My first one would not work below a heart rate of 95, so I exchanged it. The replacement did the same thing, no readings at all until you get up to the 90's. Once there, it seems to work well in the higher numbers to 145 [my own max--I'm old]. Coming back down, it does the same thing, responds well and works until HR drops below 90, then very erratic and finally stops working. It was so cheap I will keep it because it does work on higher HR, but I would not recommend it.