Looking for comfort, sleekness and low-profile is why Polar created the slim H1 heart rate sensor. It not only measures your heart rate but also seamlessly adapts to your body shape, so you can forget you are wearing it and focus on your exercise. Soft fabric material feels like clothing on the skin Slim and streamlined connector design where you won't feel any poking or discomfort New improved user-replaceable battery mechanism 5kHz coded transmission so less interferance in gym classes, bike rides, etc Polar GymLink equipped exercise machines.