Unweighting exercise treadmill

An unweighting system for unweighting a user while using an exercise machine is provided that includes a treadmill exercise device having a treadmill frame and a rotatable belt, an unweighting assembly for applying unweighting force to the user, a system frame, and a substantially airtight chamber. The system frame includes the treadmill frame, frame elements structurally attached to the treadmill frame, and a top surface region adapted for user engagement during exercise having a surface width and a surface length. The top surface region includes a movable running surface defined by the rotatable belt that extends a run length, and a stationary deck extending about at least a portion of the movable running surface. The stationary deck defines a sealing surface having a maximum seal width greater than the belt width and less than the surface width, to which the substantially airtight chamber is attached.