- Heart-rate zones approximate intensity domains.
- Individual thresholds are more useful than generic percentage charts.
- Heat, fatigue, dehydration and prolonged exercise can shift heart rate at the same pace.
Why training zones exist
Endurance intensity can be described broadly as work below a first physiological threshold, between first and second thresholds, and above the second threshold. Research uses ventilatory thresholds, lactate thresholds, critical speed and related markers. Heart rate is a practical field proxy for those domains.
LT1 / first threshold: the easy domain
Below and around the first threshold, runners can accumulate substantial aerobic work with relatively manageable metabolic stress. This area is central to easy running and many long-run segments.
Threshold / second threshold: controlled hard
Around the higher threshold, training stress rises substantially. Work here can be very valuable for distance runners, but it is not a sensible default pace for nearly every run.
Why a percentage of max heart rate is not enough
Percentage formulas can be a starting point, but different threshold methods do not line up perfectly in every athlete. Personal anchors that are repeatedly confirmed by performance, RPE and physiological response are more useful than blindly trusting a generic chart.
Heart rate is an anchor, not the only judge.
PACE combines personal HR anchors such as max HR, resting HR, easy/LT1 and threshold references with pace, RPE and workout type. Under heat, wind or demanding terrain, interpretation can lean more heavily on internal effort instead of forcing nominal pace.
When context matters more than one value
An unusually high heart rate during heat, poor recovery or dehydration may reflect real additional stress. A single spike can also be sensor noise. Better training control looks for agreement between several signals.
Scientific sources
Casado et al. (2022): Periodization and Intensity Distribution in Distance Runners.