Have you ever had one of those days at the gym where everything’s just extra hard (yeah, I know, “that’s what she said!”)?  (Advertisement) This is why I wear the Polar RS400 heart rate monitor (End Advertisement)!  Seriously, I felt totally broken yesterday.  On a normal day I can do cardio for between 60 and 90 minutes and bounce between zones 4-5 the whole time.  Most machines have a heart rate program where they will do that for you, which is great because I hate monitoring my watch or watching a timer for the entire workout.