Techniques and Considerations for Monitoring Swimmers’ Passive Drag
Authors: Raffaele Scurati, Giorgio Gatta, Giovanni Michielon, Matteo Cortesi
DOI / Source: https://doi.org/10.1080/02640414.2018.1547099
Date: 07 November 2018
Reading level: Intermediate
Why This Matters for Freedivers
If you’re doing dynamic apnea (with or without fins), your “free speed” comes from good streamlining and low drag. The less water resistance you create, the less oxygen you burn for the same distance—meaning longer dives, calmer CO₂ buildup, and a bigger safety margin.
Synopsis
“Drag” is the water resistance that fights you every time you move. In swimming research, scientists separate drag into active drag (while you’re stroking/kicking) and passive drag (while you’re gliding in a stable streamlined position). Active drag is hard to measure cleanly because your limbs are moving and constantly changing body shape. Passive drag, on the other hand, is much easier to study—and it matters a lot because glide phases can make or break performance: the push-off after a turn or start, the glide after a breaststroke kick, and any moment where you’re trying to hold speed without spending extra energy.
This paper is a practical “methods guide” that reviews how researchers measure passive drag during gliding, and what can go wrong if the setup is sloppy. The authors group the main approaches into two families:
1) Direct methods (you try to measure drag force more directly)
- Towing method: the swimmer is pulled at a controlled speed while holding a tight streamline; force in the tow line is measured. This is widely used because it’s conceptually simple and doesn’t rely heavily on mathematical assumptions—if your speed is steady, the measured force closely reflects drag. The downside is that small changes in posture (head position, arm overlap, scapula position, ankle “flop,” tiny hip bend) can change results, so the “human factor” is huge.
- Flume method: you put the swimmer in a controlled current (like a treadmill for water) and measure forces while they hold position. It offers a stable flow environment, but flumes aren’t common, and the setup can be more complex.
2) Indirect methods (you estimate drag from how speed changes, or from computer models)
- Glide decay: you push off and then track how quickly the swimmer slows down. From the speed–time curve you estimate drag. This is appealing because it’s easy to do in a pool, but it depends on assumptions (for example: how “clean” the glide is, how consistent the underwater posture is, how accurately velocity is measured).
- Computational Fluid Dynamics (CFD): you scan/model the body and simulate water flow to estimate drag. It’s powerful for “what-if” questions (changing head position, suit, arm angle), but it’s only as good as the model and assumptions, and it’s not a quick coaching tool.
A useful message of the review is that different methods can still produce broadly consistent passive-drag values if the testing is done carefully. The paper highlights the big “gotchas”: body alignment, depth (surface wave effects can change drag), speed selection, how the swimmer is stabilized, and how repeatable the posture really is between trials. The authors also point out that towing tends to be the most commonly used approach because it’s relatively straightforward and less assumption-heavy than glide decay or CFD.
Practical takeaway: if you want a real-world, repeatable way to track streamlining improvements, you don’t need a lab. A simple pool protocol (same push-off, same depth, same posture rules, same measurement method like time-to-mark or video speed tracking) can act as a “drag fitness test.” What you’re really measuring is how well you can hold a stable, low-drag shape—which is exactly what efficient freediving dynamics demands too.
Abstract
Drag is the resistant force that opposes a swimmer displacing through water and significantly affects swimming performance. Drag experienced during active swimming is called active drag (Da), and its direct determination is still controversial. By contrast, drag experienced while gliding in a stable streamlined body position is defined as passive drag (Dp), and its assessment is widely agreed upon. Dp reduction preserves the high velocity gained with the push-off from the starting block or wall after starting and turning or improves the gliding phase of the breaststroke cycle. Hence, this paper reviewed studies on swimming that measured Dp under different conditions of gliding. In the present research, accurate descriptions of the main methods used to directly or indirectly determine Dp are provided and the main advantages, limitations and critical features of each method are discussed. Since Dp differs in methods but not in reported values and is consistent regardless of the measuring method, the information provided in this paper might allow coaches and practitioners to identify the most suitable method for assessing and determining the drag of their swimmers.