the stationary bike, the treadmill

[guess]

hopefully, i'll be clear. i've noticed that when i run, compared to when i bike, i'm feel as if i've worked harder--but, when i bike it's harder to finish the session (more often during biking i really want to stop but i don't). what does this mean w/ regard to doing cardio work? is that at all clear?

[Brock Samson]

You are using less energy to cycle but you will probably get a higher avg HR from cycling. I have had many runners tell me that cycling has made better runners due to an increase in VO2 max. I don't know if this helps explain your circumstances but you are improving your VO2 by cycling.