Effect of leg kick on active drag in front-crawl swimming: Comparison of whole stroke and arms-only stroke during front-crawl and the streamlined position
Introduction
In front-crawl swimming, which is the fastest human swimming stroke among the four techniques used in swimming competitions, the arms and legs move repeatedly across the water surface, creating additional resistance from waves and splashes. Therefore, it is extremely difficult to evaluate the resistance force acting on a swimmer who propels the water surface with moving limbs.
The resistance force acting on a swimmer maintaining a streamlined position (SP), which is known as ‘passive drag’, has been widely measured and used as an index to evaluate swimmers (Chatard et al., 1990, Havriluk, 2005) and swimsuit performance (Gatta et al., 2013, Mollendorf et al., 2004). Passive drag remains similar regardless of measurement environment if swimmers maintain the same posture and shape (Havriluk, 2005, Havriluk, 2007). Measuring resistance force during swimming (known as ‘active drag’) is more difficult, and various methods have been suggested to estimate it.
Hollander et al. (1986) developed the measurement of active drag (MAD) approach and attempted to directly measure actual drag during swimming. The MAD approach measures the force of a swimmer pushing off fixed pads placed under the water surface. Active drag is then estimated based on the precondition that resistance force is equal to the force exerted on the pads by the swimmer's hands when the swimming velocity is constant. This means that only the arm stroke can be evaluated using the MAD approach. The values of active drag obtained using the MAD approach are reported to be similar to those of passive drag (Hollander et al., 1986, Van der Vaart et al., 1987). Previous studies that used trunk incline and projected frontal area to evaluate the active drag of front-crawl with kicking found it to be larger than passive drag (Gatta et al., 2015, Zamparo et al., 2009). Similar results were reported in studies using an energetics approach (Di Prampero et al., 1974, Zamparo et al., 2005). Narita et al. (2017) estimated drag during swimming using measured values of residual thrust (MRT) and found similar results. From data reported in previous studies, it is evident that the active drag in front-crawl swimming is larger than that in passive conditions, except when the data were obtained using the MAD approach. In MAD approach, the swimmer does not use leg movement unlike other studies. Therefore, the lack of leg movement is thought to be one of the reasons why the values of drag measured with the MAD approach were similar to those of passive drag (i.e. we assume that the kicking motion increases active drag). On the other hand, previous studies have reported that kicking during front-crawl reduced the resistive force acting on the whole body by elevating the legs and counteracting the sinking moment produced by the arm motion (Nakashima, 2007, Yanai, 2001). However, no studies have yet evaluated the influence of kicking on active drag using the same method and swimmers. Furthermore, it is unclear whether the differences in reported values are due to the addition of lower limb motion or to differences in methodology. To address these shortcomings, it is necessary to verify the effect of kicking on active drag by evaluating active drag with and without lower limb motion using the same method and swimmers. Moreover, by comparing the active drag evaluated using the MRT method with the passive drag, it is possible to evaluate the changes in the resistance force acting on the swimmer’s body in response to any active movements due to self-propelling. These comparisons would promote understanding of the resistance forces that act on swimmers as they move their limbs to propel themselves through the water surface.
Therefore, the purpose of this study was to investigate the effect of leg kick on resistance force in front-crawl swimming. Moreover, we compared active drag and passive drag in SP for the same swimmers. The active drag in front-crawl swimming with and without kicking was evaluated using the MRT method. We hypothesized that active drag in front-crawl swimming is larger than passive drag regardless of leg kicking because (1) the projected frontal area of the swimmer during front-crawl swimming is larger than that with SP and (2) the active movements of the upper and lower limbs cause additional resistance force owing to waves and splashes. In addition, we hypothesized that the active drag with leg kicking is larger than that without because leg kicking creates additional drag. As Clarys (1979) reported, active drag is mainly influenced by changes in body shape and movements of the body segments; therefore, the use of the lower limbs would deform the streamlined posture of the lower limbs and increase resistance force.
Section snippets
Participants
Seven male competitive swimmers participated in this study. They all trained six days per week and had experience of participating in national competitions. The anthropometric data and long-course front-crawl performance of swimmers are given in Table 1. The test procedures were approved by the University of Tsukuba Ethics Committee (approval number: 25–57), and each participant signed an informed-consent form.
Experimental design
To compare active drag with and without kicking, each swimmer performed front-crawl
Results
The WS, AS and SP drag values of all swimmers at 1.1 and 1.3 m s−1 are given in Table 2. Active drag in WS and AS conditions was found to be 1.6–1.9 times larger than that in passive conditions at both swimming velocities. In addition, the values of UTre0 (or U for the SP) and CD for each condition are shown in the upper panel of Fig. 1. We found no main effect in the swimming velocity (UTre0 for WS and AS or U for SP) at 1.1 or 1.3 m s−1 in each condition (at VS1.1: F = 0.183, p = 0.84; at V
Effects of leg motion on active drag in front-crawl swimming
The present study is the first to examine the drag caused by leg motion during front-crawl swimming. We achieved this using the same swimmers and methodology throughout. The results indicate no significant differences between the WS and AS values of CD for the VS1.1 and VS1.3 trials.
Previous studies reported that kicking in front-crawl reduced the resistive force acting on the whole body by elevating the legs and counteracting the sinking moment produced by the arm motion (Nakashima, 2007,
Conflict of interest statement
There are no conflicts of interest to declare.
Acknowledgements
This study was supported by a Grant-in-Aid for Scientific Research [15K12641] from the Japan Society for the Promotion of Science.
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