I mean, we all know that if you go running daily, you’re gonna be exhausted the first day but will do much better after 2 weeks. But what happens in your body when your endurance improves? Does stuff grow? Is it all muscles? What changes so you’re not out of breath as quickly anymore? What actual physical changes happen?
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first, as you work out your muscles, you are tearing them to some extent. they redevelop stronger after recovery, which is why you feel sore afterwards.
lactic acid builds up in the muscles as you workout as well, which is the sting you feel while working out. as you train, your lactic acid tolerance increases, helping you go longer.
with stronger muscles, oxygen and energy is used more efficiently at the end.
People have written books about all of the processes involved in improving endurance performance, but I’ll take a quick shot.
The biggest effects are on improving the strength of your heart, increasing the number of red blood cells to carry oxygen, increasing the amount of capillaries in your muscles, and similar changes, all of which get more oxygenated blood to the muscles more quickly.
Endurance athletes also get better at converting sugar into energy. Muscle cells develop more mitochondria, muscles store more glycogen (i.e. sugar, which convert to energy much faster than stored fats), the body adapts to produce more enzymes that aid the sugar-to-energy conversion and clear waste products that build up from the process (i.e. avoiding lactic acid build up), etc.
Smaller effects come from developing the muscles and tendons to be more powerful in returning energy through the running stride, which makes them more efficient at generating power from the same amount of oxygen/fuel use. Then there are also other smaller effects, such as the body adapting to produce less of certain hormones that signal fatigue as people run longer distances (primarily a marathon and longer issue).
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