that is approximately such) whose flat side is attached to the center of the board's underside. This allows the board to pivot in all directions, through 360 degrees. Standing on a wobble board exercises muscles that are not exercised by standing on boards that tilt in only two (opposite) directions. In almost all models, the board's length and width are about the same size; a circle is the usual shape. Wobble boards are widely used in child development, gymnasiums, sport training, prevention of injuries to the ankle and knee, rehabilitation after ankle, knee and hip injuries and physiotherapy. The basic use of a wobble board is to stand on it with both feet, and tilt in any direction without letting the board tilt so far that its edge touches the ground. Other common exercises are squats, standing on the board with one foot while keeping the other foot off the ground, push-ups (pressing down on the board with the hands while lying face-down with only the knees or toes contacting the ground), and sit-ups (with the board under the buttocks). Any exercise is much more work when a person's weight is on a wobble board than when supported by a stable and level base such as a floor. For additional muscle exercise while wobbling, some models can have an elastic stretch band attached: each hand pulls up one of the bands ends. The ends of the band fit through two opposed holes near the rim of the board, for quick attachment and detachment. A wobble board offers full rotation about the vertical axis (i.e., yawing, i.e., twisting), part rotation about the transverse/lateral axis (i.e., pitching, i.e., backward-forward tilting) and part rotation about the longitudinal axis (i.e., banking, i.e., left-right tilting). Additional movement, translation (i.e., sliding or skidding, usually unintended and unwanted), across the supporting surface is possible, except for the few wobble boards that have a stationary base. Sliding occurs much less often and usually across a shorter distance than in the case of a rocker-roller board and sphere-and-ring board. Most wobble boards are made of plastic. Wooden models are better able than plastic ones to withstand long use, such as in a gym. Some plastic models are more durable than others. Wobble boards are made by manufacturers of gym, sports and physical therapy equipment. Sphere-and-ring A sphere, either an inflatable rubber ball (such as a basketball) or a solid