The human is the power plant for all human-powered vehicles. Although the human engine is often compared to an internal combustion engine, its operation and performance characteristics are quite different. Both engines convert the chemical energy of fuel into mechanical energy, and both produce about four times as much waste heat as useful power. The similarity ends here, however. The internal combustion engine is a heat engine, operating on a thermodynamic cycle and subject to the second law of thermodynamics. This cycle is limited by the difference in temperatures between the hottest and coolest parts of the cycle. In contrast,...