I think it is "Running vs. Cycling" and "Walking vs. Bicycle Riding"

If you go for an easy 30 minute run you will use more energy than if you go for an easy bicycle ride. You need to be determined to ride at a pace that will elevate your heart rate to the same level as simply running. If I run around the block, I will return winded and my HR around 135. I can ride my bike around the block and return with no significant change in respiration or heart rate.