The Hard Ankle Board for the Pro Fitter 3D Cross Trainer is well suited for use in early rehab and for seniors. It provides a more stable base for standing, upper body or core exercises with the Pro Fitter. The Hard Ankle Board 2 is suitable for use on a Generation 2 Pro Fitter with the black skate. Original PF has a white skate plate and PF2 has a Black skate plate with a slightly wider stance position. If you are unsure of which model you have, please contact Customer Service at 1-800-FITTER1 for more information.