Humans are tremendously flexible when converting almost any food into energy, but they are both inflexible and insatiable in their demand for mobile energy in another form: electricity. In the developed world, many people seem more concerned with their cell phone battery life than with their next meal. Given the plentiful and, in many cases, increasing supply of stored onboard energy (that is, fat), could humans not generate the necessary electricity themselves? Hand-operated generators are both inexpensive and effective for short-term use. But a less distracting alternative for the long term might be to generate elec- tricity from walking, given that walking is how humans already expend much of their daily energy. On page 1725 in this issue, Rome et al. (1) describe a new backpack device that harvests far more energy from locomotion than other methods of obtaining energy from walking, while costing the wearer a surprisingly low amount of metabolic energy. It works by extracting energy through an oscillating sprung mass. Why it works so well is unclear. Perhaps the device reduces the mechanical work required of muscles to walk while carrying a load.
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