Other Causes of Marathon Fatigue
The length of the marathon opens up doors for other fatigue-inducing problems. For example, running for so long can cause psychological or neural fatigue, the latter of which is due to changes in the levels of the brain neurotransmitters serotonin and dopamine. These neurotransmitters increase the perception of effort, cause you to feel tired, and inhibit the central nervous system command to the muscles.
When you sweat a lot, you become dehydrated, which decreases the plasma volume of the blood; this causes the heart's stroke volume and cardiac output to decrease. When these key characteristics of the heart decrease, oxygen flow to your muscles is compromised, and your pace slows. The relentless pounding on the pavement causes muscle fiber damage, which decreases muscle force production. Finally, since your muscles produce heat when they contract, running for long periods of time is a threat to your body temperature; the resulting hyperthermia decreases blood flow to the active muscles (since more blood is directed to the skin to increase convective cooling), reducing the ability to regenerate ATP via aerobic metabolism.
How can you combat marathon fatigue to reach your first line or chase a PR?
More: 3 Surprising Ways to Run a Faster Marathon
Run Higher Mileage
A high training volume improves many aspects of aerobic metabolism, including the number of red blood cells, hemoglobin concentration, muscle capillary and mitochondrial volumes and aerobic enzymes. Together, these improvements to your aerobic metabolism result in a greater oxygen-carrying capability and ability to use the available oxygen.
More: Should You Run More Miles?
High mileage also seems to improve running economy, or the oxygen cost of maintaining a given pace. Research has shown that runners who perform high volumes of endurance training tend to be more economical, which has led to the suggestion among scientists that running high mileage—greater than 70 miles per week—improves running economy.
Economy is improved largely from increases to capillary and mitochondrial density, the former facilitating oxygen diffusion into your muscles and the latter increasing aerobic metabolic capacity. It is also possible that the countless repetitions of the running movements result in optimized biomechanics and muscle fiber recruitment patterns. Additionally, economy may be improved by the weight loss that often accompanies high mileage. The improved body composition that often results from high-mileage training leads to a lower oxygen cost; the hypertrophy of slow-twitch skeletal muscle fibers, which are more suited for aerobic metabolism; and a greater ability for tendons to store and utilize elastic energy.
More: How Runners Can Lose Fat and Improve Body Composition
Because it's hard to prove cause and effect, it is not entirely clear whether high-mileage runners become more economical by running more miles or are innately more economical and can therefore handle higher mileage without getting injured.
