Swimming: what changes between pool and open water
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What was done
Fourteen elite triathletes (10 men and 4 women, about 23 years old) swam two 1500 m tests at race pace: one in open water and one in a 25 m pool. Researchers measured speed, stroke rate (SR), length (SL) and index (SI), heart rate, blood lactate and end-exercise oxygen uptake.1
What came out
In open water stroke length and index were lower and stroke rate higher than in the pool, and speed was lower. Heart rate, lactate and end-exercise oxygen uptake did not differ. The respiratory exchange ratio was lower in open water, consistent with a longer test. The triathletes fastest in open water were also the fastest in the pool.1
The authors link the physiological invariance partly to the participants' open-water experience: it may not hold for people who rarely swim there.1
A second study: master swimmers
Fifteen trained male master swimmers (about 40 years old, mainly open-water competitors) swam a 2 km test in the pool and in open water. In the pool speed, stroke length and arm-stroke efficiency were higher; in open water stroke rate was higher. Perceived effort was similar, but internal load was higher in open water.2
The load result differs from the elite study, where physiology and effort were similar: groups, distances and methods differ, so the two studies do not add up to a single number. The authors of the second study suggest adding open-water sessions focused on technique.2
Teaching example: how to observe the difference
This is an example inspired by the study design, not a validated protocol or a plan. The idea: swim the same distance at the same perceived effort in the pool and in open water, in a safe setting (see Open water and safety), and compare time and strokes per length or per stretch. If open water takes more strokes for the same distance, it is the same kind of change seen in the study. It gives no paces, thresholds or targets: it shows what changes for you.
What we know and what we do not
We know that in this group technique changed in open water at the same effort. We do not know whether the same size of change applies to amateurs, in a wetsuit, with waves or currents, or over other distances; the study gives no change size to use as a target.
How we reviewed it
Document review of two experimental studies (14 elite triathletes; 15 master swimmers).
Limits. Elite athletes and trained male masters, one test each; load results not matching; no applicable target or pace.
Sources
- Swimming Performance in Elite Triathletes: Comparison Between Open Water and Pool Conditions. Scandinavian Journal of Medicine & Science in Sports. doi.org/10.1111/sms.14702 (abstract and parts of the text)
- Guedes, Zacca, Castro. Swimming Front Crawl in Pool and Open Water: Comparisons of Kinematics, Arm-stroke Efficiency and Internal Load in a 2 km Test. Revista Portuguesa de Ciências do Desporto, 2024. doi.org/10.5628/rpcd.24.01.68 (abstract and excerpts)
Source rechecked on 28 October 2026.