Millions of people around the world are running, cycling and rowing on machines that devour electricity. Now, eco-savvy gyms are harnessing the energy their exertions generate to power the equipment, a change made possible by evolving technology. A company named SportsArt has been working for more than four decades to develop the potential for human-powered machines to produce energy. “It wasn’t until recently that people really understood what human power could do and understand how much energy that they were currently wasting by having equipment that pulled energy as opposed to producing it,” said Britt Harris, SportsArt’s marketing manager. More than a dozen gyms worldwide now have SportsArt equipment.