Although this article is called "The Truth About Injuries," it is also a fantastic writeup of how and why exercise makes you stronger and fitter.
The Truth About Injuries
By all means read the whole article. But especially scroll down to the header "Adaptation."
An excerpt of that section:
"You see, it all comes back to the fact that the body isn’t really a big fan of stress in general. The more stress the body is under, the harder it has to work to maintain homeostasis and the greater the stressor – whatever is placing the demands on the body – is perceived as a threat.
In the context of training, the stressor, of course, is in the form of the lifting weights, running, jumping, skill practice, etc. because all these require a ton of muscular work.
In order to try to make sure it doesn’t have to work as hard next time to same stressor is faced, and thus homeostasis is less disrupted, the body responds by making physiological changes to the mechanical and/or metabolic tissues that were stressed. These tissues are made stronger and/or more metabolically efficient and thus they become better equipped to handle the same level of mechanical and/or metabolic stress it previously was faced with."
That in a nutshell is how exercise works. You stress the body, and the body adapts to ensure that next time it's more ready for that stress. It super-compensates so that it's ready for that stress and a little bit more, just in case. And that's how you get stronger. You lift 135 five times, and your body reacts by getting strong enough to pick up a bit more than 135 five times, and to pick up that 135 a bit more than five times. So next time maybe you get 145 for 5, or you get 135 for 6 or 7. And the stress/adaptation cycle continues.
Of course this doesn't continue linearly or forever. As you train the amount of stress you need goes up. As you age your body's ability to recover (or adapt to stress) goes down. But in principle this is how it works. Knowing this will help you understand why you are training, and why it "works."
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