OnCourt OffCourt Z-Ball - Speed Up Reflexes / Tennis Training Aid FOR BEGINNERS: Our patented Z-Ball speeds up reflexes! Ideal for beginning to intermediate athletes ages 7 to 13. IMPROVE REFLEXES: The 7-knob design creates unpredictable bounces for athletes with beginning through intermediate skills. PRO-TIP: Use the red/yellow ball on darker surfaces. PROFESSIONALLY MADE: All our products are designed by Tennis pros with years of experience and expertise. ONCOURT OFFCOURT: We are the leading creative resource for youth tennis, training aids, court equipment and ball machines. Over 10 million players have improved using our products in over 100 countries.