TECHNICAL REPORT
EFFECT OF SWIM CAP SURFACE ROUGHNESS PASSIVE DRAG GIORGIO GATTA,1 MATTEO CORTESI,1
AND
ON
PAOLA ZAMPARO2
1
Department for Life Quality Studies, Rimini, School of Pharmacy, Biotechnology and Sport Science, University of Bologna, Bologna, Italy; and 2Department of Neurological, Neuropsychological, Morphological and Movement Sciences, University of Verona, Verona, Italy ABSTRACT
Gatta, G, Cortesi, M, and Zamparo, P. Effect of swim cap surface roughness on passive drag. J Strength Cond Res 29(11): 3253– 3259, 2015—In the last decade, great attention has been given to the improvements in swimming performance that can be obtained by wearing “technical swimsuits”; the technological evolution of these materials only marginally involved swim caps production, even if several studies have pointed out the important role of the head (as main impact point with the fluid) on hydrodynamics. The aim of this study was to compare the effects on passive drag (Dp) of 3 swim cap models: a smooth silicon helmet cap (usually used during swimming competitions), a silicon helmet cap with “dimples,” and a silicon helmet cap with “wrinkles.” Experiments were performed on 10 swimmers who were towed underwater (at a depth of 60 cm) at 3 speeds (1.5, 1.7, and 1.9 m$s21) and in 2 body positions: LA (arms above the swimmer’s head) and SA (arms alongside the body). The Dp values obtained in each trial were divided by the square of the corresponding speed to obtain the speed-specific drag (the k coefficient = Dp/v2). No differences in k were observed among swim caps in the LA position. No differences in k were observed between the smooth and dimpled helmets also in the SA position; however, the wrinkled swim cap helmet showed a significant larger k (4.4%) in comparison with the model with dimples, when the swimmers kept their arms alongside the body (in the SA position). These data suggest that wearing a wrinkled swim cap helmet can be detrimental to performance at least in this specific position.
KEY WORDS swimming, hydrodynamic gliding, performance INTRODUCTION
I
n human locomotion, at constant speed, propulsive forces equal resistant forces; when the former are larger than the latter, the body accelerates, and when the latter are larger than the former, the body decelerates. In
Address correspondence to Matteo Cortesi, [email protected]. 29(11)/3253–3259 Journal of Strength and Conditioning Research Ó 2015 National Strength and Conditioning Association
water locomotion, the major force resisting motion is hydrodynamic resistance (D, drag); thus, a swimmer able to produce a given propulsive force would improve his/her performance (would proceed at a larger speed) if he/she can reduce the resistance forces at stake. Even a small decrease in drag could make the difference; indeed, in elite competition, very short winning time margins are generally observed. To reduce D, pressure and/or wave and/or friction drag should be reduced. Although the friction drag contribution remains approximately constant when swimming speed increases, pressure and wave drag increase with it; studies on passive drag indicate that when swimming at the water surface, pressure drag is the major determinant of D accounting for 74, 55, and 51% at 1.0, 2.0, and 2.2 m$s21, respectively, whereas friction (24, 25, and 23%) and wave (2, 20, and 26%) drag have a lower influence on D at the corresponding speeds (15). Reducing pressure drag is thus expected to result in the greater improvement in performance (5). Wave drag, besides on speed, depends also on the level of immersion (21): it is lower than 5% of total passive drag at depths .0.5–0.7 m; in these conditions, therefore, the effects of pressure and friction drag on total drag can be more easily appreciated. In the last decades, studies on the “evolution” of technical swimsuits have drawn attention on the role of friction drag on swimming performance (3). This component of D can be reduced by changing the interface between the swimmer’s body and the surrounding fluid by making the body surface as smooth as possible—e.g., by shaving body air, as shown by Sharp and Costill 1989 (18) or using smooth silicone helmet swim caps as shown by Gatta et al. (9) or by using vortex generators and/or riblets (24,22). In both cases, the transition of the “boundary layer” (the fluid in direct contact with the body) from laminar to turbulent is delayed. In the boundary layer, the flow velocity at the body surface is considered to be nil because of no-slip conditions; at increasing distances from the body surface, the flow velocity increases until it reaches the “free-stream” velocity. Because of the Bernoulli’s principle, when the fluid encounters impact points along the swimmer’s body (i.e., head, shoulders, glutei), it increases its speed, losing “local pressure” (17); this could induce the fluid to detach from the VOLUME 29 | NUMBER 11 | NOVEMBER 2015 |
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Effects of Swim Cap Roughness body resulting in the generation of vortexes, with an increase in turbulence along the body and hence in pressure drag. Thus, some authors argue that changes in the body-fluid interface have main effects on pressure drag rather than on friction drag (13,14). In any case, the surface characteristics of a body moving in a fluid influence fluid mechanics (total drag), and therefore, the effects of different surfaces on hydrodynamic resistance are largely studied in fluid dynamics. Shark skin was the first to attract the attention of swimsuits manufacturers. This surface is covered by small denticles (dermal tooth-like elements) equally spaced in a linear-arrayed pattern (23,2) that create local changes in the surrounding fluid (10). In a recent study, Wen et al. (25) designed and fabricated a flexible biomimetic shark skin membrane using 3D printing; they tested this surface in water using a flapping device and showed that it allows to increase swimming speed of about 6.6% compared with a smooth control model (without denticles). Vortex generators and riblets on swimsuits have also been suggested to reduce drag (24). FastSkin swimsuits use a microscopic system of riblets equally spaced in a lineararrayed pattern (mimicking shark skin); in some (but not all) studies, these suits have been shown to reduce passive drag (13) and to improve performance (19). By applying turbulators (trip wires) to specific points along a swimsuit, Pendergast et al. (14) have shown that passive drag can be reduced from 11% (1 turbulator) to 16% (3 turbulators). These authors have calculated that the use of trip wires do not affect friction or wave drag (at the investigated speeds: 0.4–2.2 m$s21) but have a significant effect on pressure drag; the latter was indeed significantly reduced (19–41%) when turbulators were used because they seem to disrupt flow separation, thus reducing pressure drag. Whereas turbulators, riblets, or denticles protrude from the object/body surface, dimples are depressions on the object/body surface. Dimples of different sizes in golf balls were shown to delay the transition of the “boundary layer” from laminar to turbulent (1,20); even if this effect is well known (and studied) in fluid dynamics, it was investigated in other sports (tennis, golf, and cricket), but to our knowledge, data on these matters are not reported in the swimming literature. Given that the head of a swimmer is an important point of impact with the fluid (13,14,16,26), the swim cap has an important role on hydrodynamic resistance, even if it covers only a small portion of the swimmer’s body. The few studies on swim caps reported in the literature indicate that wearing a swim cap can reduce hydrodynamic resistance up to 15% compared with a no-swim cap condition (the study by Marinho et al. (12) was performed with computational fluid dynamic analysis) and that the swim cap model has a significant effect on passive drag (8); a rigid silicon helmet swim cap allows for a 5–6.5% reduction in hydrodynamic resistance compared with a textile swim cap model.
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The aim of this study was to further investigate the effects of different swim caps on hydrodynamic resistance (passive drag) by focusing on swim cap roughness as a possible performance factor.
METHODS Experimental Approach to the Problem
In the literature, only few data are reported on the effects of different swim caps on passive drag (Dp); a previous study (9) indicated that swim cap texture has an effect on Dp; passive drag is indeed reduced (the more so, the larger the speed) when silicone caps are used instead of textile caps (Lycra); a further reduction in Dp can be obtained using rigid silicone caps (helmets) without seams. It could be of interest, therefore, to investigate whether further decreases in Dp can be obtained by changing the surface roughness of a helmet silicon cap, the model that, in the previous study (9), was shown to allow for the largest decrease in drag. To investigate these matters, we decided to use the same method to assess passive drag (towing experiments) that we used in the previous study (9), and we decided to compare the swim cap that was previously shown to be the “best model” (3D Race–91554, Arena, Macerata, Italy; with a smooth surface) with 2 new models, conceived and designed ad hoc by Arena (Macerata, Italy): one with bumps and one with dimples. As we expected small differences in drag between these 3 swim cap models, we decided to modify/improve the protocol (in comparison with the previous study) to avoid some confounding factors and to improve the accuracy of our measurements. In the previous study, we asked the swimmers to maintain a “standard prone streamlined position,” with the arms outstretched in front of the head; however, in this position, the impact point is actually on the hands, rather than on the head of the swimmer. Considering that the effects on Dp of different swim cap surfaces could be better observed if the head is the point that actually “breaks the water,” we tested also a second body position: prone, with the arms alongside the body. Furthermore, we decided to perform the towing experiments not at the water surface but at a depth of 60 cm; this to avoid wave drag (a possible confounding factor) and to be closer to the typical conditions during a race (e.g., gliding after starts and turns) when Dp differences should matter. The main hypotheses of this study, therefore, were (a) that Dp would change with the swim cap surface roughness and (b) that the differences in Dp among swim caps would be larger in the swimmer’s position with the arms alongside the body. In more general terms, we wanted (a) to draw attention on a possible performance factor (swim cap roughness) and (b) to verify if today’s technology has the potential to further improve swimming performance: to decrease passive drag (and hence to increase swimming speed) at least during the gliding phases of a race (after starts and turns).
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Ten male swimmers (age: 21 6 2 years; stature: 1.80 6 0.06 m; body mass: 75.9 6 6.9 kg) participated to the study. Their average swimming experience was 10 6 3 years, and their average volume of training was 25 6 10 km$wk21; all swimmers were competing at regional level and were specialized in short crawl swimming events. The study conformed to the standards set by the Declaration of Helsinki, and the procedures were approved by the bioethics committee of the local university. All swimmers signed a provided written informed consent before involvement in the study. Procedures
Testing was performed while the swimmers were in a regular training period; the sessions were conducted in a 25-m indoor swimming pool (average water temperature: 28.0 6 0.58 C), and each session (for each subject) lasted about 2.5 hours. The subjects were asked to refrain from alcohol, caffeine, and strenuous exercise in the 24 hours preceding the test. Data collection was divided into 3 steps: Anthropometric measures and 15-minute warm-up period (freestyle swimming). Habituation trials to allow the subjects to practice to maintain a “stable prone position” during the towing test. These trials were repeated several times at the 3 different speeds and in the 2 body positions (Figure 2). Main session: the measurements of passive drag were performed with the swimmer linked (by a nonelastic wire) to a low-voltage isokinetic engine positioned at the edge of the pool (Swim-Spektro; Talamonti Spa, Ascoli Piceno, Italy) that dragged the subject at constant velocity (1.5, 1.7, and 1.9 m$s21); the resistance force
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was measured by a dynamometer that was calibrated before the tests. After the experiments, data were downloaded to a PC and further analyzed by means of a dedicated software (DB:4, Talamonti Spa). Three rings (90 cm in diameter) were positioned in the swimming pool at 10, 15, and 20 m from the pool wall (the starting point); these rings were anchored to the swim floor so that their center was at a depth of 60 cm from the water surface. The tow line was fed, although a pulley positioned at the same water depth (60 cm); the swimmers were asked to adopt a stable prone position and to hold their breath, after full inspiration; they were then towed underwater through these rings (i.e., they performed their glide at a controlled water depth of 60 cm); the trials in which a swimmer was not able to follow this trajectory were discarded (and repeated). Average values of force (in newton) and speed (in meters per second) were calculated between the 10th and 20th meter after the start when stability was attained in these values. Overall, 900 trials were randomly performed: 5 repetitions, at the 3 swimming speeds (1.5, 1.7, and 1.9 m$s21), in the 2 body positions, with the 3 swim cap models for each of the 10 participants. For further analyses, the Dp values obtained in each trial (Dp = k$v2, N) were divided by the square of the corresponding speed (v2, m2$s22) to obtain the speed-specific drag (k coefficient = Dp/v2, N$m22$s22). Swim Caps and Body Positions
The swim caps used in this study were: A “smooth” helmet silicone swim cap without seams (SC1, 3D Race–91554; Arena); the swim cap surface is smooth (Figure 1); A “dimpled” helmet silicone swim cap without seams (SC2, 3D GForce–1E169; Arena); the swim cap surface presents small dimples (,1 cm in diameter), regularly spaced (Figure 1); A “wrinkled” helmet silicone swim cap without seams (SC3, 3D Ultra X– 1E 168; Arena); the swim cap surface presents irregularly spaced wrinkles (,1 cm in thickness) (Figure 1). When wearing the swim caps, the dimensions of wrinkles and dimples could change slightly, according to the “head size” of each swimmer. An Figure 1. The swim caps used in this study. SC1 = Smooth helmet silicone cap; SC2 = helmet silicone cap with operator helped the swimmers dimples; SC3 = helmet silicone cap with wrinkles. See text for details. to fit the swim caps tightly over VOLUME 29 | NUMBER 11 | NOVEMBER 2015 |
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Effects of Swim Cap Roughness Types: different types of materials may be used on the same cap, and they can be of different thicknesses; however, differences in thickness and material shall not be used to form shapes. These prototypes were approved by FINA from January 1, 2015 (http://www.fina.org/ H2O/index.php?option=com_ content&view=article&id=2217: arena&catid=304:fina-approvedswimwear& Itemid=1006). As indicated in the Aims, we Figure 2. The body positions used in this study. LA position = arms above the swimmer’s head; SA position = wanted to test these models to arms alongside the body. understand if swim cap roughness could have an effect on performance whether these the head and to remove (eventual) air bubbles. Swimmers swim caps could be used in competitions or not. were provided with the same swim goggles (Swedic; Arena) The experiments were repeated in 2 (prone) body and were requested to wear the strap under the cap. positions (Figure 2): LA position (arms above the swimmer’s head): the SC2 and SC3 swim caps prototypes were manufactured according to the FINA rules (6) that indicate the characterelbow and wrist are completely extended and the arms istics of the swim caps that can be used in swimming races: are outstretched in front of the swimmer (in contact Health: the material used must not put the health of the with the sides of the head), one hand is on top of the athletes at risk. other and both hold onto the wire (Figure 2). Hardness: no hard material can be used (the material SA position (arms alongside the body): the elbow and can have some rigidity but shall be able to follow the wrist are completely extended and the arms are posishape of the head; no hard helmets). tioned along the body (the hands in contact with the
TABLE 1. Individual data of passive drag for each swim cap model and speed condition in the SA position (arms alongside the body). Passive drag (N) in SA condition 1.5 (m$s21)
1.7 (m$s21)
1.9 (m$s21)
Subject
SC1
SC2
SC3
SC1
SC2
SC3
SC1
SC2
SC3
1 2 3 4 5 6 7 8 9 10 Mean SD
88.1 89.7 74.4 75.7 61.4 83.5 79.6 72.6 70.4 60.8 75.6 9.9
88.4 87.7 70.3 72.0 58.1 83.1 78.9 73.1 70.4 58.2 74.0 10.7
87.4 92.5 72.7 78.2 64.5 80.3 84.3 80.4 70.3 71.6 78.2 8.6
101.0 108.6 93.7 91.8 70.5 100.0 96.9 85.8 87.9 68.6 90.5 12.9
96.1 106.7 87.3 88.9 71.1 97.2 97.0 82.8 82.5 65.7 87.5 12.5
107.5 108.7 85.8 89.7 75.0 98.8 99.7 91.1 85.4 72.7 91.5 12.3
125.5 124.3 110.1 113.8 81.9 109.4 114.2 111.0 100.1 85.1 107.5 14.6
113.1 122.9 104.6 110.4 82.2 115.4 111.9 109.7 95.4 78.8 104.4 14.5
127.5 130.1 106.1 108.9 86.2 116.1 115.8 105.4 100.3 80.8 107.7 15.9
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TABLE 2. Individual data of passive drag for each swim cap model and speed condition in the LA position (arms above the swimmer’s head). Passive drag (N) in LA condition 1.5 (m$s21)
1.7 (m$s21)
1.9 (m$s21)
Subject
SC1
SC2
SC3
SC1
SC2
SC3
SC1
SC2
SC3
1 2 3 4 5 6 7 8 9 10 Mean SD
65.9 64.3 56.2 64.4 44.7 77.8 65.8 57.5 54.4 50.4 60.1 9.4
65.9 63.2 57.6 58.5 43.1 69.5 70.6 59.2 56.5 48.4 59.2 8.7
70.5 65.9 57.5 60.8 46.8 70.9 70.5 54.8 53.5 50.8 60.2 8.8
80.5 74.6 71.6 79.4 52.0 90.5 81.9 69.3 65.5 58.7 72.4 11.6
80.7 77.6 72.0 72.4 52.6 87.2 81.8 74.2 68.3 61.4 72.8 10.2
79.5 78.7 71.9 75.3 55.6 82.6 85.6 68.2 67.7 63.2 72.8 9.3
89.1 93.1 79.6 85.1 65.7 102.9 93.2 81.3 80.8 73.4 84.4 10.7
91.6 88.0 84.4 92.0 60.2 96.8 90.9 88.4 80.2 71.8 84.4 11.0
94.2 89.9 84.6 89.7 60.3 95.6 91.2 84.8 80.7 75.4 84.6 10.5
sides of the thighs). The wire, in this position, is passed under the armpits (Figure 2). In both SA and LA, the swimmers were requested to maintain the position of the head as fixed as possible and to look down at the swimming pool floor. Statistical Analyses
Data are reported as mean 6 SD. To quantify the agreement between the 5 trials (for each swimmer, swim cap model,
body position, and speed condition) the coefficient of variation (% CV) was calculated; the mean absolute value was quite low (2.1%); thus, the average value of the 5 trials (for each swimmer, speed, and condition) was computed and used in further analysis. A 2-way repeated-measures ANOVA was used to assess the effect of body position and swim cap model (and their interaction) on the k coefficient (k = Dp/v2). Pairwise multiple comparison procedures were performed with a Tukey’s post hoc test to compute the differences in k among the swim cap models. Differences were considered statistically significant at p # 0.05, and Cohen’s effect size (ES) was reported for significant pairwise comparisons as a measure of effect size (4). Threshold values for Cohen’s ES statistic were .0.2 (small), .0.5 (moderate), and .0.8 (large). Statistical analysis was performed with SPSS Statistics Rel. 14.0.0 (SPSS, Chicago, IL, USA).
RESULTS Figure 3. Average values of k coefficient for the 3 swim cap models (SC1 = diamonds, smooth helmet silicone cap; SC2 = circles, helmet silicone cap with dimples; SC3 = triangles, helmet silicone cap with wrinkles) in the 2 body positions (*p # 0.05: SC2 vs. SC3).
In Tables 1 and 2, the passive drag values (Dp, N) are reported at the 3 investigated speeds, for the 3 swim cap
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Effects of Swim Cap Roughness models, and the 2 body positions. In Figure 3, the average values of the drag coefficient (k = Dp/v2) are showed for the 3 swim cap conditions (SC1: diamonds; SC2: circles; SC3: triangles) and the 2 body positions. Analysis of variation revealed significant differences in swim cap condition (F(2, 18) = 4.801, p # 0.05), in body position (F(1, 9) = 124.682, p , 0.001), and a significant interaction for the swim cap condition 3 body position (F(2, 18) = 4.723, p # 0.05). No differences in k were observed among swim caps in the LA position. No differences in k were observed between the smooth and dimpled helmets in the SA position; however, the wrinkled swim cap helmet showed a significant larger k (4.4%) in comparison with the model with dimples when the swimmers kept their arms alongside the body (in the SA position); in this case, the Cohen’s ES was rated as small (0.33). These data suggest that wearing a wrinkled swim cap helmet can be detrimental to performance in this specific position.
DISCUSSION Even if, during a race, active drag (rather than passive drag) is the factor that influences performance, 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 (passive) hydrodynamic resistance. Data reported in this article indicate that the “classical” streamlined body position (LA) is characterized by far lower values of passive drag in comparison with SA and thus that the former should be the position of choice in these phases, as previously shown in the literature (11,21). Data reported in this article also indicate that differences in passive drag can be observed when the swimmers (towed underwater) wear helmet silicone caps with different surface roughness. These differences, however, can be detected only if the swimmer’s head represents the first point of impact with the water (when the arms are alongside the body, in the SA position). As previously shown by Mollendorf et al. (13) in the “classical” streamlined body position (e.g., with the arms outstretched in front of the swimmer head, in the LA position), the arms, rather than the head, impact the water first, so that the flow changes from laminar to turbulent already at the level of the elbows. Therefore, the differences in swim caps surface roughness could be better appreciated in the SA position because the water flow is still laminar when it reaches the head (and hence the swim cap). Indeed, our data show that no differences in k (and hence in Dp) could be appreciated in the LA condition and that, in the SA position, surface roughness does not represent an advantage compared with a smooth surface: wrinkled swim caps are associated with an increase in k (and hence in passive drag) and dimpled swim caps show no difference in k in comparison with a smooth helmet. It can be hypothesized that the wrinkled cap increases total drag because of an increase in friction drag that is greater than any possible benefit from a reduction in pressure drag.
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Although our previous study (9) was focused on the effects on drag, in the classical streamlined (gliding) position, of the swim caps that are normally used during training or competitions, the focus of this study was, instead, to investigate the effects on drag of unconventional swim caps prototypes that can be designed and manufactured based on the observations reported in the literature about the different strategies adopted by marine animals to reduce drag (7). Data reported in this article show that swim cap surface dimples could actually decrease hydrodynamic resistance, and this suggests that further improvements in performance could be expected by wearing these prototypes.
PRACTICAL APPLICATIONS Swimming performance improves when the resistance forces can be reduced; even a small decrease in drag could make the difference because, in elite competitions, very short winning time margins are observed. Data reported in this article show that in the gliding phases of a race, it is better to assume the classical streamlined position (LA) to reduce drag, as previously found in the literature (8). In our previous work (9), we have shown that rigid helmet silicone caps could reduce passive drag compared with other swim cap models, and indeed, these are the swim caps that are nowadays used by elite swimmers in international competitions. Contrary to our hypothesis, swim cap roughness does not seem to be a possible performance factor and no further improvements in speed can be foreseen using rigid silicone helmets presenting wrinkles (or dimples) on their surface. Our swimmers reported, however, that in the LA position, the roughness of the swim cap surface helped them in maintaining the arms close to the head and thus to stabilize their streamlined position; this aspect could be analyzed in future studies as, for both swim cap prototypes, no detrimental effect on drag was observed in this condition.
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EFFECT OF SWIM CAP SURFACE ROUGHNESS PASSIVE DRAG GIORGIO GATTA,1 MATTEO CORTESI,1
AND
ON
PAOLA ZAMPARO2
1
Department for Life Quality Studies, Rimini, School of Pharmacy, Biotechnology and Sport Science, University of Bologna, Bologna, Italy; and 2Department of Neurological, Neuropsychological, Morphological and Movement Sciences, University of Verona, Verona, Italy ABSTRACT
Gatta, G, Cortesi, M, and Zamparo, P. Effect of swim cap surface roughness on passive drag. J Strength Cond Res 29(11): 3253– 3259, 2015—In the last decade, great attention has been given to the improvements in swimming performance that can be obtained by wearing “technical swimsuits”; the technological evolution of these materials only marginally involved swim caps production, even if several studies have pointed out the important role of the head (as main impact point with the fluid) on hydrodynamics. The aim of this study was to compare the effects on passive drag (Dp) of 3 swim cap models: a smooth silicon helmet cap (usually used during swimming competitions), a silicon helmet cap with “dimples,” and a silicon helmet cap with “wrinkles.” Experiments were performed on 10 swimmers who were towed underwater (at a depth of 60 cm) at 3 speeds (1.5, 1.7, and 1.9 m$s21) and in 2 body positions: LA (arms above the swimmer’s head) and SA (arms alongside the body). The Dp values obtained in each trial were divided by the square of the corresponding speed to obtain the speed-specific drag (the k coefficient = Dp/v2). No differences in k were observed among swim caps in the LA position. No differences in k were observed between the smooth and dimpled helmets also in the SA position; however, the wrinkled swim cap helmet showed a significant larger k (4.4%) in comparison with the model with dimples, when the swimmers kept their arms alongside the body (in the SA position). These data suggest that wearing a wrinkled swim cap helmet can be detrimental to performance at least in this specific position.
KEY WORDS swimming, hydrodynamic gliding, performance INTRODUCTION
I
n human locomotion, at constant speed, propulsive forces equal resistant forces; when the former are larger than the latter, the body accelerates, and when the latter are larger than the former, the body decelerates. In
Address correspondence to Matteo Cortesi, [email protected]. 29(11)/3253–3259 Journal of Strength and Conditioning Research Ó 2015 National Strength and Conditioning Association
water locomotion, the major force resisting motion is hydrodynamic resistance (D, drag); thus, a swimmer able to produce a given propulsive force would improve his/her performance (would proceed at a larger speed) if he/she can reduce the resistance forces at stake. Even a small decrease in drag could make the difference; indeed, in elite competition, very short winning time margins are generally observed. To reduce D, pressure and/or wave and/or friction drag should be reduced. Although the friction drag contribution remains approximately constant when swimming speed increases, pressure and wave drag increase with it; studies on passive drag indicate that when swimming at the water surface, pressure drag is the major determinant of D accounting for 74, 55, and 51% at 1.0, 2.0, and 2.2 m$s21, respectively, whereas friction (24, 25, and 23%) and wave (2, 20, and 26%) drag have a lower influence on D at the corresponding speeds (15). Reducing pressure drag is thus expected to result in the greater improvement in performance (5). Wave drag, besides on speed, depends also on the level of immersion (21): it is lower than 5% of total passive drag at depths .0.5–0.7 m; in these conditions, therefore, the effects of pressure and friction drag on total drag can be more easily appreciated. In the last decades, studies on the “evolution” of technical swimsuits have drawn attention on the role of friction drag on swimming performance (3). This component of D can be reduced by changing the interface between the swimmer’s body and the surrounding fluid by making the body surface as smooth as possible—e.g., by shaving body air, as shown by Sharp and Costill 1989 (18) or using smooth silicone helmet swim caps as shown by Gatta et al. (9) or by using vortex generators and/or riblets (24,22). In both cases, the transition of the “boundary layer” (the fluid in direct contact with the body) from laminar to turbulent is delayed. In the boundary layer, the flow velocity at the body surface is considered to be nil because of no-slip conditions; at increasing distances from the body surface, the flow velocity increases until it reaches the “free-stream” velocity. Because of the Bernoulli’s principle, when the fluid encounters impact points along the swimmer’s body (i.e., head, shoulders, glutei), it increases its speed, losing “local pressure” (17); this could induce the fluid to detach from the VOLUME 29 | NUMBER 11 | NOVEMBER 2015 |
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Effects of Swim Cap Roughness body resulting in the generation of vortexes, with an increase in turbulence along the body and hence in pressure drag. Thus, some authors argue that changes in the body-fluid interface have main effects on pressure drag rather than on friction drag (13,14). In any case, the surface characteristics of a body moving in a fluid influence fluid mechanics (total drag), and therefore, the effects of different surfaces on hydrodynamic resistance are largely studied in fluid dynamics. Shark skin was the first to attract the attention of swimsuits manufacturers. This surface is covered by small denticles (dermal tooth-like elements) equally spaced in a linear-arrayed pattern (23,2) that create local changes in the surrounding fluid (10). In a recent study, Wen et al. (25) designed and fabricated a flexible biomimetic shark skin membrane using 3D printing; they tested this surface in water using a flapping device and showed that it allows to increase swimming speed of about 6.6% compared with a smooth control model (without denticles). Vortex generators and riblets on swimsuits have also been suggested to reduce drag (24). FastSkin swimsuits use a microscopic system of riblets equally spaced in a lineararrayed pattern (mimicking shark skin); in some (but not all) studies, these suits have been shown to reduce passive drag (13) and to improve performance (19). By applying turbulators (trip wires) to specific points along a swimsuit, Pendergast et al. (14) have shown that passive drag can be reduced from 11% (1 turbulator) to 16% (3 turbulators). These authors have calculated that the use of trip wires do not affect friction or wave drag (at the investigated speeds: 0.4–2.2 m$s21) but have a significant effect on pressure drag; the latter was indeed significantly reduced (19–41%) when turbulators were used because they seem to disrupt flow separation, thus reducing pressure drag. Whereas turbulators, riblets, or denticles protrude from the object/body surface, dimples are depressions on the object/body surface. Dimples of different sizes in golf balls were shown to delay the transition of the “boundary layer” from laminar to turbulent (1,20); even if this effect is well known (and studied) in fluid dynamics, it was investigated in other sports (tennis, golf, and cricket), but to our knowledge, data on these matters are not reported in the swimming literature. Given that the head of a swimmer is an important point of impact with the fluid (13,14,16,26), the swim cap has an important role on hydrodynamic resistance, even if it covers only a small portion of the swimmer’s body. The few studies on swim caps reported in the literature indicate that wearing a swim cap can reduce hydrodynamic resistance up to 15% compared with a no-swim cap condition (the study by Marinho et al. (12) was performed with computational fluid dynamic analysis) and that the swim cap model has a significant effect on passive drag (8); a rigid silicon helmet swim cap allows for a 5–6.5% reduction in hydrodynamic resistance compared with a textile swim cap model.
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The aim of this study was to further investigate the effects of different swim caps on hydrodynamic resistance (passive drag) by focusing on swim cap roughness as a possible performance factor.
METHODS Experimental Approach to the Problem
In the literature, only few data are reported on the effects of different swim caps on passive drag (Dp); a previous study (9) indicated that swim cap texture has an effect on Dp; passive drag is indeed reduced (the more so, the larger the speed) when silicone caps are used instead of textile caps (Lycra); a further reduction in Dp can be obtained using rigid silicone caps (helmets) without seams. It could be of interest, therefore, to investigate whether further decreases in Dp can be obtained by changing the surface roughness of a helmet silicon cap, the model that, in the previous study (9), was shown to allow for the largest decrease in drag. To investigate these matters, we decided to use the same method to assess passive drag (towing experiments) that we used in the previous study (9), and we decided to compare the swim cap that was previously shown to be the “best model” (3D Race–91554, Arena, Macerata, Italy; with a smooth surface) with 2 new models, conceived and designed ad hoc by Arena (Macerata, Italy): one with bumps and one with dimples. As we expected small differences in drag between these 3 swim cap models, we decided to modify/improve the protocol (in comparison with the previous study) to avoid some confounding factors and to improve the accuracy of our measurements. In the previous study, we asked the swimmers to maintain a “standard prone streamlined position,” with the arms outstretched in front of the head; however, in this position, the impact point is actually on the hands, rather than on the head of the swimmer. Considering that the effects on Dp of different swim cap surfaces could be better observed if the head is the point that actually “breaks the water,” we tested also a second body position: prone, with the arms alongside the body. Furthermore, we decided to perform the towing experiments not at the water surface but at a depth of 60 cm; this to avoid wave drag (a possible confounding factor) and to be closer to the typical conditions during a race (e.g., gliding after starts and turns) when Dp differences should matter. The main hypotheses of this study, therefore, were (a) that Dp would change with the swim cap surface roughness and (b) that the differences in Dp among swim caps would be larger in the swimmer’s position with the arms alongside the body. In more general terms, we wanted (a) to draw attention on a possible performance factor (swim cap roughness) and (b) to verify if today’s technology has the potential to further improve swimming performance: to decrease passive drag (and hence to increase swimming speed) at least during the gliding phases of a race (after starts and turns).
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Ten male swimmers (age: 21 6 2 years; stature: 1.80 6 0.06 m; body mass: 75.9 6 6.9 kg) participated to the study. Their average swimming experience was 10 6 3 years, and their average volume of training was 25 6 10 km$wk21; all swimmers were competing at regional level and were specialized in short crawl swimming events. The study conformed to the standards set by the Declaration of Helsinki, and the procedures were approved by the bioethics committee of the local university. All swimmers signed a provided written informed consent before involvement in the study. Procedures
Testing was performed while the swimmers were in a regular training period; the sessions were conducted in a 25-m indoor swimming pool (average water temperature: 28.0 6 0.58 C), and each session (for each subject) lasted about 2.5 hours. The subjects were asked to refrain from alcohol, caffeine, and strenuous exercise in the 24 hours preceding the test. Data collection was divided into 3 steps: Anthropometric measures and 15-minute warm-up period (freestyle swimming). Habituation trials to allow the subjects to practice to maintain a “stable prone position” during the towing test. These trials were repeated several times at the 3 different speeds and in the 2 body positions (Figure 2). Main session: the measurements of passive drag were performed with the swimmer linked (by a nonelastic wire) to a low-voltage isokinetic engine positioned at the edge of the pool (Swim-Spektro; Talamonti Spa, Ascoli Piceno, Italy) that dragged the subject at constant velocity (1.5, 1.7, and 1.9 m$s21); the resistance force
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was measured by a dynamometer that was calibrated before the tests. After the experiments, data were downloaded to a PC and further analyzed by means of a dedicated software (DB:4, Talamonti Spa). Three rings (90 cm in diameter) were positioned in the swimming pool at 10, 15, and 20 m from the pool wall (the starting point); these rings were anchored to the swim floor so that their center was at a depth of 60 cm from the water surface. The tow line was fed, although a pulley positioned at the same water depth (60 cm); the swimmers were asked to adopt a stable prone position and to hold their breath, after full inspiration; they were then towed underwater through these rings (i.e., they performed their glide at a controlled water depth of 60 cm); the trials in which a swimmer was not able to follow this trajectory were discarded (and repeated). Average values of force (in newton) and speed (in meters per second) were calculated between the 10th and 20th meter after the start when stability was attained in these values. Overall, 900 trials were randomly performed: 5 repetitions, at the 3 swimming speeds (1.5, 1.7, and 1.9 m$s21), in the 2 body positions, with the 3 swim cap models for each of the 10 participants. For further analyses, the Dp values obtained in each trial (Dp = k$v2, N) were divided by the square of the corresponding speed (v2, m2$s22) to obtain the speed-specific drag (k coefficient = Dp/v2, N$m22$s22). Swim Caps and Body Positions
The swim caps used in this study were: A “smooth” helmet silicone swim cap without seams (SC1, 3D Race–91554; Arena); the swim cap surface is smooth (Figure 1); A “dimpled” helmet silicone swim cap without seams (SC2, 3D GForce–1E169; Arena); the swim cap surface presents small dimples (,1 cm in diameter), regularly spaced (Figure 1); A “wrinkled” helmet silicone swim cap without seams (SC3, 3D Ultra X– 1E 168; Arena); the swim cap surface presents irregularly spaced wrinkles (,1 cm in thickness) (Figure 1). When wearing the swim caps, the dimensions of wrinkles and dimples could change slightly, according to the “head size” of each swimmer. An Figure 1. The swim caps used in this study. SC1 = Smooth helmet silicone cap; SC2 = helmet silicone cap with operator helped the swimmers dimples; SC3 = helmet silicone cap with wrinkles. See text for details. to fit the swim caps tightly over VOLUME 29 | NUMBER 11 | NOVEMBER 2015 |
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Effects of Swim Cap Roughness Types: different types of materials may be used on the same cap, and they can be of different thicknesses; however, differences in thickness and material shall not be used to form shapes. These prototypes were approved by FINA from January 1, 2015 (http://www.fina.org/ H2O/index.php?option=com_ content&view=article&id=2217: arena&catid=304:fina-approvedswimwear& Itemid=1006). As indicated in the Aims, we Figure 2. The body positions used in this study. LA position = arms above the swimmer’s head; SA position = wanted to test these models to arms alongside the body. understand if swim cap roughness could have an effect on performance whether these the head and to remove (eventual) air bubbles. Swimmers swim caps could be used in competitions or not. were provided with the same swim goggles (Swedic; Arena) The experiments were repeated in 2 (prone) body and were requested to wear the strap under the cap. positions (Figure 2): LA position (arms above the swimmer’s head): the SC2 and SC3 swim caps prototypes were manufactured according to the FINA rules (6) that indicate the characterelbow and wrist are completely extended and the arms istics of the swim caps that can be used in swimming races: are outstretched in front of the swimmer (in contact Health: the material used must not put the health of the with the sides of the head), one hand is on top of the athletes at risk. other and both hold onto the wire (Figure 2). Hardness: no hard material can be used (the material SA position (arms alongside the body): the elbow and can have some rigidity but shall be able to follow the wrist are completely extended and the arms are posishape of the head; no hard helmets). tioned along the body (the hands in contact with the
TABLE 1. Individual data of passive drag for each swim cap model and speed condition in the SA position (arms alongside the body). Passive drag (N) in SA condition 1.5 (m$s21)
1.7 (m$s21)
1.9 (m$s21)
Subject
SC1
SC2
SC3
SC1
SC2
SC3
SC1
SC2
SC3
1 2 3 4 5 6 7 8 9 10 Mean SD
88.1 89.7 74.4 75.7 61.4 83.5 79.6 72.6 70.4 60.8 75.6 9.9
88.4 87.7 70.3 72.0 58.1 83.1 78.9 73.1 70.4 58.2 74.0 10.7
87.4 92.5 72.7 78.2 64.5 80.3 84.3 80.4 70.3 71.6 78.2 8.6
101.0 108.6 93.7 91.8 70.5 100.0 96.9 85.8 87.9 68.6 90.5 12.9
96.1 106.7 87.3 88.9 71.1 97.2 97.0 82.8 82.5 65.7 87.5 12.5
107.5 108.7 85.8 89.7 75.0 98.8 99.7 91.1 85.4 72.7 91.5 12.3
125.5 124.3 110.1 113.8 81.9 109.4 114.2 111.0 100.1 85.1 107.5 14.6
113.1 122.9 104.6 110.4 82.2 115.4 111.9 109.7 95.4 78.8 104.4 14.5
127.5 130.1 106.1 108.9 86.2 116.1 115.8 105.4 100.3 80.8 107.7 15.9
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TABLE 2. Individual data of passive drag for each swim cap model and speed condition in the LA position (arms above the swimmer’s head). Passive drag (N) in LA condition 1.5 (m$s21)
1.7 (m$s21)
1.9 (m$s21)
Subject
SC1
SC2
SC3
SC1
SC2
SC3
SC1
SC2
SC3
1 2 3 4 5 6 7 8 9 10 Mean SD
65.9 64.3 56.2 64.4 44.7 77.8 65.8 57.5 54.4 50.4 60.1 9.4
65.9 63.2 57.6 58.5 43.1 69.5 70.6 59.2 56.5 48.4 59.2 8.7
70.5 65.9 57.5 60.8 46.8 70.9 70.5 54.8 53.5 50.8 60.2 8.8
80.5 74.6 71.6 79.4 52.0 90.5 81.9 69.3 65.5 58.7 72.4 11.6
80.7 77.6 72.0 72.4 52.6 87.2 81.8 74.2 68.3 61.4 72.8 10.2
79.5 78.7 71.9 75.3 55.6 82.6 85.6 68.2 67.7 63.2 72.8 9.3
89.1 93.1 79.6 85.1 65.7 102.9 93.2 81.3 80.8 73.4 84.4 10.7
91.6 88.0 84.4 92.0 60.2 96.8 90.9 88.4 80.2 71.8 84.4 11.0
94.2 89.9 84.6 89.7 60.3 95.6 91.2 84.8 80.7 75.4 84.6 10.5
sides of the thighs). The wire, in this position, is passed under the armpits (Figure 2). In both SA and LA, the swimmers were requested to maintain the position of the head as fixed as possible and to look down at the swimming pool floor. Statistical Analyses
Data are reported as mean 6 SD. To quantify the agreement between the 5 trials (for each swimmer, swim cap model,
body position, and speed condition) the coefficient of variation (% CV) was calculated; the mean absolute value was quite low (2.1%); thus, the average value of the 5 trials (for each swimmer, speed, and condition) was computed and used in further analysis. A 2-way repeated-measures ANOVA was used to assess the effect of body position and swim cap model (and their interaction) on the k coefficient (k = Dp/v2). Pairwise multiple comparison procedures were performed with a Tukey’s post hoc test to compute the differences in k among the swim cap models. Differences were considered statistically significant at p # 0.05, and Cohen’s effect size (ES) was reported for significant pairwise comparisons as a measure of effect size (4). Threshold values for Cohen’s ES statistic were .0.2 (small), .0.5 (moderate), and .0.8 (large). Statistical analysis was performed with SPSS Statistics Rel. 14.0.0 (SPSS, Chicago, IL, USA).
RESULTS Figure 3. Average values of k coefficient for the 3 swim cap models (SC1 = diamonds, smooth helmet silicone cap; SC2 = circles, helmet silicone cap with dimples; SC3 = triangles, helmet silicone cap with wrinkles) in the 2 body positions (*p # 0.05: SC2 vs. SC3).
In Tables 1 and 2, the passive drag values (Dp, N) are reported at the 3 investigated speeds, for the 3 swim cap
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Effects of Swim Cap Roughness models, and the 2 body positions. In Figure 3, the average values of the drag coefficient (k = Dp/v2) are showed for the 3 swim cap conditions (SC1: diamonds; SC2: circles; SC3: triangles) and the 2 body positions. Analysis of variation revealed significant differences in swim cap condition (F(2, 18) = 4.801, p # 0.05), in body position (F(1, 9) = 124.682, p , 0.001), and a significant interaction for the swim cap condition 3 body position (F(2, 18) = 4.723, p # 0.05). No differences in k were observed among swim caps in the LA position. No differences in k were observed between the smooth and dimpled helmets in the SA position; however, the wrinkled swim cap helmet showed a significant larger k (4.4%) in comparison with the model with dimples when the swimmers kept their arms alongside the body (in the SA position); in this case, the Cohen’s ES was rated as small (0.33). These data suggest that wearing a wrinkled swim cap helmet can be detrimental to performance in this specific position.
DISCUSSION Even if, during a race, active drag (rather than passive drag) is the factor that influences performance, 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 (passive) hydrodynamic resistance. Data reported in this article indicate that the “classical” streamlined body position (LA) is characterized by far lower values of passive drag in comparison with SA and thus that the former should be the position of choice in these phases, as previously shown in the literature (11,21). Data reported in this article also indicate that differences in passive drag can be observed when the swimmers (towed underwater) wear helmet silicone caps with different surface roughness. These differences, however, can be detected only if the swimmer’s head represents the first point of impact with the water (when the arms are alongside the body, in the SA position). As previously shown by Mollendorf et al. (13) in the “classical” streamlined body position (e.g., with the arms outstretched in front of the swimmer head, in the LA position), the arms, rather than the head, impact the water first, so that the flow changes from laminar to turbulent already at the level of the elbows. Therefore, the differences in swim caps surface roughness could be better appreciated in the SA position because the water flow is still laminar when it reaches the head (and hence the swim cap). Indeed, our data show that no differences in k (and hence in Dp) could be appreciated in the LA condition and that, in the SA position, surface roughness does not represent an advantage compared with a smooth surface: wrinkled swim caps are associated with an increase in k (and hence in passive drag) and dimpled swim caps show no difference in k in comparison with a smooth helmet. It can be hypothesized that the wrinkled cap increases total drag because of an increase in friction drag that is greater than any possible benefit from a reduction in pressure drag.
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Although our previous study (9) was focused on the effects on drag, in the classical streamlined (gliding) position, of the swim caps that are normally used during training or competitions, the focus of this study was, instead, to investigate the effects on drag of unconventional swim caps prototypes that can be designed and manufactured based on the observations reported in the literature about the different strategies adopted by marine animals to reduce drag (7). Data reported in this article show that swim cap surface dimples could actually decrease hydrodynamic resistance, and this suggests that further improvements in performance could be expected by wearing these prototypes.
PRACTICAL APPLICATIONS Swimming performance improves when the resistance forces can be reduced; even a small decrease in drag could make the difference because, in elite competitions, very short winning time margins are observed. Data reported in this article show that in the gliding phases of a race, it is better to assume the classical streamlined position (LA) to reduce drag, as previously found in the literature (8). In our previous work (9), we have shown that rigid helmet silicone caps could reduce passive drag compared with other swim cap models, and indeed, these are the swim caps that are nowadays used by elite swimmers in international competitions. Contrary to our hypothesis, swim cap roughness does not seem to be a possible performance factor and no further improvements in speed can be foreseen using rigid silicone helmets presenting wrinkles (or dimples) on their surface. Our swimmers reported, however, that in the LA position, the roughness of the swim cap surface helped them in maintaining the arms close to the head and thus to stabilize their streamlined position; this aspect could be analyzed in future studies as, for both swim cap prototypes, no detrimental effect on drag was observed in this condition.
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