2. Energy Supply and 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 to minimize the risk of further damage.
More: 8 Core Exercises Every Cyclist Should Do
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 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 and 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.
Summary
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.
READ THIS NEXT: 7 Things to Do After Every Ride
Find your next ride.