Swimmers Best Power Bags are the most versatile resistance training you will find. Use on feet, calves, forearms, hips, parachute, & kickboard drag (Note only the PB40 size fits on Kickboards, not PB25 light). Each set of Power Bags Include: 2 bags 2 strap belts Each bag has 2 straps – 1 red non-elastic (ideal for around ankles) and 1 black elastic (best for the top of calves or over elbows) Sizing Info PB25 - Light For novice swimmers Medium resistance Ideal for timed sets and to train a new kick Best for ages 12 and under PB40 - Medium For advanced swimmers Great for most workouts Ideal for very strong competitive swimmers such as advanced high school and college Best for ages 13 and up that swim 10+ hours per week PB50 - Expert For expert swimmers Only recommended for college and Olympians