Training Foundations

Why Your Heart Rate Lies by Day Three: Trusting Effort Over Numbers in Multi-Day Events

By the third consecutive climbing day, your watch and your body are no longer telling the same story.

On day one of a multi-day mountain event, pace and heart rate roughly agree with how hard something feels. By day three or four, that relationship quietly breaks down. The same climbing effort that produced a heart rate of 140 on day one might read 150 or higher by day three, for no change in actual output. Pushing to hit that same familiar number now would mean running well past your real capacity, and it is one of the more common ways strong athletes unravel late in a stage event.

This shift has a name: heart rate drift, the progressive rise in heart rate that occurs during sustained effort or across a multi-day event even when the underlying effort stays constant. It is driven by rising core temperature, accumulating fatigue and, for many women, hormonal factors that raise resting and exercise heart rate independent of anything happening in the legs. The mechanism differs slightly day to day, but the practical effect is the same: heart rate becomes a progressively less honest narrator of effort the longer an event runs.

This is precisely why perceived effort, RPE on a 0 to 10 scale, is built as the primary pacing tool rather than a backup to heart rate or pace. RPE integrates everything that distorts the other numbers: terrain, heat, hormones, fatigue and altitude, all in one felt signal that travels with you regardless of what the device on your wrist is doing. Pivarnik and Sherman (1990) found that women report higher perceived effort than men on equivalent uphill gradients, a calibration point worth knowing about yourself rather than a weakness to correct. Whatever your personal RPE-to-output relationship looks like on a fresh day one, know that it will shift as the days accumulate, and that shift is expected, not a sign that something has gone wrong.

The talk test remains the simplest practical check. Zone 1 to 2, the effort that should dominate the easier stretches of any long day, is defined by the ability to hold a full, relaxed conversation. Once full sentences give way to phrases and then single words, you have crossed into threshold and above, regardless of what pace or heart rate the moment reads. This holds true on flat ground, on a steep grade, in heat, and on the fourth consecutive morning of climbing. It is also worth naming that hormonal contraception and cycle phase both shift the accuracy of heart rate as a marker specifically for women, which is one more reason RPE deserves to be the primary tool rather than a secondary check.

The practical takeaway for any multi-day event: decide before day one that from day two onward, effort leads and the numbers follow, not the other way around. If a familiar heart rate zone suddenly feels much harder to hold, that is useful information about accumulated fatigue, not a signal to chase the same number by working harder. Use the talk test to anchor your easier climbing, reserve genuinely hard efforts for where they belong in the day's plan, and trust what your body is actually reporting over what the screen says it should be reporting. By the final day, that trust is often the difference between finishing well and finishing broken.

Sources: Pivarnik and Sherman (1990), PMID 2304407. Her Trails Coaching Database, RPE Guide and Glossary entries.