The 55cm Stability Exercise Ball strengthens and tones the abs, core, and back and supports balance, flexibility, and recovery work across a commercial floor. Use it for Swiss-ball crunches, planks, push-ups, squats, and lunges, or for posture and physical-therapy programming. This 55cm size suits users between 4'11" and 5'4"; size up to the 65cm ball for taller athletes.