The Reactor Pro Exercise Ball will specifically target abs, back and glutes to strengthen and tone. Advanced users position the ball for upper and lower body workouts. Ideal for stretching and warm-up exercises. Sizes: 45cm for individuals 4'6" to 5'0" 55cm for 5'1" to 5'7" 65cm for 5'8" to 6'1" 75cm for 6'2" to 6'7" 85cm for 6'8" and up