Power Systems Ball Chair Kit, Includes 50cm Stability Ball and Swivel Caster Ball Chair with Removable Backrest, 22 x 22 x 31 Inches, Gray (80625) Includes 50 cm ball, chair with backrest and swivel casters, and air pump Strengthens the core back, abs, hip flexors and obliques Backrest is removable and promotes active sitting Chair measures 22 x 22 x 31 inches high. Ball is burst-resistant up to 300 pounds Balance and core exercise training aid for speed, agility, strength, power, running, football, volleyball, basketball, baseball, soccer and all other athletic, fitness, weight, or cross training