Swimming Heart Rate Zones

Heart rate in the water runs lower than heart rate on land at the same effort, which means every zone chart built from running is wrong the moment you get in the pool. Start with an age-based ceiling — then read what has to happen to it before it can govern a swim set.

What the measurement actually says

The cleanest number comes from a 2019 study of elite swimmers tested maximally both on land and in the pool — and the sample is smaller than the headline suggests. Twelve swimmers enrolled; only ten completed the land protocol, and three of those ten were unable to run and rode a cycle ergometer instead. The swim-versus-run comparison therefore rests on about seven athletes.

In them, maximal heart rate came out 6.7 ± 5.3 bpm lower for swimming than for running (p = 0.015). Against cycling the gap was 5.7 ± 6.5 bpm and did not reach significance — running was the only land modality with a significantly higher maximum than the water (p = 0.028), so the bike does not belong in the same sentence as the treadmill here. Running peaked at 199.9 ± 8.9 bpm; the pool step-tests at 50, 100 and 200 m landed at 191.9, 190.8 and 192.0 bpm.

Notice what is doing the work there. The average gap is small, and the spread around it is almost as large as the gap itself — some swimmers barely change, others drop far more. That is why the authors explicitly rejected the old shortcut of subtracting a fixed 12 bpm from a running or predicted maximum, and recommended testing maximal heart rate separately in each sport you train.

If you have seen much bigger gaps quoted, they exist too, and they come from different people: in groups whose training is not centered on the pool, the ceiling in the water falls far further. Our page on max heart rate by sport covers that comparison across disciplines, including an occupational group of helicopter rescue paramedics whose pool peak came in about 24 beats below their treadmill peak — nearly four times the gap measured in elite swimmers. This page is about the narrower question of what to do with your zones once you accept that the ceiling moved.

Why the water lowers it

Several mechanisms stack, and none of them is about fitness:

• You are horizontal. Blood does not have to be pushed uphill, so each beat returns and ejects more, and fewer beats deliver the same output.
• Less muscle mass is doing the work. Buoyancy means almost nothing is required to hold you up, so the large lower-body musculature that running recruits sits idle. Less active muscle, less cardiovascular demand.
• Breathing is constrained. Your breath is timed by the stroke, not by demand, and external pressure on the chest works against you.
• Water pulls heat away faster than air. On land a real share of cardiac output goes to the skin to shed heat. Immersed, the water handles it.

Building zones that work in the pool

Three routes, in increasing order of how much you get out of them.

1. Test your swimming maximum. A hard, well-recovered set of progressively faster 100s to exhaustion, with a chest strap, gives you the ceiling that swim zones should be built from. This is what the researchers recommend, and it costs one session.

2. Borrow the published swimming bands, knowing what they are. A 2024 perspective article in Frontiers in Sports and Active Living prints heart rate as one column of a five-zone table: below about 140–150 bpm for low aerobic work, 150–160 for moderate, 160–170 at threshold, 170–190 for aerobic power and above 190 for the anaerobic band. Two things about that table matter before you copy it. It is not a measurement: the authors say it was assembled from the literature they had just reviewed plus “our own knowledge and experience”, and the paper reports no sample and no n. And it is addressed to a narrow population — the legend restricts it to front crawl high-level swimmers. For a recreational swimmer it is orientation, not a prescription.

3. Prescribe by pace instead. For most swimmers this is the practical answer. Pace is read off the wall clock, needs no sensor at all, and the optical sensor on your wrist is fighting cold water and a moving arm. A threshold pace from the critical swim speed calculator comes in seconds per 100, which is already the unit sets are written in. Note that this is a matter of what you can execute on deck, not of heart rate being useless: the same 2024 paper keeps heart rate in its table and recommends combining it with pace, stroke rate and perceived effort rather than choosing one. The swimming training zone chart puts both scales side by side.

Whichever route you take, the rule underneath does not change: never carry a running zone chart into the pool without correcting it, and never treat a low pool heart rate as evidence the set was easy. Ask the effort, not the watch.

This content is informational and does not replace medical evaluation. If you have a heart condition or take medication that affects heart rate, ask your clinician what your numbers should be before using any of this.