A swimming pool cover assembly including a frame, at least one cover, and an adjustment means. The frame including at least one inflatable portion and an interconnected three-dimensional (3D) lattice of inflatable straight tubes. The at least one cover is supported by the frame to define an exercise chamber including at least part of a swimming pool. The adjustment means reduces an oxygen content of air in the exercise chamber compared with air external the exercise chamber. The exercise chamber has elongate sidewalls.