Exercise is a major stressor. But it’s a major acute stressor, rather than a chronic one. It hits us, then it’s over, and we recover. When the next session rolls around, we’re better/faster/stronger/fitter. We adapt. At least, that’s how exercise is supposed to go if you have enough buffer time between sessions. Most people do provide enough buffer time between their exercise sessions to promote recovery. Many provide too much, leading to detraining. (This Mark's Daily Apple article was written by Mark Sisson, and is narrated by Tina Leaman)