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Technical Report

Effect of Swim Cap Model on Passive Drag

Gatta, Giorgio1; Zamparo, Paola2; Cortesi, Matteo1

Author Information
Journal of Strength and Conditioning Research: October 2013 - Volume 27 - Issue 10 - p 2904-2908
doi: 10.1519/JSC.0b013e318280cc3a
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In aquatic locomotion, at constant speed, propulsive forces equal resistive forces such that a reduction of hydrodynamic resistance makes it possible to attain higher speeds for a given propulsive force. The determination of drag is therefore a key issue in evaluating swimming performance.

Even if the hydrodynamic resistance is determined by the so-called active drag (Da) during swimming, passive drag, according to several authors, is a good index of the hydrodynamic attitude of the swimmer during competitions (2). This, in any case, holds true for the phases of the race that the swimmer covers in a gliding position (e.g., after starts and turns) (13).

Even though several methods to estimate Da have been proposed in the literature [for a review, see (14)], a direct determination of Da in swimmers is still quite difficult to perform and also quite debated; in contrast, several studies have investigated passive drag (Dp) during swimming by means of towing tests on a “rigid swimmer” at constant speeds. Since the 1930s, it has been known that pressure, wave, and friction drag interact to determine the total hydrodynamic resistance acting on a swimmer (4). Studies of passive drag indicate that pressure (form) drag is the major determinant of Dp accounting for 74, 55, and 51% at 1.0, 2.0, and 2.2 m·s−1, respectively; whereas friction (24, 25, and 23%) and wave (2, 20, and 26%) drag have a lower influence on Dp at the corresponding speeds (8). Even if other authors have found different contributions of pressure, wave, and friction drag to Dp (12,13), it is generally acknowledged that pressure drag contributes most to Dp at low-intermediate speeds, whereas the wave drag contribution increases with increasing speed, and friction drag is rather small and unaffected by speed changes.

It is commonly believed that drag-reducing suits reduce friction drag because the surface characteristics of a body moving in a fluid have an effect on fluid mechanics. However, as shown by Mollendorf et al. (6), the effect of drag-reducing suits depends on the transition-to-turbulence Reynold's number locations on the swimmer's body, which are derived from trade-offs between the induced changes in pressure, wave, and friction drag.

It has been shown (for surface swimmers) that laminar flow ends at the largest circumference of the head and that turbulent flow occurs below the knees, with the remainder of the body being in transitional flow (6,7); as a consequence, the largest reductions in overall drag are seen for the swimsuits that cover the upper part of the body (shoulders and trunk); the lower-body coverage suits have very small effects on Dp. Moreover, it has been shown (6) that upper-body swimsuits increase rather than decrease friction drag, but they allow the flow to remain attached to the body, thus lowering pressure and wave drag.

Because laminar flow is disrupted at the level of the head, tripping the boundary layer at the head should give the greatest reductions in drag. Studies using computational fluid dynamics indeed have shown that the position of the head has a marked influence on Dp, strongly modifying the wake around the swimmer (9,10,15). Moreover, Marinho et al. (5) have shown that a reduction of Dp of up to 15% can be obtained by wearing a swim cap (compared with the “no-cap” condition), even if this “intervention” affects a very small surface of the swimmer's body.

To our knowledge, no studies have been conducted to investigate the effects of passive drag on wearing different swim cap models. Therefore, the aim of this study was to investigate whether differences in the material they are made of and in their “structure/composition” can be associated with differences in hydrodynamic resistance.

The choice of one swim cap over another should be considered with care because it could have an effect on swimming performance, allowing for a reduction in drag (and therefore for an increase in speed) at least during the gliding phases of a race (after starts and turns).


Experimental Approach to the Problem

To verify the hypothesis that different swim caps could affect hydrodynamic resistance, whole-body passive drag was measured by means of the passive towing method (3,16) at 3 speeds that within the range of those attained during the majority of swimming competitions: 1.5, 1.7, and 1.9 m·s−1.

We decided to test the swim cap conditions with this method because the methods proposed in the literature so far to determine Da are difficult to perform and are also much debated (16); in contrast, passive drag measurements are quite reliable and widely accepted by the scientific community (e.g., 2, 6, 7, 14, and 16). Furthermore, by means of the passive method, the variability because of the effects of each swimmer's technique on his hydrodynamic resistance (a problem that could affect Da measurements) is avoided, and the specific effect of different swim cap conditions could be better investigated.

Because we expected small differences in drag between conditions, we decided to ask each swimmer (16) to repeat the measurements at each velocity (3) and swim cap condition (3) 5 times for a total of 720 observations.


Sixteen male highly trained swimmers participated to the study. Their average physical characteristics (mean ± SD) were as follows: 26.5 ± 8.6 years of age, 1.80 ± 0.05 m in stature, and 75 ± 5 kg of body mass. The average swimmer's experience was of 11 ± 3 years, and the average volume of training was of 5.5 ± 2.4 km·d−1. Written informed consent was obtained from the participants and their parents.

The study conformed to the standards set by the Declaration of Helsinki, and the Bioethics Committee (Institutional Review Board) of the local university approved the procedures.


The test sessions were conducted in a 25-m indoor swimming pool (average water temperature: 28.0 ± 0.5° C). For each swimmer, the testing procedure was completed within 2 hours, and the test sessions were conducted in a period of the year when the swimmers were not involved in intense training or in competitions. Before each trial, the participants carried out a 20-minute warm-up period and were habituated to the experimental conditions.

The measurements of passive drag were performed with the athletes linked via a nonelastic wire to a low-voltage isokinetic engine (Ben Hur; ApLAb, Rome, Italy) positioned at the edge of the pool (3,16) that dragged the swimmer via a cable at a constant velocity. The towing was repeated at 3 velocities (1.5, 1.7, and 1.9 m·s−1), and these velocities were repeated 5 times for each swim cap condition. The swimmers were asked to adopt a prone position with the arms completely flexed at the shoulders and extended at the elbow and wrist, to position the upper arms in contact with the sides of the head (one hand on top of the other), to hold onto the wire and to hold their breath after a maximal inspiration; the lower limbs were passively lifted using a pull buoy (21 × 12 × 8-cm foam figure-eight-shaped pull buoy; Finis, Livermore, CA, USA). The engine rolled up the wire at a constant speed, and the resistance force was measured by a dynamometer (Ben Hur; ApLAb) that was calibrated before each experimental session. After the experiments, the data were downloaded to a PC and further analyzed by means of dedicated software (DAQ; ApLab). Average force and speed were calculated between the 10th and 20th m from the start, i.e., after they had attained constant values.

Cap Conditions

The described protocol was randomly repeated for the 3 swim cap “conditions”: (a) a lycra swim cap (LSC: Unix cap; Arena, Macerata, Italy) made of nylon and elastomer, (b) a classic silicone swim cap (CSC: Classic Silicone cap; Arena), and (c) a helmet silicone swim cap (HSC: 3d Race cap; Arena) without seams (see Figure 1). All swim caps were purchased from local suppliers and were provided to the swimmers according to their fit.

Figure 1:
The swim cap models tested in this study. LSC = made of nylon and elastomer; CSC = a classic silicone cap; HSC = a helmet silicone cap without seams.

The 3 cap condition trials (LSC, CSC, and HSC) were conducted in random order to avoid any potential order effect. Participants were requested to fit the swim caps tightly over their head and to tuck the hair under the cap; they were provided with the same swim goggles (Swedix; Arena) and were requested to wear the strap under the cap.

Statistical Analyses

To quantify the agreement between the 5 trials (for each swimmer, cap, and speed condition), the coefficient of variation (CV) was calculated for each velocity and swim cap condition: the mean absolute value was 3.5%. The average value of the 5 trials (for each swimmer, cap, and speed condition) was then computed and used in further analyses.

A 2-way repeated-measure analysis of variation (ANOVA) (speed and cap condition) was used to assess the effects of velocity, swim cap condition, and their interaction on each dependent variable. Pairwise multiple comparison procedures were carried out with a Tukey test. Data in tables and figures are reported as mean ± SD. Differences were considered statistically significant at p < 0.05.


The mean and SD (all swimmers at each velocity) of passive drag are reported in Figure 2. Analysis of variation revealed significant differences in velocity (F2,15 = 460, p < 0.001) and in swim cap condition (F2,15 = 16.1, p < 0.001) and a significant interaction for the speed × cap condition (p < 0.001).

Figure 2:
Mean and SD of passive drag for 3 swim cap conditions at the 3 considered velocities. LSC = made of nylon and elastomer; CSC = a classic silicone cap; HSC = a helmet silicone cap without seams.

The passive drag was significantly different at the 3 levels of speed (1.5 vs. 1.7 m·s−1, 1.5 vs. 1.9 m·s−1, and 1.7 vs. 1.9 m·s−1; p < 0.001), and it increased significantly with speed (1.5 vs. 1.7 m·s−1, 1.5 vs. 1.9 m·s−1, and 1.7 vs. 1.9. m·s−1; p < 0.001) for all swim cap conditions.

The statistical analyses revealed no differences in passive drag between LSC and CSC at the 3 investigated speeds; in contrast, ANOVA revealed significant differences in passive drag between LSC and HSC at all speeds (see Table 1). The values of hydrodynamic resistance in CSC and HSC were found to differ significantly at the highest tested speed only.

Table 1:
Significant differences in the passive drag among swim cap conditions at the examined velocities.

The percentage differences in the values of drag (at p < 0.05) ranged from 5 to 6.6%; i.e., approximately twice the value of the average CV (3.5%).


During the gliding phases of a race (after starts and turns), swimmers have to keep their body in a streamlined position to reduce, as much as possible, their hydrodynamic resistance. During these phases, the position of the head is of fundamental importance because it determines the dynamics of the impact of the body with the fluid and the vortexes that propagate along the swimmer's body (9,10,15). Swim caps could affect these factors and thus could have an impact on swimming performance (5).

As shown in this study, differences in passive drag among swim caps are indeed observable (especially at high swimming speeds), and this indicates that the choice of one model over the other has very practical implications. This choice not only has an impact on swimming performance but can also affect the outcome of research studies. As an example, not taking into account these differences could be a “confounding factor” in those studies in which the effects on passive drag when wearing different swim suits are investigated.

Even though the literature contains several papers that have investigated the effect of wearing different swimsuits on hydrodynamic resistance (6,7,11), research regarding the effects of different swim cap models on drag is scarce. To our knowledge, the only study that has investigated this topic is that of Marinho et al. (5), who by using a numerical simulation around a swimmer model (computational fluid dynamics on one female swimmer) have shown that wearing a cap can reduce drag by approximately 15% (at speeds of 1.5–2.5 m·s−1) compared with the no-cap condition. Data reported in this study indicate a difference in passive drag (as assessed during towing experiments on the 16 swimmers) for different swim caps that is approximately half this value (approximately 6%) over a similar speed range (1.5–1.9 m·s−1).

Our results are relevant because (a) all swimmers wear swim caps during competitions, (b) they can choose the model of swim cap they prefer, and (b) the types of swim cap investigated in this study are largely used during training and competitions.

Our results indicate that wearing a helmet silicone swim cap (HSC: 3d Race cap; Arena) allows for a significant reduction in whole-body passive drag in comparison with a lycra swim cap (LSC: Unix cap; Arena, made of nylon and elastomer) and a classic silicone swim cap (CSC) even if, in the latter case, the difference is significant only at the highest tested speed. The differences in drag between swim caps can thus be attributed to the following causes:

  1. The type of material (silicone vs. lycra: HSC vs. LSC)
  2. The presence/lack of seams (HSC vs. CSC, both made of silicone)

The differences in drag observed in this study between HSC and LSC indicate that rubber material (silicone) possesses advantages over non-rubber material (e.g., lycra) at all tested speeds. This confirms what has been previously shown in the literature for swimsuits: those made of rubber allow for a significant reduction in hydrodynamic resistance (compared with those made of textile), allowing for a significant increase in speed (1). However, no differences in drag were found in this study between LSC and CSC, indicating that differences in the material itself are not sufficient to determine differences in drag.

Indeed, the differences between HSC and CSC do not depend on the material they are made of (because both swim caps are made of silicone) but rather on their “structure”; indeed, HSC is made of several layers of silicone, and this makes it a more rigid and compact structure. This most likely makes this cap more resistant to impact forces and reduces the formation of wrinkles compared with the other two types of caps.

The use of hydrorepellent materials (such as silicone) for swim caps allows for a significant reduction in passive drag when compared with more “compliant caps” of different material (such as lycra). This is especially so in the case that silicone caps are rigid, adhere completely to the swimmer's head, and do not allow for the formation of wrinkles.

Practical Applications

Swimmers should choose carefully when selecting their equipment because the choice of one swim cap over another has a significant impact on hydrodynamic resistance (at least during the gliding phases of a race). It is therefore relevant to choose a swim cap model that allows for a “larger” reduction in drag (the most important features for a “good swim cap” are as follows: to be made of silicone, to be rigid, and to be without seams) and to take particular care to fit the swim cap properly to reduce the formation of wrinkles and to improve the fluid dynamics at the “leading edge” of the body.

Furthermore, the choice of one model over the other has an impact not only on swimming performance but can also affect the outcome of research studies: not taking into account these differences could be a confounding factor in those studies in which the effects of wearing different swim suits on passive drag are investigated.


The authors would like to thank Arena Italia for technical support and the swimmers for their willingness to participate. We also would like to thank Dr. Silvia Pogliaghi and Dr. Alessandro Lubisco for their help with the statistical analyses. The results of the present study do not constitute endorsement of the product by the authors or the NSCA.


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swimming; drag; hydrodynamics; gliding; performance

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