I’m a fairly serious cyclist. When I train and improve, what is happening in my body? It’s obviously not as simple as “less fat, more muscle mass.” More muscle mass doesn’t help me go 100 miles when I previously couldn’t go 50.
More specific, related, questions: What changes in the human body = cardiovascular fitness? Does something about your heart and your blood vessels actually physically change? Does your body somehow become more efficient at carrying oxygen? What about endurance? How does your body “learn” to endure exercise for a long time? Besides loss of fat and gain of muscle, what are the physical changes that happen in endurance training?
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Your body adapts to withstand exercise as you repeatedly expose it to exercise. The type of adaptation depends on the type of exercise.
Your example is cycling, an aerobic exercise. This requires application of a sub maximal force repeatedly over time (unlike, say, powerlifting). You don’t need big muscles to apply that force, you just need those muscles to be able to apply it over and over again for a long time. How do you do that? By providing those muscles with the most efficient fuel: oxygen and hydrocarbons line sugar and fat. Between these two, over the course of a typical exercise bout, you’re usually shorter on oxygen while fat is abundant. So basically you need to deliver more oxygen to your muscles to allow them to sustain higher repetitive workloads.
How do you deliver more oxygen to muscles? Well, oxygen is carried by red blood cells through arteries! So more red blood cells is better, right? Sort of. Blood doping is an artificial (and illegal) method of raising the number of RBCs in the body. This does improve aerobic exercise capacity. However, natural training doesn’t change red blood cell count. Your body knows better: too many RBCs makes the blood too viscous, predisposing to strokes and heart attacks.
What about lungs? Lungs load oxygen onto RBCs. There are some changes in lungs that take place as part of adaptation to aerobic exercise but they are minor. This is because lungs are already incredibly efficient in loading oxygen onto RBCs. A healthy persons arterial oxygen saturation is 98-100% even without training. Can’t get much better than that, so better lungs don’t really change aerobic capacity much.
How about the heart? The heart takes oxygenated blood and pumps it to the muscles. So if the heart can pump more blood per unit time, the muscles get more oxygen per unit time and they can sustain higher workloads. How do we get the heart to pump more blood? Not by making it go faster, but by making the chambers bigger! This is a major contributor to your body’s adaptation to exercise: the ability to increase cardiac output (the number of litres per minute that your heart can pump). Your heart chambers enlarge. This is really easy to document with ultrasound, and is very well studied in athletes at all levels. Google “exercise induced cardiac remodelling” for more reading.
TL;DR: a major contributor to aerobic fitness is change in the size of the heart, allowing it to supply critically needed oxygen to muscles and thereby enable them to sustain higher workloads.
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