An exercise apparatus includes a substantially flat, rigid platform for supporting a person's weight. A first inflatable tubular support includes a fastener for clamping the platform to a top surface of the first tubular support. A second inflatable tubular support includes a fastener for fastening a top surface of the second inflatable tubular support to a bottom surface of the first inflatable tubular support. The first inflatable tubular support is inflatable to a first inflation pressure and the second inflatable tubular support is inflatable to a second inflation pressure separately and independently from the first inflation pressure to effect a vertical instability and a horizontal instability of the rigid platform relative to a support surface under the second tubular support.