a higher workload primarily through aerobic metabolism without going too deep into anaerobic metabolism. This focus on maximizing your cardiovascular efficiency is also one of the primary rationales behind prolonged base training prior to high-intensity training.
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2. Energy Supply / Energy Depletion: In this model, fatigue is directly due to insufficient adenosine tri-phosphate (ATP—the basic energy currency unit in the body) being supplied to the working muscles. While the body is generally very good at protecting its store of ATP inside the muscles, it's possible that low ATP levels causes sensors within the muscle to send fatigue signals to the brain. A parallel model to this is the depletion of glycogen (storage form of carbohydrate) in the muscles and liver to fuel the active muscles. Consuming at least 16 to 32 ounces of water and 30 to 60 grams of carbohydrates for every hour you spend on the bike can help to prolong energy depletion.
3. Neuromuscular Fatigue: The first two models are primarily on the supply side of the chain. With neuromuscular fatigue, we're talking about the output component. Namely, even if oxygen and fuel delivery remains adequate, prolonged exercise and repeated stimulation may cause the muscles themselves to be incapable of contracting as fast or as strong as necessary to maintain the desired workload.
4. Muscle Trauma: Above and beyond the muscles becoming "tired" or incapable of the same strength of contraction, the high force output in cycling may cause local damage to muscle cells. In addition to the actual mechanical damage, local inflammation and chemical imbalances may further limit force output and send fatigue signals to the brain in order to minimize the risk of further damage.
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5. Biomechanical Model of Fatigue: We've spent a lot of time in Toolbox over the years on efficiency, pedaling dynamics, and optimal cadence, and it all comes down to good pedaling. In some ways, this is both an individual mechanism and also an integrated model. This biomechanical model proposes that non-optimal pedaling will cause an increase in energy required to generate a particular power output, which then creates the domino effect of increased cardiovascular and energy demand, along with increased stress on the muscles.
6. Thermoregulatory Model: Over the two decades or so, a lot of research has found that individuals seem to become exhausted and voluntarily terminate exercise at a very consistent core temperature no matter what else we do to them (hydration, heat acclimation, starting core temperature, etc.). So a proposal has arisen that elevated temperature directly affects the brain's ability to recruit muscles and "drive" the body's ability to exercise.
7. Psychological/Motivational Model of Fatigue: The brain, even more so than the body, is the true black box in the whole equation of fatigue. This model is another attempt at integrating a number of mechanisms, and is predicated on the body integrating a whole host of inputs from throughout the body and then having a psychological response in order to minimize the risk of damage. In simple terms, the body screams out at the brain to slow down. How the brain deals with this demand from the body is at the heart of sport psychology.
Fatigue and our body's ability to maintain prolonged effort is built on a complex house of cards. One or more of these systems becoming weaker can cause the whole system to collapse. In the coming months, we'll examine each of these systems in greater detail to see what we can learn about improving the body and our training to delay fatigue as long as possible.
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