system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: why my sides hurt after going for a short run but not after an hour bike ride.
gpt
Bikes have wheels to keep momentum, so you don't always have to pedal.  When you run, you have to constantly maintain your pace.
 Your feet follow a circular path on a bike, so you really only have to push down with your legs to generate acceleration.  The motion of one leg will bring the other foot passively back into place for the next cycle.   A jogging stride involves a plant, swing, and push motion, which is much more intensive.