It&0s more than a heart rate monitor! The Scosche RHYTHM turns your smart phone into a comprehensive & versatile fitness device, enabling it to monitor your pulse, intensity levels, calories burned, distance & speed to help you get the most out of every training session! It even motivates you & controls your music... Lightweight & comfortable The RHYTHM is very light at only 30 grams; when it&0s on your arm, you hardly feel it, & there&0s no need for cumbersome chest straps as it takes your pulse via integrated sensors. It is supplied with three different Velcro straps to suit your arm size.