Harness the Energy of Exercise?

Notions of Expenditure is devoted to proposals to harness the energy expended in exercise. Yesterday's Wired article sets the stage:

Back in the 19th century, researchers studied prisoners at Bellevue Prison in New York as they walked on a treadmill as punishment. They produced about 100 watts each, with the energy used to grind grain to make bread for the prisoners, said Steve Vogel, professor of biology at Duke University.

"Apparently they hated it roundly," he said. Now, of course, people pay good money to do the same thing -- and use up electricity in the process.

Okay, so NoE is more of an odd art project than anything serious - but the proposals are amusing and a couple are even practical. Raj Pandian's entry includes a plausible model to support the claim that

When large numbers of children play on equipment such as a teetertotter, swing, or merry-go-round, part of the energy of their play can be converted into energy to power low-power equipment such as lights, fans, etcetera.

So let's think about this. Treadmill-walking and riding an exercise bike is easy, but how could, say, stepping on a DDR pad be made to perform useful work? Ideas? Anyone? Bueller?