The difference in heart rate is due to the amount of muscles being incorporated into the movements that provide propulsion. Mainly just the legs, buttocks, some back muscles. Running utilizes a lot more, most notably the above muscles and then the arms/shoulders. he level of exertion cannot be calculated solely from heart rate. Just because someone is only at 150bpm when dying up le Alpe d'Huez doesn't mean they aren't busting their ass.
Comparing the 2 wrote:
My brother is a great cyclist but the only reason he started doing it was because he was not a good at running.
And you would get your ass handed to you at basketball, baseball, wrestling, etc. Some people are just built for different sports. I am short, have a barrel chest, small hips, and large quads. Great power/weight ratio. Cycling was a natural progression for me.
*Side note: I still run, and am faster at the mile and up than I was in high school*