An exercise ball is an example of what type of surface?

• A. Stable
• B. Foam
• C. Firm
• D. Unstable

Answer: (D)

Balance and muscle control depend on how stable the surface is. An exercise ball provides an unstable surface because its round shape and air-filled interior let it move in multiple directions. When you sit or stand on it, your body must constantly adjust to keep your center of gravity over the base of support. Those continuous adjustments recruit stabilizing muscles around the core and hips, improving balance and coordination. A stable surface wouldn’t require that ongoing effort; a firm surface is rigid, and a foam surface is soft and compressible but not inherently unstable in the same way as a ball. The defining feature here is the instability of the surface.