4. Long races are best run at times of the month when estrogen is high.
Estrogen is a runner-friendly hormone. It influences many aspects of a runner's performance, including a shift in metabolism toward a greater reliance of fat when running at a submaximal pace. Relying more on fat means your muscles' limited store of high-octane fuel—carbohydrate—is conserved.
When male rats are given estrogen, they rely more on fat when running on a rat treadmill and can run for longer periods of time (for obvious reasons, this research can't be done on humans).
More: How to Use Your Menstrual Cycle to Become a Better Runner
5. Your lung capacity has nothing to do with your ability to run.
At first glance, distance running seems to have everything to do with big, strong lungs. After all, it's through our lungs that we get oxygen. If the size of our lungs mattered, you would expect the best distance runners to have large lungs that can hold a lot of oxygen. However, the best distance runners in the world are quite small people, with characteristically small lungs. Total lung capacity—the maximal amount of air the lungs can hold—is primarily influenced by body size, with bigger people having larger lung capacities. There is no relationship between lung capacity and how fast you run a 10K. I've measured this myself in the lab.
Studies show that the lungs don't adapt to training or limit the ability to perform endurance exercise, especially in untrained people. That limitation rests on the shoulders of the cardiovascular and metabolic systems, with blood flow to and oxygen use by the muscles the major culprits.
Apart from lung volume, there are other aspects of the pulmonary system that affect running performance, gas exchange being the most important. However, in healthy people, the lungs are more than adequate for this gas exchange to occur.
If you've been told to take deeper breaths when you run to get in more oxygen, don't listen. At sea level, your blood is nearly 100 percent saturated with oxygen, even when running fast. Taking deeper breaths doesn't get more oxygen from the lungs into the blood. At sea level, the main stimulus to breathe is the partial pressure of CO2, not O2. In some elite runners, there is a diffusion limitation between the alveolar wall and pulmonary capillaries because of a very high cardiac output, which leads to a desaturation of oxygen on hemoglobin. However, in non-elite runners without pulmonary pathology, the lungs do not limit exercise performance.
6. Your muscle fiber type will dictate which races you'll be best at.
Humans have three major muscle fiber types, with gradations between them, the proportions of which are genetically determined. If you have lots of slow-twitch aerobic muscle fibers, you'll be good at long-distance races; if you have a lot of fast-twitch anaerobic muscle fibers, you'll be good at sprints. If you have a 50/50 mix or 60/40 mix of slow-twitch and fast-twitch, you'll be good at middle-distance races, like the 800 meters and mile.
Since it's impossible to know what your dominant fiber type is without getting a muscle biopsy, the only way you can gain some insight is by running many different races and doing different types of workouts over a number of years to see what you're best at. But once you know, train according to your fiber type. If you have 70 percent fast-twitch fibers and 30 percent slow-twitch fibers, you could get through a marathon if you really want to run it, but it's going to be tough road to hoe. You always want to train to your strengths; you'll be most successful when you listen to the genes your parents gave you.
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