Swimming heart rate zones: why the water reads lower

Your watch reads lower in the pool than on the road, at an effort that feels harder. Nothing is broken. Heart rate genuinely falls in water at the same oxygen cost, maximal heart rate in swimming sits below maximal heart rate in running, and the size of that gap is not a fixed number — it depends almost entirely on how good a swimmer you are.

Below is what has actually been measured, why the drop happens, the arithmetic that goes wrong when you carry running zones into the pool, and the two ways out: a swim-specific maximal heart rate test, or dropping heart rate for pace. If you want the short version, the second route is the one most swimmers should take, and critical swim speed is how it works.

The short answer, in beats per minute

Three studies, three populations, one direction. Every one of them found maximal heart rate lower in the water than on land — but the gap ran from under seven beats to twenty-four.

In elite swimmers the gap was small. Olstad and colleagues took twelve national-level swimmers through three maximal swim protocols and a treadmill test: the best swim figure was 193.6 ± 7.5 bpm against 199.8 ± 7.7 bpm on land, a mean paired difference of 6.7 ± 5.3 bpm. Two of the twelve were actually higher in the water.

In people who swim without being swimmers, the gap was four times larger. Fourteen helicopter rescue paramedics reached 162 ± 11 bpm in a pool test and 186 ± 10 bpm on a treadmill — 13% lower, and the difference was not marginal. Ten triathletes landed in between: 174.8 ± 9.0 bpm swimming against 188.6 ± 7.5 bpm cycling.

So the honest answer to “how much lower is my heart rate when I swim?” is a band, not a figure: somewhere between about 5 and 25 beats below your running maximum, and the better your stroke, the nearer the bottom of that band you sit.

Why the number drops when you get in

Four things stack up, and only one of them is about swimming being hard.

You are horizontal, and you are immersed. Water pressure pushes blood from the legs toward the chest, so the heart fills better on every beat. Boussuges measured a 12% rise in cardiac output after ten minutes of quiet head-out immersion in 26 volunteers, driven entirely by stroke volume — heart rate did not change at all. More blood per beat means fewer beats are needed for the same delivery.

And that holds under load, not just at rest. This is the measurement that matters most, because it isolates the water from the stroke. Sheldahl and colleagues had nineteen men cycle on land and then cycle immersed to the shoulders, at 40%, 60% and 80% of maximal oxygen uptake. Same exercise, same intensities. Stroke volume was higher in water at every stage, and at 80% the heart rate was significantly lower. Identical work, lower number on the screen.

You are using less muscle. Freestyle is driven largely by the upper body, which is a smaller slice of your muscle mass than the legs you run with. A smaller active mass asks for less flow, and the heart rate you can drive is bounded by what the working muscle demands.

Your breathing is on a schedule. You cannot pant freely with your face in the water. Breathing is tied to stroke rate, which constrains the ventilation-driven part of the heart rate response — one reason a settled breathing pattern changes how a set feels without changing the pace.

The gap measures your swimming, not your fitness

Line the three studies up by swimming ability and they sort themselves perfectly: elite swimmers 6.7 beats, triathletes 13.8, non-specialist swimmers 24. The paramedics were fit — their treadmill maximum of 186 bpm and a maximal blood lactate of 13.4 mmol/L say so. They simply could not drive their own hearts hard in water.

That is what an inefficient stroke does. If you are fighting the water, you fatigue locally — shoulders, forearms, the grip on the catch — and stop before your cardiovascular system is anywhere near its ceiling. The limit is mechanical, not central. It shows up as a heart rate that stubbornly refuses to climb however hard the set looks on paper.

Which inverts the usual reading of the number. A swimmer whose heart rate will not rise in the pool is not necessarily lazy or unfit; more often the stroke is cashing the effort before the heart gets a chance to spend it. The fix for that is technique and stroke efficiency, not a harder set — and the number to watch while you fix it is distance per stroke, which the SWOLF calculator will track for you.

What breaks when you use running zones in the pool

Here is the arithmetic, using the paramedics' measured figures. Say your running maximum is 186 bpm and your true swimming maximum is 162.

Both ends fail, and they fail in opposite directions. Your easy days come out too hard, because a land-derived recovery number is a real aerobic load in the water. Your hard sets come out impossible, because the land-derived threshold figure sits above your swimming ceiling — so you chase it, never see it, and either overreach or quietly conclude the monitor is faulty.

For a trained swimmer the same error is minor: 6.7 beats on a maximum near 200 is about 3%, inside the noise of any zone scheme. The people most likely to use heart rate zones in the pool — adult-onset swimmers and triathletes coming from running — are exactly the people for whom the error is largest. That is the trap.

One thing does carry across, and it is worth knowing: Roels found the ventilatory threshold, expressed as a percentage of maximal oxygen uptake, did not differ between swimming and cycling in either group. The physiological fraction travels. The heart rate that marks it does not.

And 220 minus age was never your number

If you are planning swim zones off a formula, you are stacking two errors. Tanaka and colleagues pooled 351 studies covering 492 groups and 18,712 people, and the relationship they found was 208 − 0.7 × age, cross-validated on a further 514 people in the lab at 209 − 0.7 × age. The familiar 220 − age underestimates maximal heart rate in older adults.

Note what that result is: a regression line through group means. It describes populations. It was independent of sex and of habitual activity level, which sounds reassuring until you realise it means the formula knows nothing about you specifically. Feed it into a land-to-water conversion you also guessed, and the zones you train in are two assumptions deep.

The swim-specific maximal heart rate test

Olstad's group ran three protocols — built on 50 m, 100 m and 200 m repeats — and found no meaningful difference between them: 191.9 ± 8.4, 190.8 ± 8.3 and 192.0 ± 6.9 bpm. Any properly structured step test will do the job, so use the one you can pace. Here is their 50 m version, which is the most forgiving to swim.

One to two minutes of passive rest between steps, every repeat started in the water, and a full warm-up first — theirs was 2,000 m. Your maximal heart rate is the highest single value recorded, which almost always lands in the minute after the final 100, not during it. If you do not know your threshold pace to set step 1, get it from the CSS calculator first; the send-offs come from the interval calculator.

Shorter on time? Swim 1,000 m of real warm-up, then 6 × 100 m getting faster, then one all-out 200. It is less rigorous than the published protocol and you should expect a slightly lower figure, but it beats a formula.

Where the sensor has to sit

Worth saying plainly, because it explains most “my watch shows nothing in the pool” complaints: in the Olstad study nobody measured from a wrist. Each swimmer wore a chest strap underneath the swimsuit — the men wore a suit covering the chest so the push-off would not drag the strap down — plus an optical sensor at the temple, under the cap, held by a headband cut from an old strap.

That is what it takes to get a trustworthy second-by-second trace while swimming. A wrist is the worst available site: it is the fastest-moving part of your body, the band loosens as water passes over it, and the hand pushes against water on every stroke. If your watch is giving you a swim heart rate you have not checked against a strap, treat it as a mood, not a measurement.

A practical consequence that catches people out: the calorie figure your watch reports for a swim is often derived from heart rate. Feed it a number that is 20 beats low and the estimate inherits the error, which is one reason pool sessions read as suspiciously cheap.

Use pace, which the water does not distort

Swimming has something running does not: a cheap, repeatable, lane-marked measure of output. Pace is the output. Heart rate is a response to it, and the water bends that response.

The anchor is critical swim speed, from Wakayoshi's 1992 method — two timed swims, one subtraction, and you have the pace that sits at your aerobic threshold. From there the zones are percentages of speed, so a pace is divided by them, not multiplied:

Three advantages over heart rate, in the water specifically. Pace responds instantly, while heart rate lags by 30 to 60 seconds, which makes it useless for a 50 that lasts 40 seconds. Pace is unaffected by water temperature, caffeine, sleep or how hot the building is. And pace is the thing you are trying to improve, so it doubles as the test. Run the numbers on the CSS calculator, convert any distance with the pace calculator, and build the week around it with the rest of the training guides.

Water temperature moves the number as well

This is the part where the evidence runs out, so here is where it stops. The three immersion studies used three different temperatures: Olstad's pool was about 27 °C, Sheldahl's immersed cycling 31 °C, Boussuges' resting immersion 34.5 °C. Each is inside what a pool or a wetsuit swim might present, and none of them was designed to isolate temperature.

What follows is coaching consensus rather than proof: cool water tends to hold heart rate down at a given pace, and warm water drives it up as the body diverts blood to the skin to shed heat. A race-pace effort in a 31 °C hotel pool and the same effort in 25 °C open water will not produce the same trace. If you train across both — most triathlon swimmers do — that alone makes heart rate a poor week-to-week yardstick, and it is another reason to anchor on pace.

Treat this as a reason for caution, not as a correction factor. Anyone offering you beats-per-degree is making it up.

Where heart rate still earns its place

None of this makes the strap useless. It makes it a different instrument than the one you used for running.

• Keeping easy days easy. The commonest error in adult swimming is that recovery swims are not recovery. A ceiling you must not exceed is the one zone heart rate polices well, and lag does not matter over a continuous 400.
• Open water, where pace is unknowable. No walls, no clock, current and sighting corrupting your speed. Effort is the only honest input you have — see the open water guide.
• Drift within a long set. If heart rate climbs while pace holds, you are decoupling: fatigue is costing you efficiency. That trend is informative even when the absolute figures are not.
• Illness and fatigue. A resting heart rate 8 to 10 beats above your own normal is a reason to swim easy or not at all. That is a comparison against yourself, which is the one use of the number that needs no zones at all.

What it should not do is set your intervals. That is pace work, and it is the backbone of building swimming endurance.

What to do this week

1. Stop using your running zones in the pool today. Of everything here, this is the change that removes an actual error rather than adding a refinement.
2. Swim a CSS test. A 400 and a 200, both hard, with 5 to 10 minutes between them. Put the times in the calculator and you have your five pace zones — no strap, no formula, no maximal effort required.
3. Build the week from pace. Threshold sets at 95% of CSS, easy swimming at 70 to 80%, and send-offs that leave real rest. If you are unsure how much to swim, frequency has its own answer.
4. If you want heart rate zones anyway, measure them. Get a chest strap, run the step test above with the safety conditions respected, and use the figure you actually recorded. Retest once a season.

Your watch was not lying. It was answering a different question than the one you were asking it.