Heart rate becomes increasingly unreliable as an intensity metric the longer a workout continues, so athletes should use heart rate to set intensity early in runs (within the first hour) and abandon it as a guide beyond one to two hours due to cardiac drift.
normativepending
Evidence Quote
“it's a good idea to use heart rate early on in your race or training session and that sets the tone for the intensity that you want to run at and as you go beyond one hour and in particular two hours you become less and less reliant on heart rate leading your intensity”
Created: 8/13/2026, 10:26:22 AM
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