Can fitness gyms across the World use their treadmills to generate Green power?

Could gyms across the World use their treadmills, bikes & weight machines to generate electricity? And provide power to their own lights and maybe even to the grid – practical or impractical?

Much of the homeless have always been Green by recycling cans. Maybe homeless shelters could get treadmilss from Gyms acquiring new ones and have an option for the homeless to make some money by generating electricity.