To understand this, we need to view everything in life through the lens of work/stress/energy. Every breathing moment of your life is work/stress/energy (let's just call it stress). When you breathe, that's stress on your lungs. When you sit, that's stress on your skeleton and muscles. When you stand up, that's even more stress on your skeleton and muscles. When you walk, when you think, when you speak, when you digest food; everything is stress on your system. It's work your system has to do, and it can do it better or worse, more efficiently or less efficiently. A crucial distinction is chronic stress vs short-term stress. Chronic stress occurs all the time. That's the stress of everyday life. That's the stress you want to reduce the effects of, because it's constant. Working out on the other hand is short-term stress. It only makes up a small fraction of your day (1/24th if you work out 1 hour a day). It's in the larger scheme of things not very important (only if you train e.g. 12 hours a day). Now, what working out does is it makes you more capable of handling stress in general, and also chronic stress, the stress you want to reduce the effects of. When you train your ability to pump blood to the right organs, when you train the ability of your cells to perform metabolic processes, to breathe, to walk, to stand; when you train your body to respond to stress, you will adapt in such a way that you will be better able to handle stress. And because working out only induces short-term stress, which again is only a small fraction of your day, and because it reduces the effects of chronic stress, the stress of being alive as a human, working out will have a positive effect on your lifespan (as stress is virtually synonymous with reducing your lifespan; it's your life-clock). Again, there are cases where working out too much could start to tip the balance in the wrong direction, which could be the case for those who work out intensely for say 12 hours a day (which is no one); I've done the math on this and it seems to add up. But if you are 99.999% of people who work out somewhere below that, working out will positively impact your lifespan. This is also why I think you can probably never replace working out with e.g. drugs like Mike Israetel might wish could happen with things like myostatin inhibitors and the like. Working out produces a holistic response on the system, it's not something you can fix with one receptor or one chemical. It's the entire system responding to something the entire system is doing. It will probably not be reproduced by anything, or something which is indistinguishable from magic. Or anything which could reproduce it would look something like working out in a big way (taking a streak from Bernardo Kastrup's "when we'll be able to create conscious AI is when we'll be able to create life from non-life; abiogenesis", essentially deconstructing the AI vs life distinction). So maybe you could sit in a chair that makes your muscles contract spontaneously without you having to do anything, but this also means you lose things like coordination, balance, movement patterns. In reality, it seems like you have to be willing to get off your ass at some point and work out.