@Pilot - When you exercise, you're putting stress on the body (specific and general stress).
After exercise, your body recovers and (hopefully) adapts to that stress, making you stronger, faster, more fit, etc. The vast majority of the actual improvement (adaptation) occurs
after the exercise, during the recovery from that exercise.
The key to getting the best results is to target the right type and amount of exercise to maximize your gains while minimizing the accumulated stress or time spent.
At one extreme, too much exercise and not enough recovery can lead to injury, overtraining, or more commonly: plateauing and slow progress, as your body is not able to sufficiently adapt to the additional stress before additional breakdown occurs.
On the other end of the spectrum, too little exercise (or movement in general) leads to stagnation and eventually deterioration. Your body is designed for energy efficiency. To not waste energy, it won't retain muscle, bone, or tissue that doesn't get used (adaptation!).
How much and which types of exercise are best for you?
A quick morning check with HRV (and some smart context) can tell you how much exercise your body can handle on a given day or week to maximize your gains while minimizing the time spent or the likelihood of pushing too far.
For some people, true overtraining is a real concern - like our Olympians and pro athletes, for example.
For most people, you can get better results by exercising less and experimenting with different types and intensities of exercise - while monitoring your fitness levels and progress with some basic tracking + HRV monitoring. The concept of "just do more" just leads to a bunch of wasted effort and possibly feeling like you never make gains even though you work hard at it.
Hope this helps!
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@ c-jags - Right! And so much more, if people want to use it for other things. Also, without the huge price tag and no need to replace your favorite watch or wear anything 24/7.