But that open-water swimming example aside, pound for pound, men still generally outrun, outwalk, and outcycle us. Female world records from the 800 meter to the marathon are about 11 percent slower than those held by men.
Why? Well, for the same reason that a Prius will have to pull some wily moves if it wants to race against a Mustang—we start with a smaller engine. As a woman, you have a smaller heart, smaller heart volume, smaller lungs (25 to 30 percent less capacity than men), and lower diastolic pressure (the pressure in the arteries when the heart is resting between beats and the ventricles fill with blood), which predisposes us to have lower maximum heart rates and greater problems with dehydration in the heat. This also means we pump out less oxygenated blood with every beat—about 30 percent less cardiac output than men.
Less oxygenated blood means we have to breathe more often, and as a consequence, our respiratory muscles—such as the diaphragm and intercostals between our ribs—need to work harder and use a lot of energy. Like other skeletal muscles, the contracting respiratory muscles require enough bloodflow to meet oxygen demand. If you have a greater oxygen cost of breathing, you also likely dedicate a greater amount of bloodflow toward your respiratory muscles during maximal exercise. When you push the pace and breathe hard, it can be difficult to race against the guys because less bloodflow is going to your legs.
Testosterone also gives men a bit of an edge because the male sex hormone increases the production of red blood cells, which absorb and carry oxygen to working muscles. On average, men have 6 percent more red blood cells and 10 to 15 percent more hemoglobin (which is the molecule in red blood cells that carries the oxygen) concentration than women.
Our combined smaller heart and lungs and lower oxygen-carrying capacity means we have a lower max VO2 (the maximum amount of oxygen your body can use to make fuel) than men, about 15 to 25 percent lower on average, as shown in the chart below. So if two athletes are doing the same amount of work, the woman will have a higher heart rate and need more oxygen to get the job done.
Because of our hormones, we also use energy differently during aerobic exercise. We’ll get into this in much greater detail in the following chapters, but in general, because of our high estrogen levels, we rely less on carbs and more on fat than our male counterparts. That sounds like a good thing, and in some ways it is, since fat is the main fuel for aerobic exercise. But it’s not such a good thing when we need to go really hard, because that tendency to spare glycogen (which is really strong during the high-hormone phase of your menstrual cycle right before your period) can make it harder to hit high intensities. We really need those carbs to fuel the anaerobic energy system when we push past our threshold. If you’re running low on carbs in your bloodstream, it may mean slamming on the brakes instead of hitting the gas because your body just can’t get the glycogen stores it needs to make the energy you want.
Speaking of energy, because men have bigger type II fibers and the energy-producing enzymes that go with them, they have a higher glycolytic capacity than women, which is a fancy way of saying that they can burn through more glucose in the absence of oxygen. That helps them outperform us in short-intense bursts of effort, but it also means they accumulate more lactate (a chemical your body makes and uses for energy during very high-intensity efforts; accumulating more than you can use leads to muscle acidity or “the burn” and forces you to slow down) and need longer recovery time for all-out efforts. Women, on the other hand, have a greater advantage in the endurance world, as our type I fibers are much more efficient at using fat as fuel and sparing glucose.
Finally, women are also more likely to sweat out excess amounts of sodium and are more likely to eat into their muscles for energy. We also have a harder time rebuilding and repairing those muscles after exercise during the high-hormone premenstrual time in the cycle when progesterone levels are high.
What’s a woman to do? Well, let’s go back to that wily Prius for a moment. Sure, that Mustang is going to beat her in a drag race. Maybe even in a race across New Jersey. But that efficient little vehicle will hum along much longer on less fuel and may even beat the high-horsepower vehicle in the long run.
On the pointy end of the field where the very elite athletes are, the fastest woman probably won’t ever break the tape in front of the fastest man because they are too close in body size (top marathoners—male and female—often weigh within 5 pounds of each other). But for the rest of us, it means hanging with and passing or “chicking” the dudes is very much in the realm of possibility, so long as we know and work with our unique physiology. In this case, it’s a matter of building up your plasma (the watery part of your blood) volume through training and feeding your body what it needs to keep your metabolism humming, which we’ll cover in great depth in the chapters to come.
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