Regarding the RPM (revolutions per minute) while bicycling: I have heard that a too low RPM is bad for your knees. Both too low and too high is a waste of energy. What is the optimal RPM when cycling?
The general wisdom is that 80-90 rpm is ideal for reduction of muscular fatigue and 80-100 rpm (possibly as low as 60 rpm) is best for ideal oxygen usage. But those are values for trained cyclists. More recreational cyclists have been found to have the longest cycling time around 50 rpm. Basically, the faster your cadence, the better the blood flow, but the more that you have to actively "push" to keep pedaling at that speed, creating a tradeoff.