Heart Rate Zones: A Useful Guide, Not a Verdict

The monitor is the most worn and least examined instrument in the boathouse. It gives a number, the number gives a zone, the zone gets written in the log, and three months later the log is used to decide whether training is working. Very little of that chain gets questioned. This article is about the two links in it that are weaker than they look.

Zones are arithmetic on a number you have not measured

A zone boundary is a percentage of maximal heart rate. The maximum is usually predicted from age. That prediction has a pedigree: Tanaka and colleagues assembled group maximal heart rates from 351 studies covering 492 groups and 18,712 subjects, found maximal heart rate strongly related to age, and produced the equation 208 − 0.7 × age, which then reproduced almost exactly in a laboratory study of 514 adults ([1] Tanaka). It replaced the older 220 − age shortcut, and the authors noted that the old equation underestimates maximum heart rate in older adults.

That is a serious piece of work and it is worth being precise about what it does. It fits a line through group averages. A line through group averages is not your maximum heart rate. It is the best available estimate of what the average person of your age reached when somebody actually measured them — and the distance between that line and an individual is real, measured in beats per minute, and larger for some people than others.

Multiply that distance by 80% and you have a zone boundary that is off by a meaningful number of beats. Every zone label inherits it, and the error does not shrink because the arithmetic looks precise. This is the part worth internalising: the percentage is not the uncertain quantity — the maximum underneath it is.

The same split is not the same effort

A second assumption sits underneath most zone thinking: that pace determines intensity. On the erg it does not, because rate is a free variable.

Seventeen trained female rowers rowed at three different stroke rates under constant flywheel power. Gross efficiency did not differ significantly between them ([5] Hofmijster). Work in, work out — the flywheel absorbed the same power regardless of how fast the handle came back. What changes is the physiological cost of moving the handle that much faster, and heart rate is where that cost shows up.

So two sessions can show the identical 1:55 average split, produce genuinely different amounts of work, and land in different zones. If your zone 3 interval was twenty-eight strokes and your zone 3 steady piece was twenty-two, they were not the same session. This is also why a zone number without its stroke rate is not a measurement of anything reproducible.

The number moves while the effort does not

Even with pace and rate held constant, heart rate is not steady. It climbs through a long piece while the work stays identical, and the mechanism is well characterised: as skin blood flow increases to shed heat, it displaces volume from the central circulation, stroke volume falls, and heart rate rises to compensate ([4] Souissi). The alternative account has sympathetic activity driving the rise directly. Neither is settled, but both agree on the observable, which is that drift is real and is not a sign you are slipping.

This is why the last third of a steady piece reads harder than the first third at the same split. It also means the order of your intervals changes what they mean. On a day when the erg room is hot, the same piece sits a few beats higher than it did in March, and the logbook will tell you your fitness declined when nothing about your fitness moved.

What the number is being asked to reflect

None of this is an argument against wearing a monitor. It is an argument about what is underneath the reading, and the gap between the two is larger than most training plans assume.

The machine is not measuring effort directly. It is measuring a cardiovascular response to a mechanical action, and the action is a repeating sequence that every rower performs differently ([7] Soper). Within that sequence the force itself changes shape: in race simulations, handle force peaked at 46 percent of the stroke cycle in men and 43 percent in women ([6] Ledergerber), and the biomechanical determinants of rowing performance vary between rowers doing the same split ([8] Baudouin). Two athletes can hold the same average split, spend different amounts of time at different force levels, and arrive at the same zone number.

The monitor's own documentation is worth reading against what the machine actually records, because a zone on screen is a derived value rather than something a sensor picks up off the handle ([9] Concept2). So is the distance you ran the piece over: the indoor distances most clubs test at — 500 m, 1,000 m, 2,000 m, 30 minutes ([10] World Rowing) — are the ones a zone number is supposed to be interpreted against.

The practical version: the reading is a summary of a session, not a property of you. Two laps of the same 2K produce the same number when the work was the same, and for reasons that have nothing to do with fitness when it was not.

The HRV signal that elite rowers inverted

If heart rate is the instrument you should be sceptical of, resting heart-rate variability is the one where scepticism pays most — because the folk reading of it is wrong for the best athletes in the world.

Plews and colleagues recorded daily resting HRV in four rowers over the seven weeks before they won at the 2015 World Rowing Championships. The time course of change in Ln rMSSD was athlete-dependent, with stagnation and decreases apparent in some of them. But when each athlete's rMSSD was read against their own R-R interval — a ratio rather than a raw number — all four showed substantial reductions. The authors interpreted that combination as parasympathetic saturation, meaning three of the four championship-winning rowers were showing decreases in raw HRV while their parasympathetic activity was actually increasing ([2] Plews).

In other words, the athletes who were in the best condition available had the falling numbers. A drop in resting rMSSD in one of those rowers would have been read, under the usual rules, as overreaching.

The reason is that rMSSD is scale-dependent — it falls as resting heart rate rises, whether or not anything has changed for the better. Correcting for that scale is what the ratio does, and it is why the authors concluded that a combination of indices is needed rather than a single number ([2] Plews).

What elite rowers' HRV did track

The same group looked at nine elite rowers across the 26-week build to the 2012 Olympics, two of whom won gold and two bronze. Weekly rMSSD correlated moderately with time spent below the first lactate threshold (r = 0.43), was unclear against total training time (r = 0.37 with very wide confidence limits), and was trivial against time between the two thresholds (r = 0.01) ([3] Plews).

The shape of that is the useful part. What predicted a healthier autonomic state was not more training and not harder training. It was more easy training specifically. Time above the second threshold carried a small negative relationship. The authors read this as supportive of periodised low-intensity work, and it is a much more specific instruction than "monitor your HRV" — it says which part of the week the signal belongs to.

What to do with this article

Measure your real maximum once, if the club has a step test or a field test you trust, and use it in place of the age equation. If you cannot, understand that every zone you compute is carrying an error in beats per minute, and stop treating the boundary as a fact about you.

Always record the stroke rate beside the zone. Same split at a different rate is a different session, and the zone number will move without the work changing.

Compare sessions to yourself on the same machine, in similar conditions, and read drift as information about the room rather than about your fitness. A rising number in warm conditions with steady effort is the expected response.

Treat a single HRV reading as one noisy sample of a scale-dependent quantity. Look at a trend against your own baseline, and check it against resting heart rate before reading it as fatigue. The best rowers in the world produced falling numbers while improving.

Let pace, rate and how the piece felt decide what the session is for. Use heart rate to confirm what you already know, and to notice when the same effort has started costing more than it did. That is the job it is good at, and it is a genuinely useful one.