Adaptive resistance exercise (ARX) Fit uses carefully matched, motorized resistance to give you a perfect workout every time. The machine matches your resistance in contracting (concentric) and relaxing (eccentric) motions, so you receive optimal resistance throughout your training. ARX’s isokinetic exercise provides an effective workout in as little as 15 minutes. ARX Fit is scientifically proven to deliver quantifiable results by combining computer-controlled, motorized resistance that can match the force you generate 100% of the time.