Zone 2 on Hills: Why Heart Rate Lies on Trails

A runner sends me a long run file. Average heart rate 158. Their easy ceiling is 145. They tell me the run felt easy the whole way and they are worried something is wrong with them.

Nothing is wrong with them. They ran up a mountain, and heart rate is a bad instrument for that job.

The three ways heart rate misleads you on trails

1. Lag

Heart rate is a downstream response. When you hit a 20% pitch, the muscular and metabolic demand changes instantly; your heart rate takes 30-60 seconds to catch up, and another 30-60 to come back down when the gradient relents.

On a road, where gradient is roughly constant for long stretches, this barely matters. On rolling trail, where the gradient changes every ninety seconds, your heart rate is essentially always describing the terrain you were on rather than the terrain you are on. You end up chasing a number that is structurally behind you.

2. Cardiac drift

At a genuinely constant workload, heart rate climbs over time - typically 5-10% over a few hours, more in heat, more when you are underfuelled or dehydrated. Plasma volume drops, core temperature rises, stroke volume falls, and heart rate compensates.

So the same easy effort that read 138 in the first hour reads 150 in the fourth. If you hold the number, you are progressively slowing down all afternoon for reasons that have nothing to do with the training you intended.

That said, drift is useful information - it just is not a target. The gap between your first-half and second-half heart rate at constant effort (decoupling) is a decent index of aerobic durability. Under about 5% on a long run is a good sign. Consistently over 10% suggests either the run was too hard, your aerobic base is thin, or you are underfuelling.

3. It cannot see eccentric load

Descending causes substantial muscle damage at low heart rates. A long technical descent might read 125 bpm while doing more to determine how your race goes than the climb that preceded it. Heart rate measures cardiovascular strain. It does not measure mechanical cost, and in trail running the mechanical cost is often the one that matters.

What to use instead

The talk test (free, and better than your watch)

For easy running, this is the standard:

• Easy / Zone 2: you can speak in complete sentences without gasping. Nasal breathing is possible, even if you choose not to. You could hold this for hours.
• Steady / moderate: short sentences. Uncomfortable to talk but possible. This is the grey zone - the place most runners live and the place that produces the least adaptation per unit of fatigue.
• Threshold: three or four words. Rhythmic, controlled, hard.
• Hard: single words, or nothing.

Say a sentence out loud every ten minutes on an easy run. It is faintly ridiculous and it is the most accurate intensity tool available to you.

Breathing rhythm

Many runners settle into a consistent breath-to-step pattern - often around 3:3 (three steps in, three out) when easy, dropping to 2:2 when moderate and 2:1 when hard. Learn yours. When the pattern shortens on a climb, you have left easy, regardless of the number on your wrist.

Running power

Power meters - footpods or watch-based estimates - solve the lag problem directly. Your power output at a given metabolic cost is roughly consistent whether you are climbing, descending, or flat, and it responds instantly to a gradient change with no lag and no drift.

The honest caveats: different manufacturers compute running power differently, so your numbers are not comparable to anyone else's; accuracy on very steep and technical terrain is imperfect; and it adds a device and a calibration process to your training. But if you want a single number that behaves sanely on rolling terrain, it is the only real option.

Heart rate as a ceiling

Do not throw the strap away. Use heart rate as a guardrail: set an upper limit for easy runs and treat crossing it for more than a couple of minutes as a signal to back off or hike. That works well, because the failure mode you are guarding against is running too hard, and heart rate - lag and all - catches sustained overreach reliably.

Hike the steep sections. That is the answer.

Here is the practical version of everything above.

On a gradient over about 15%, most runners cannot stay at an easy effort while running. The physics do not allow it. You have two options: run and accept that this is now a hard session, or hike and stay easy.

For an easy run, hike. Almost every runner who tells me their easy runs come out too hard is refusing to walk on climbs. Walking on an easy run is not a failure of fitness or discipline. It is the mechanism by which the run stays easy.

The same applies in reverse: on an easy run, do not hammer the descents. Cruise them.

What 80/20 actually means for trail runners

The polarized model - roughly 80% of training time at low intensity, 20% genuinely hard, with as little as possible in between - holds up well for endurance athletes, and it holds up especially well for ultra runners, whose event is overwhelmingly aerobic.

Three things people get wrong about it.

It is 80% of time, not 80% of sessions. A week with four easy runs and one interval session might still be 85/15 by time, because the interval session's warm-up and recovery are easy minutes. Count minutes.

The failure is usually the easy days, not the hard days. The classic pattern is not too much interval work. It is five runs a week at "moderate" - too hard to be recovery, too easy to be a stimulus - producing a permanently slightly tired runner who never adapts to anything. The trail-specific version of this is running every hill because it feels lazy not to.

The 20% still has to be hard. Polarized means genuinely polarized. If your hard sessions are also moderate, you have not polarized anything, you have just made all your training the same. Two hard sessions a week during a build, done properly, is plenty.

Setting your zones without a lab test

If you want heart rate zones despite everything above - as a ceiling, which is a legitimate use - do not use 220 minus your age. It has a standard deviation of roughly 10-12 beats, which makes it useless for an individual.

Better options, in order:

1. A lab test. Gold standard, gives you lactate or ventilatory thresholds directly.
2. A 30-minute time trial. Run 30 minutes as hard as you can sustain, on a runnable climb or flat ground. Your average heart rate over the final 20 minutes approximates your lactate threshold heart rate. Easy running is roughly 75-85% of that number.
3. Maximum heart rate from a hard field test. Several hard hill repeats to exhaustion, take the highest value you see. Easy is roughly 65-75% of that.

Then, having set them, treat them as a ceiling and steer by breathing. The zones exist to stop you overreaching, not to be hit.

A worked example

A runner doing a 3-hour mountain long run with 1,200 m of gain, easy effort:

• Flat and gentle rolling ground: run, conversational, heart rate settles wherever it settles.
• Climbs under 10%: run, slower, still full sentences.
• Climbs over 15%: hike, hands on quads, breathing steady.
• Descents: run relaxed, do not chase the pace, use it to eat.
• Heart rate: glanced at maybe four times, mostly to confirm nothing strange is happening.
• Post-run check: was the decoupling under 5%? Could I have gone another hour? If yes to both, that was an easy run, whatever the average said.

The average heart rate on that file might be 15 beats above the runner's "Zone 2 ceiling", and the session was still exactly right.

If you want to see how your intensity distribution has actually looked over a training block rather than how you think it looked, the race readiness score breaks your last twelve weeks into endurance, vertical, speed, and taper and tells you which one is the weak link.