The Polar Pro Chest Strap works with all Polar heart rate sensors to measure your heart rate activity. With a soft textile material, slip-preventing silicone dots, and a secure buckle, the Pro Strap is comfortable to wear and stays in place even through intense workouts. The strap features interference-preventing electrodes that make it extra sensitive to your heart's electrical signals, while providing a precise readout. The strap is designed to fit people with chests 20.1 to 26.0" around. The Pro Strap requires a separately available heart rate sensor to work. It cannot measure your heart signals without one.