Speed and explosivness are key factors in most sports to reach maximum performance. The most effective and natural training method has been proven to be air resistance running. The Dragonfly trainer consists of a winglike foil which unfolds behind the back while running. The size of the foil (i.e. amount of resistance) is easily controlled from the belt while running, folding in/out by a single switch. After use the foil is pulled up into a soft back-holder - for maximum performance - out of the way but ready for next run.