Remember the scene from the Arnold Scwharzenegger Conan the Barbarian where Conan becomes a musclebound superhero because some evil bastards make him push a grinding wheel for ten years?
Let’s say Conan has to apply 50 pounds of force. That’s 222 newtons. That lever-arm looks to be about 2 meters long, for 444 newton-meters of torque at the axle. The wheel isn’t turning very fast: I’d judge about 1 rotation per minute, which is 2 * pi radians divided by 60 seconds, or 0.1 radians per second. 444 newton-meters * 0.1 per second (radians isn’t an actual unit, so we can drop it) means 44.4 Watts total power, or a pathetic 0.06 horsepower (Hey, it’s a kid pushing a grindstone. What did you expect?)
Really I just linked to this to be able to include The Wheel of Pain video. Best time-lapse montage ever.