i do love bicycles. and it was actually spin classes back in the late 90’s that got me into it. once i was aerobically back in shape after a few years of complete inertia,. i had the confidence and wherewithal to get going ona regular bike. it’s just not the same as an epic real ride though. for one, you’re not going to spend a whole day out and about on a spin bike, unless you’re mental. the bicycle is just different.
that said, you can definitely get your heart rate up on a spin bike. i used to wear a heart rate monitor and watch it climb climb climb! scary!
but not quite like a farmer’s walk could get it. that’s a more intense overall effort….
i’m not sure high heart rate is the whole story.
then there’s the problem of just frail bird like bones from excess cycling.
we were meant to be yoked to the earth, to heavy items. man was not meant to fly on wheels.