Air Bikes
Air bikes use a large fan for resistance, so the harder you push, the harder it gets. Arms and legs work together, which makes them a staple for conditioning in CrossFit boxes, bootcamp studios, and athletic training rooms. They need no power and hold up to brutal use. When comparing models, look at the fan housing, belt drive versus chain drive, seat adjustability, and the monitor's interval programs. Used air bikes are common and easy to inspect, though you'll want to check bearings and the crank for play.