ELI5- Physiologically, how does one increase “endurance” and what does that look like in our body?
In: Biology
I’ll write a somewhat-complex response to this, but I’ll try to keep it accessible, and if you have any questions or need further clarifications, feel free to ask!
Whenever you use a muscle, it contracts, and every time a muscle contracts, it requires energy. All the energy a muscle uses comes from a molecule called adenosine triphosphate, or ATP. However, different muscle fibers generate ATP using different methods, this ultimately determines many things, including how much strength and stamina your muscles have.
I’m not going to go into great detail about ATP production, although if you would like an indepth explanation, I’m happy to provide. The short of it is that there are two major processes that can produce ATP:
1. As a byproduct of a chemical reaction called glycolysis (glycose-lysis, or the breaking down of glucose)
2.As the end product of a chain of reactions called oxidative phosphorylation.
Glycolysis requires several specific enzymes to occur, and oxidative phosphorylation requires oxygen and takes place in organelles called mitochondria.
Some muscle fibers have large amounts of glycolytic enzymes and relatively few mitochondria. These are called glycolytic fibers, and they produce ATP, or energy, mostly through glycolysis. Other muscle fibers have large amounts of mitochondria and produce ATP through oxidative phosphorylation. These muscle fibers are called oxidative fibers.
The tl;dr here is that glycolysis is a lot less efficient than oxidative phosphorylation at making ATP. Glycolytic fibers are larger and stronger than oxidative fibers, but they’re also less good at operating for long periods of time. This is in part because lactic acid is a byproduct of glycolysis, and that soreness you feel when you exercise is in part caused by lactic acid build up.
However, aerobic exercise will convert glycolytic fibers to oxidative fibers, which increases their resistance to fatigue (since oxidative phosphorylation doesn’t produce lactic acid). This is seen in an increase in the mitochondria count in glycolytic fibers, as well as an increase in the amount of capillaries that surround them (more oxygen to conduct oxidative phosphorylation, and blood is what provides your muscles with oxygen). You also see a decrease in the average diameter of the fibers, which makes it easier for oxygen to move into cells (which is why distance runners don’t tend to be super bulky people).
Additionally, I mentioned above that glycolytic fibers are mobilized in high intensity responses. However, oxidative fibers are mobilized in low-intensity movements. As you get better at running, more of these movements are “low-intensity” for you, so you have less glycolytic fiber activity in general.
tl;dr the muscles change up how they make energy.
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