A triathlon is harder than the sum of swimming, cycling and running. But how much does a 2-km swim effect your cycling performance? Can you still use your cycling power meter values and functional threshold power? Or should you throw them overboard? Let’s find out what science tells us!
You might think your threshold intensity is a fixed value. But if you don’t take fatigue due to swimming into your equation, you will have an empty tank before you get towards the finish line. Let’s look at exactly how much a 2-km swim influences your cycling performance.
Results study 01
That is exactly what researchers wanted to test in their recent study, that got published in April 2020. They got eighteen trained triathletes performing a cycling test twice on separate days. One test was done immediately after a 2-km swim and one without prior swimming.
The cycling test contained a 3-minute all out test. Now you might think a 3-minute all out test is a strange way to test for endurance. If you do, keep reading for the next study. But research actually shows a 3-minute all out test can predict threshold power, when the effort is not paced. That means the athlete just starts sprinting and does not stop until 3 minutes are over. Don’t try this at home, rather watch how we let someone else do it :).
The study results contained 3 parameters. First, total work done, which is a combination of the blue and red surface. Second, end-test power, which is about 350 watts in this example. Third, work done above end-test power, which is the blue surface in this graph.
The 2-km swim significantly decreased total work done by 6%. That means athletes produced less power after swimming. End-test power (4%) and work done above end-test power (13%) also decreased, but not significantly.
Results study 02
In another study (2019) they tried to find the same answers but took a little different testing approach. They again looked at the effect of a 2-km swim on cycling performance. Instead of using a 3 minute all out test, they went for an incremental ramp test. Many of you know this test as a classic VO2max test.
They looked at 2 sub-maximal parameters and 2 maximal parameters. The first and most important sub-maximal (4-mM blood lactate) result showed a power decrease of 3.8% after swimming. That means that the power you can sustain for a longer period of time is lower after swimming. The second sub-maximal (220 W for males, 150 W for females) result showed an increased heart rate of 4.0% compared to cycling without prior swimming. That means your body has to work harder for a sub-maximal effort after swimming.
When looking at the maximal parameters, maximal oxygen uptake and peak power decreased after swimming by an average of 4.0% and 4.5% respectively.
Practical implications and future technology
Roughly speaking these studies show you should lower your expectations about 5% when it comes to cycling power after a 2-km swim. Once you know your cycling power for a certain distance, you can now calculate your cycling power after a swim by subtracting at least 5%.
We’ve also seen higher cycling heart rates at the same power output after swimming. So, if you don’t use a power meter and target a specific heart rate, you will produce less power after a swim then in an isolated cycling time trial.
Generally speaking, your threshold power is affected by fatigue. A threshold range might therefore be a better way to understand your body than a specific threshold value. Wouldn’t it be cool if there was a device that tells us what our threshold is at that moment? A tool that takes into account whether we are fatigued or not? That tool probably uses a combination of internal intensity like heart rate or RPE, and external intensity like power or pace. (Read more about internal and external intensity here).
You might say 5% is not a lot, and I agree that I expected bigger changes. But if you don’t take this drop of power into account, I’m sure you’ll regret it. Oh, and don’t forget: after you finished swimming and cycling, your running shoes are waiting for you… Don’t forget to enjoy! 
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Founder of Molab, Human Movement Scientist and Freelance Content Marketer.