What is "cardiac drift" during zone 2 training and should I care?

Cardiac drift is the gradual rise in heart rate during a prolonged zone 2 session (typically 45+ minutes) without a corresponding increase in effort or pace, driven by dehydration (reduced plasma volume requiring higher heart rate to maintain cardiac output), thermal stress (core temperature rise increasing sympathetic tone), and glycogen depletion shifting fuel substrate, and it is a normal physiological phenomenon that does not invalidate a zone 2 session if the pace remains consistent (Fritzsche & Coyle, Appl Physiol, 2000).

Practically: if your heart rate rises from 125 to 140 bpm over a 60-minute easy run while your pace and perceived effort remain constant, this is cardiac drift, not a meaningful change in training zone. Hydrating adequately (approximately 400–500 ml per hour of zone 2 exercise) reduces cardiac drift significantly. For training zones purposes, the most accurate approach is to use pace (for running) or power (for cycling) as the primary zone 2 monitor rather than heart rate alone, particularly in sessions lasting more than 45 minutes.

Honesty Scale: Solid (1) for cardiac drift as a physiological phenomenon. Solid (1) for hydration partially mitigating it.

What to do: For zone 2 sessions above 45 minutes, drink 400–500 ml of water per hour. If you have a running power meter or cycling power meter, use that as your zone anchor rather than heart rate. Accept that heart rate will drift upward in long easy sessions, maintain your effort level, not a rigid heart rate ceiling.

For the full picture, read The VO2max/Zone 2 Deep Dive