Heart rate isn’t a good metric to be iron clad to. It’s far to variable as your stroke volume changes in response to: stimulants, sleep, stress, external heat,

Heart rate isn’t a good metric to be iron clad to. It’s far to variable as your stroke volume changes in response to: stimulants, sleep, stress, external heat, blood pressure, body temperature, and time spent running. For all of the fitness tracking that exists, perceived exertion and relative pace to your max is better.