Power and speed
The efficiency of humans, animals, and automobiles, defined as the work-on-the-world divided by the chemical potential energy ingested, is about 20-25%. Energy is consumed in waste heat and in running the body and the engine. Work-on-the-world is the measured work output, at pedals, tires, wings, feet ,etc. Power is work per unit time. Power in humans and animals is developed in two separate pathways, aerobic and anaerobic. Anaerobic power is self-limiting and short duration because of several problems, including buildup of lactic acid, a byproduct. Maxiumum anaerobic power is inversely related to duration. Some individuals are capable of very short and very high power outputs. It is difficult to measure this very short term anaerobic power output (like lifting a heavy weight) because sometimes complex body movements store energy momentarily and then release it. An example is a baseball pitcher, who may be delivering over 1500 watts to the baseball at delivery, but some of this is stored energy from the windup. Aerobic, steady state power is easier to measure, and much lower. Road racing bicycles may require almost 500 watts at 33 mph in still air, near the highest sustainable speed. Above 22 mph, 90% of the drag is aerodynamic drag, which increases as the square of the speed. Power required to overcome aerodynamic drag increases as the cube of the speed. Selection of gearing and crank length produce foot speeds and forces which attempt to optimize the impedence matching between legs and machine. Power is force times speed, so the two can be traded off. Racing cyclists report that at high power outputs crank RPM of roughly 110 is optimal. Laboratory testing and some theoretical concerns show that much lower RPM, say 60, and correspondingly higher torque produces the least energy waste, but cyclists report fatigue and lactic acid buildup at high power and low RPM. Skaters and skiiers, of course, maintain cadence near 60 in admittedly different movements. The speed record for human powered vehicles on level ground is over 125 mph, a shocking speed, but the bicycle was "motor paced" and so was traveling in the wake of a pace vehicle. Drag on the bicycle is very hard to compute because the flow around the pace vehicle would be complex. Fun stuff...
