Why heart rate climbs at a steady pace
Cardiac drift is the gradual rise in heart rate that occurs during prolonged steady-state exercise even when pace, power, and external workload are held constant. It is commonly observed in efforts lasting longer than roughly 30 to 60 minutes, particularly in heat. The drift means the same physical output costs progressively more cardiovascular effort as time goes on.
Two contributors are usually cited: fluid loss from sweating reduces blood plasma volume, and rising body temperature shifts blood toward the skin for cooling. Both can lower stroke volume, so heart rate increases to maintain cardiac output.
What it means for pacing and training data
Because heart rate drifts upward independent of effort, relying on heart rate alone to gauge intensity during long sessions can be misleading; power or pace is often a steadier reference. Good hydration and heat management reduce the size of the drift.
This is general educational information about exercise physiology, not personalized hydration or training advice.