An elastic ball filled with air to a diameter of approximately 35 to 85 centimeters (14 to 34 inches) and intended to be used as an unstable surface on which to perform exercise to improve balance and posture. In contrast to exercising on a hard flat surface, the instability of the ball engages more muscles to maintain balance; it may also improve the strength and flexibility of muscles over time. The core body muscles, the abdominal and back muscles, are typically the focus of physical therapy programs that utilize this device which is also referred to as a Swiss ball, birth ball, Pilates ball, and body ball.