Good point about the "significance" thing. However, the word "exercise" probably covers many different regimes here. If you take a pair of twins, put one on th

Good point about the "significance" thing. However, the word "exercise" probably covers many different regimes here. If you take a pair of twins, put one on the couch for 3 years, and give me the other to train for 3 years with Starting Strength/Texas Method or similar, I will make him into an unstoppable monster in every dimension you can measure: Bone density, neuromuscular function, 1 rep max squat performance, bodyfat percentage, lean bodymass gained, whatever. He will even look conventionally "ripped" by the end of it if you let me control his diet a little bit. Same story if you subject him to the training regimen of an elite sprinter or something like that for 3 years. If that same twin does Nautilus machine circuits for three years, he will probably be ~pretty much unchanged, depending on various genetic factors pertaining to sensitivity to light exercise. Probably all of the options above would be colloquially called "exercise," but they're not even approximately created equal.