In the late 90s and early 2000s, leisure centres and exercise videos across the world were invaded by very large, bouncy balls. Initially used by physios for rehabilitation, they quickly became mainstream. Why?
Because exercise balls are an excellent tool for developing functional, ‘real-world’ strength.
With a machine and even some free weight exercises, you can only move in one direction and therefore only recruit the main muscle groups. Exercise balls, however, enable you to work the prime movers, plus all the stabiliser muscles that support the movement.