You’re still running the same amount of miles
In: 37
because your body needs to be healthier to do it in one go, assuming you run at the same pace both times. It pumps more lactic acid which taxes your muscles, and potentially wears out your muscles and joints more
In order for muscles to grow, they need to tear a tiny bit. When the body repairs those muscles, it adds a little extra muscle. That’s why they grow when you exercise.
Running, then resting, allows your muscles to recover. They may not necessarily tear a little bit; after the rest, it’s like you’re starting over.
Running two miles puts stress on the muscles without letting them recover. This “tears” them more, allowing them to grow even stronger.
But your body is not working equally hard.
The point of exercise is to strain your body’s resources. In the case of aerobic exercise like running, the main resource that runs out is oxygen*: your muscles are consuming oxygen as fast as your lungs can provide it, and it has to resort to some unsustainable processes to keep going. In particular, your muscle cells have to swap to [lactic acid fermentation](https://en.wikipedia.org/wiki/Lactic_acid_fermentation) instead of the usual and more efficient [Krebs cycle](https://en.wikipedia.org/wiki/Citric_acid_cycle) to generate energy. This produces by-products that have to be cleared out later, but if you continually push your muscles, they build up more and your body stays short on oxygen for longer.
That forces your body to develop higher oxygen capacity, which improves your health overall. It does so by developing your heart muscle, by producing more red blood cells, and by increasing the amount of “ready to go” energy (in the form of glycogen) stored in your muscles.
(EDIT: * Strictly speaking, *extra* oxygen. Your body won’t divert oxygen away from your brain to keep your muscles going, for example. So while there’s still oxygen in your blood, it isn’t available in sufficient amounts to your muscles during aerobic exercise.)
I think it’s important to define what “healthy” means in this context, since which is healthier really depends what traits you are trying to maximize for your definition of healthy. High intensity interval training is the best strategy for burning calories and burning fat. However, running for two miles at once will improve the performance and endurance of your cardiovascular system. There’s a lot more depth and complexity than that, but the point is which one is healthier is a function of what you are trying to accomplish
It’s not considered healthier, technically.
Assuming the same pace on both runs, you’re still burning a similar number of calories and you’re still getting cardiovascular benefit, just with two recovery periods. Running it at once is more time efficient in this case, but not necessarily healthier.
Where you could really improve on the two bouts vs single bout is that you can ramp up the intensity on the two single bouts, which would ultimately have a greater outcome as those bouts add up. But the argument also remains that steady state (single bout) may be better for endurance.
It really comes does to your goals. If you’re wanting to build endurance, mix the two. If you’re wanting to increase glycolytic capacity, focus on enhancing intensity. If you’re wanting to cut fat, go lift weights and do a single bout after.
It’s not necessarily “healthier”, it’s just another set of tools in your fit kit.
i haven’t seen this answer here yet so… we as mammals have several main energy storage systems that store different scales of energy. a short exercise only touches the bottom levels. in order to “fully exercise” you have to deplete each storage stage in succession. 🤔
at the lowest biological level is ATP (adenosine triphosphate) which is the battery that powers most cellular processes. space and mass wise though ATP is not efficient. we have to produce ATP continuously to keep up. i have seen it said on ye internets that we produce and use over our own weight in ATP every day! So in exercise mode your body depletes this reserve in a couple minutes. All gone need more.
I’m going to skip a few steps up to sugar. every cell in your body has a bunch of “mitochondria” which are almost their own separate life form but are also oversimplified like a parasite such that it blurs the line of what is life. these mitochondria maintain a stable temperature a couple degrees warmer than the rest of you and that helps them create an ideal environment for turning each molecule of sugar into over 30 fully charged molecules of ATP! Without enough oxygen for the mitochondria to do their thing… we eukaryotes are forced to fall back to a system that produces like… 4 ATP per sugar atom? But the sugar is far more efficient storage wise… each atom of sugar is almost 3 times lighter than a single atom of ATP. 😲
the next step is a lot more complicated. but the cliff notes is that we can stack up sugars into what are called cholesterols and fats. fully hydrogenated fats (which are the only kind made by animals) themselves stack up for storage extremely efficiently.
so… longer exercise forces your body to develop the full chain ability to go from fat to ATP in real time. if you don’t exercise long enough your body doesn’t have to do that.
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