Which technology keeps track of heart rate during a workout

Karen is waiting by the lockers before school for her friend richard. they had agreed to look over the history review sheet together to be ready for the test later that day. richard shows up late with uncombed hair and wrinkled clothes. karen: "you're five minutes late and you look like a wreck! " richard: (slams his locker) "you know, i don't have time for this." karen: "fine! figure out the test on your own! "