The relationship between front crawl swimming performance and hydrodynamic variables during leg kicking in age group swimmers

Transcription

1 Original paper The relationship between front crawl swimming performance and hydrodynamic variables during leg kicking in age group swimmers Daniel A. Marinho, Roberto C. Oliveira, Aldo M. Costa University of Beira Interior. Sport Sciences Department (UBI, Covilhã, Portugal) Research Centre in Sports, Health and Human Development (CIDESD, Portugal) Running title: Hydrodynamic variables during leg kicking in age group swimmers Abstract 1(1) : 3-12, The purpose of this study was to analyze in age-group swimmers the relationship between front crawl swimming performance and hydrodynamic variables during leg kicking. Thirty-eight age-group swimmers (10.03±0.66 years, 43.18±8.28 kg, 1.51±0.10 m) participated in this study. The 200m front crawl performance, the 200m front crawl kicking performance and the active drag during leg kicking were measured. The velocity perturbation method was used to determine active drag. The 200m front crawl performance was significantly correlated (p<0.01) with the performance in 200m kicking (0.87), the hydrodynamic drag force during leg kicking (-0.50), and with power output during kicking (-0.58). Drag coefficient values were not related to the performance in 200m front crawl. This research demonstrated a potential relationship between kicking variables and swimming velocity, suggesting the important role of kicking tasks during training in young swimmers. This study points toward the potential need for attention to kicking tasks in a swim conditioning program, especially to improve power output of the legs in young swimmers. KEY WORDS: Biomechanics, Hydrodynamics, Age group, Technique. Corresponding author: Daniel A. Marinho Universidade da Beira Interior. Departamento de Ciências do Desporto. Adress: Rua Marquês de Ávila e Bolama Covilhã. Portugal Phone: Fax: dmarinho@ubi.pt 3

2 INTRODUCTION Swimming performance is affected by several factors including swimming technique. The swimmer's technical proficiency comprises hydrodynamic variables such as hydrodynamic drag force and propelling force components. Previous investigations reported that forward propulsion in front crawl swimming is mainly achieved through the arm stroke with minimal contribution from the leg kick (e.g., Hollander et al., 1986; Toussaint & Beek, 1992). Additionally, other authors stated that the leg kick is the most inefficient action of front crawl swimming and its main function is to stabilize the trunk and keep the body in a streamlined position during swimming to reduce hydrodynamic drag (e.g., Bucher, 1974; Laurence, 1969). Konstantaki and Swaine (1999) aiming to compare the lactate and cardiopulmonary responses to simulated armpulling and leg-kicking in swimmers of different level, concluded that it is the metabolism and local muscle endurance of the arms that are enhanced with competitive swimming endurance, thus the ones determining most differences between high-level and recreational swimmers. However, Deschodt, Arsac and Rouard (1999) showed that the legs actually improve the propulsive action of the arms, thus improving the generated propulsive force of the whole body. It was shown, in a sample of women swimmers, that the kick contributes indirectly by stabilizing the trunk and streamlining the body, accounting for 9 and 6% of the total stroke velocity, respectively (Watkins & Gordon, 1983). Ogita, Hara and Tabata (1996) also reported that the total energy production during swimming was lower than simply the sum of arm only and leg kicking only swimming. It seems that the potentials of both the anaerobic and aerobic energy releasing processes in the muscle groups involved in arm and leg action cannot be fully reached during free swimming (Ogita et al., 1996). Swaine (2000), that determined the arm-pulling and leg-kicking power with the use of an isokinetic dry-land ergometer, reported that the legs could sustain greater power output than the arms during simulated swimming. This result was similar to previous research of Ogita et al. (1996), who found higher levels of anaerobic capacity (through maximal accumulated oxygen deficit) and maximal oxygen uptake (VO2max) during leg kicking than during arm stroke actions. More recently, McCullough et al. (2009) showed significant correlations between 50m swim time and 22.86m kick time (r = 0.790) in front crawl swimming. In addition to the controversy around this topic, the investigation under the importance of leg kicking to overall performance in age-group swimmers seems scarce. Moreover, most experiments used dry-land tests and/or complex procedures to assess leg-kicking performance in swimming (Ogita et al., 1996; Swaine, 1997), which are not available to be used on daily basis by coaches. In addition, to the best of our knowledge there is a lack of research on the analysis of hydrodynamic variables (drag force, drag coefficient, power output) during leg kicking and its effect on swimming performance (Marinho et al., 2011). These variables can be easily determined through the velocity perturbation method (Kolmogorov & Duplisheva, 1992), being accepted as a simple, non-invasive, and low-cost methodology, that can be used in age-group swimmers evaluations during training. 4

3 Therefore, the purpose of this study was to analyze in age-group swimmers the relationship between front crawl swimming performance and active drag, drag coefficient and power output during leg kicking. It was hypothesized that front crawl swimming is related to hydrodynamic variables during leg kicking. MATERIALS AND METHODOLOGY Sample 38 young swimmers (18 females and 20 males) volunteered to participate in this study. Their mean (± standard deviation) age, body mass, height and best swimming performance in 100m front crawl was 10.03±0.66 years, 43.18±8.28 kg, 1.51±0.10 m and, 75.19±12.57 s, respectively. All swimmers belonged to the same swimming club and were trained by the same coach for the last two years. Parents and coaches gave their consent for the swimmers participation in this study. All procedures were in accordance to the Declaration of Helsinki in respect to Human research. Procedures The performances in 200m front crawl, in 200m front crawl leg kicking and the active drag during leg kicking were measured in three consecutive days. All tests took place in a 25m indoor-swimming pool and each subject performed the tests alone with no other swimmer in the lane or nearby lanes to reduce drafting and pacing effects, affecting the drag force. Subjects performed a standardized warmup routine at the beginning of the testing sessions (about 1000 m of low intensity tasks). 200m front crawl and 200m front crawl leg kicking Each swimmer performed a maximal 200m front crawl swim with an underwater start. The time spent to cover this distance was measured with a manual chronometer (Golfinho Sports MC 815, Aveiro, Portugal) by two expert evaluators and the mean value was used for further analysis. The same procedure was used to determine the 200m front crawl leg-kicking performance. During the 200m leg-kicking test, all subjects used the same kickboard, held with their arms fully extended with their hands grasping the rounded edges of the top of the board. Active drag during leg kicking The velocity perturbation method with the help of an additional hydrodynamic body was used to determine active drag in front crawl kicking (Kolmogorov & Duplisheva, 1992; Kolmogorov, Rumyantseva, Gordon & Cappaert, 1997). Active drag was calculated from the difference between the swimming velocities with and without towing the perturbation buoy. To ensure similar maximal power output for the two sprints, the swimmers were instructed to perform maximally at both 25m trials. Between trials swimmers had a passive rest of at least 30 5

4 minutes. Each swimmer performed two maximal 25m at front crawl leg kicking with and without the perturbation device (Marinho et al., 2010). Swimming velocity was assessed during 13m (between 11m and 24m from the starting wall). The time spent to cover this distance was measured with a chronometer (Golfinho Sports MC 815, Aveiro, Portugal) by two expert evaluators and the mean value of the two times was considered for further analysis. The ICC between these two evaluators was Values greater than 0.90 can be considered reliable in the present study. Active drag (Da) was calculated as (Kolmogorov and Duplisheva, 1992): D a Db v 3 v v 2 b 3 vb Where Da represents the swimmer s active drag at maximal velocity, Db is the resistance of the perturbation buoy and, vb and v are the swimming velocities with and without the perturbation device, respectively. The drag of the perturbation buoy was calculated from the manufacturer s calibration of the buoy-drag characteristics and its velocity moving at the surface (Kolmogorov & Duplisheva, 1992). The buoy (Figure 1) has a drag characteristic of Db = 9.32v 2 and is attached with an 8m long rope to a belt around the waist of the swimmer. The approximately 8m distance between swimmer and hydrodynamic body ensured that the drag force of the buoy was unaffected by the wake created by the swimmer (Kolmogorov & Duplisheva, 1992). (1) Figure 1. The hydrodynamic buoy developed by Kolmogorov and Duplishcheva (1992) used in the current study. 1 Emerged part of the buoy; 2 Water surface line; 3 Water entry; 4 cylinder supports; 5 rope passages; 6 hydrodynamic cylinder. Active Drag coefficient (CDa) was calculated as: C Da 2 D S v a 2 Where is the density of the water (assumed to be 1000 kg/m 3 ), Da is the swimmer s active drag, v is the swimmer s velocity and S is the projected frontal surface area of the swimmers. Frontal surface area was estimated using Clarys s prediction (Clarys, 1979), according to: S = 6.93BM +3.50H (2) (3) 6

5 Where BM is the body mass and H is the swimmer s height. The power needed to overcome the drag force (PD) was computed as: P D D v a Where PD is the power to overcome drag force, Da is the swimmer s active drag and v is the swimmers velocity. (4) Statistics The normality of the distributions was assessed with the Shapiro-Wilk test. Descriptive statistics (mean and one standard deviation) from all variables were calculated. The performance in the 200m front crawl was related to active drag data (drag force, drag coefficient and power output values) and to the performance in the 200m leg kicking using Pearson correlation coefficient. The level of statistical significance was set at p RESULTS Table 1 presents the descriptive (mean ± 1 standard deviation) values of the performance in 200m front crawl, the performance in 200m front crawl kicking and the active drag parameters during leg kicking. Table 1. Descriptive (mean ± 1 standard deviation) values of the analyzed variables. Variable Mean±1SD 200m front crawl performance (s) ± m front crawl kicking performance (s) ±49.60 Drag force during kicking (N) 10.80±5.80 Drag coefficient during kicking 0.30±0.12 Power output during kicking (W) 9.00±5.00 Table 2 presents the correlation between the 200m front crawl performance and the hydrodynamic variables during leg kicking (200m performance, drag force, drag coefficient, power output). A significant correlation (p<0.01) was found between the 200m front crawl performance and the performance in the 200m kicking only (0.87), and hydrodynamic drag force during leg kicking actions (- 0.50), and power output during kicking (-0.58). Drag coefficient values during leg kicking were not related to the performance in 200m front crawl. 7

6 Table 2. Pearson correlation coefficient between 200m front crawl performance and the hydrodynamic variables during kicking. Kicking variables 200m kicking performance Drag force during kicking Drag coefficient during kicking Power output during kicking 200m front crawl performance r = 0.87, p<0.01 r = -0.50, p<0.01 r =-0.18, p=0.30 r = -0.58, p<0.01 DISCUSSION The aim of this study was to analyse in age-group swimmers the relationship between front crawl performance and hydrodynamic variables during leg kicking. Main data suggests that swimming performance is related to hydrodynamic drag force, power output and velocity during leg kicking actions. The velocity perturbation method was used to determine active drag in front crawl kicking. This method has been previously used in other investigations (e.g., Garrido et al., 2010; Kolmogorov et al., 1997), representing a simple and reliable approach to determine active drag in young swimmers. Indeed, one of the great advantages of this methodology is to allow its use in large groups of swimmers. In contrast to other methodologies, that required heavy and costly experimental procedures, the velocity perturbation method just required the use of the hydrodynamic body device and a chronometer to assess active drag, an important criterion to be considered during young swimmers evaluations (Barbosa et al., 2009). To the best of our knowledge, there is scarce research in the literature analyzing the active drag during leg kicking tests in young swimmers (Marinho et al., 2011). Therefore, the comparisons are somewhat difficult to be carried-out. Hydrodynamic drag values of the current study were much lower than data found in other experiments conducted with children for whole body front crawl swimming, assessed with this same procedure (e.g., Kjendlie & Stallman, 2008; Marinho et al., 2010). This difference was expected, since the velocity achieved during leg kicking is much lower than the one during whole body front crawl stroke. Moreover, the body position during leg kicking tends to be more stable, thus reducing hydrodynamic drag (McCullough et al., 2009). Nevertheless, when comparing to similar data, current data were very close to values found during active drag measurements in front crawl leg kicking (Marinho et al., 2011). Slightly differences could be due to gender level, since Marinho et al. (2011) only studied female age-group swimmers, whereas the present study analysed both boys and girls. Thus, as expected, our sample presented slightly higher values of drag force, drag coefficient and power output, and slightly lower values in the 200m front crawl and in the 200m leg kicking. Regarding to the relationship between front crawl performance and kicking variables, it is important to underline the significant association between the front crawl performance and the 200m kicking performance, and drag and power output during leg kicking. The swimmers who achieved better 8

7 performances in 200m front crawl presented higher values of hydrodynamic drag and power output and better performances in the leg-kicking test. McCullough et al. (2009) found similar trend (r=0.79) in short distance tests (correlation between 50m swim time and 22.86m kick time), in adult women (i.e., 20.6±1.6 years-old). Marinho et al. (2010) found also similar trends between swimming performance, drag force, power output and performance during leg kicking, when analyzing female age-group swimmers, reinforcing the association between leg kicking actions and swimming performance. Although the assumption that the contribution of leg kicking to overall performance in front crawl is reduced, when compared to arm stroke (e.g., Hollander et al., 1986; Toussaint & Beek, 1992), the results of the current study can be supported by the data of Deschodt et al. (1999), stating the relevant role of leg kicking in front crawl stroke in young swimmers. Moreover, the kick can help reducing drag during swimming by keeping the body in a more streamlined position and this aim should be emphasized by coaches (Engesvik, 1992; Onusseit, 1972). The kick can also contribute to stabilize the trunk, neutralizing the reaction to the arm motion, and keeping the position of the trunk fairly stable (Engesvik, 1992; Laurance, 1969). This is even more important when considering age group swimmers, with less capacity to stabilize swimming technique during the events. In the current study the body alignment during front crawl swimming was not measured. However, it seems that the best swimmers during leg kicking performance can achieve higher velocities during front crawl, being the power output during leg kicking a determining variable to this aim. However, one should be aware that only 200m front crawl swimming was analyzed. Different trends could be expected if shorter front crawl distances were considered. Drag coefficient values during leg kicking were not correlated with swimming performance (r=-0.18, p=0.30), thus suggesting that the shape and the position of the body during kicking is not so important as the power output generated by the legs. This seems possible due to the importance of drag force during kicking (higher velocity and higher power output values lead to higher drag force values) to the overall swimming performance (r=-0.50, p<0.01). Additionally, during leg kicking the position of the body is very stable, being similar between swimmers of different skill level, thus leading to similar values of drag coefficient between swimmers when performing kicking tests (Engesvik, 1992). Moreover, one should be aware that kicking performance is expected to differ between long and short distance tests. Thus, some differences can be expected during the active drag test in leg kicking (13m distance) and the swimming performance (200m test). Additionally, the estimation of frontal surface area using Clarys s prediction could lead to some errors, since the angle of attack and the position during leg kicking was not considered. Some limitations can be addressed regarding to this paper. These results cannot be applied to other aged swimmers (e.g., adult/elite ones) and to remaining swim strokes, rather than front crawl stroke. Although front crawl can be considered the basis of swimming training routines, it is important to analyse the role of leg kicking to overall swimming performance in butterfly, backstroke and breaststroke. Moreover, others variables can play an important role during leg kicking that were not considered in the current study. It could be interested to analysed anthropometric data, especially of the lower limbs, and ankle flexibility, and related these results to leg kicking performance as suggested previously 9

8 (McCullough et al., 2009). Additionally, the use of Clarys s equation to predict frontal surface area could lead to some bias, since Clarys used adult swimmers. It seems important to develop different equations to estimate frontal surface area in young swimmers of different age, since frontal surface area determines hydrodynamic drag. On the other hand, the angle of attack and the position of the body holding a kicking board should also be considered in further analysis. CONCLUSIONS Front crawl swimming performance seems to be associated to leg kicking actions in age group swimmers. The swimmers who presented higher values of hydrodynamic drag force and power output during leg kicking also achieved better performances in the 200m front crawl. Moreover, the correlation between the leg kicking performance and the 200m front crawl performance underlies the role of leg kicking action to overall performance in front crawl swimming at these ages. Although it is well known that correlations do not imply causations, they can give insights into potential factors, which may be associated with different components of swimming technique. This research demonstrated a potential relationship between kicking variables and swimming velocity. This study points toward the potential need for attention to kicking tasks in a swim conditioning program, especially to improve power output of the legs in young swimmers. Acknowledgement The authors would like to thank the important contribution of the swimmers who participated in this research. This work was supported by University of Beira Interior (UBI/FCSH/Santander/2010). REFERENCES 1. Barbosa, T.M., Costa, M.J., Marinho, D.A., Coelho, J., Moreira, M. & Silva, A.J. (2009). Modelling the links between age-group swimming performance, energetic and biomechanic profiles. Pediatric Exerc Sci, 22(3), august/modeling-the-links-between-young-swimmers-performance-energeticand-biomechanic-profiles 2. Bucher, W. (1974). The influence of the leg kick and the arm stroke on the total speed during the crawl stroke. In J.P. Clarys & L. Lewillie (Eds.), Swimming II (pp ). Baltimore, MD: University Park Press 3. Clarys, J.P. (1979). Human morphology and hydrodynamics. In J. Terauds, & E.W. Bedingfield (Eds), Swimming III (pp. 3-41). Baltimore: University Park Press. 10

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