Biomechanics and Medicine in Swimming XI

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Effect of Stroke Drills on Intra-cycle Hip Velocity in Front Crawl Arellano r., domínguez-castells r., Perez-Infantes e., sánchez e. University of Granada, Granada, Spain Competitive strokes have been widely analysed, while kinematics of coordinative drills, used to train or teach swimming techniques, have been scarcely studied. The aim of this work is to evaluate the differences in the intra-cycle hip velocity among freestyle swimming drills. Thirteen national and regional level swimmers performed five 25-m freestyle trials, using different stroke drills in a random order. The propulsive phase of each stroke was analysed. Mean and peak (one or two) velocity were obtained, and their corresponding phase percentage. The first peak recorded corresponded to the inward movement, which coincided with the final propulsion or downward movement of the opposite arm. The second one corresponded to the final backward-upward movement, which, is in most cases, the highest value of velocity while the non-propulsive arm is extended horizontally in front of the shoulder. The freestyle stroke drills applied to teach or train different types of inter-limb coordination reduce the mean and peak values of intra-cycle hip velocity while the percentage location of peak hip-velocity value during the underwater stroke phase is similar without significant statistical differences. Key words: swimming kinematics, swimming technique. IntroductIon Current teaching and competitive swimming programs are composed of the correct combination of skill acquisition and conditioning exercises. Many swimming books or papers describe or classify these swimming exercises proposing guidelines to use them properly or in a skill assessment context (Goldsmith et a. 2007; Guzman, 1998; Laughlin & Delves, 1996; Maglischo, 2003; Sweetenham & Atkinson, 2003). The classifications used to include coordinative drills: arm-arm coordination, arm-breathing coordination and arm-leg coordination. Lately, armarm coordination drills have become very popular applied to develop the crawl-stroke swimming technique in master swimming, triathlon and competitive swimming while they were and are broadly used in the advanced learning stages of technique development in ‘learn to swim’ programs (Figueiredo et al., 2009; Seifert & Chollet, 2009). While hip or centre of mass intra-cycle velocity has been studied in the current competitive strokes using observational or biomechanical methods (Barbosa, et al., 2005; Figueiredo, et al., 2009; Psycharakis et al. 2009), this has not been the case with the stroke drills applied to teach or train swimming technique. Stroke coordination, bi-dimensional or tridimensional kinematics, intra-cycle velocity, body-trunk rotation, stroke frequency and stroke length have been some subjects of study frequently applied to formal strokes but missed out in these drills. A first attempt was performed to analyse the differences in body rotation and 3D hand swimming path between freestyle swimming and one arm crawl stroke drills (López et al. 2002). Less body rotation and hand depth were found during the practice of formal one-arm and catch-up crawl stroke, while a modified one-arm stroke drill obtained similar values to that recorded during no breathing freestyle swimming. The purpose of this study is to reveal the differences in intra-cycle hip velocity between formal front crawl and four other front crawl swimming coordination drills. Methods Thirteen national and regional level swimmers (five males and eight females, aged 19.58±2.23) participated in this study as volunteers. All of them had trained in swimming for 6-8 years. Every participant had chaPter2.Biomechanics previous experience of the exercises. The protocol was fully explained to them and they provided written consent to participate in the study, which was approved by the university ethics committee. This investigation was performed in September, at the beginning of the competitive season. The characteristics of the participants are presented in Table 1. Table 1. Anthropometric characteristics of participants in the study (n=13). Body mass (kg) Height (cm) Max. body length (cm) Arm span (cm) Mean (SD) 67.4 (10.5) 171 (7) 228 (11) 181 (11) After a standard warm-up, swimmers randomly performed five 25-m freestyle trials, using a different stroke drill and no-breathing freestyle. They always started in the water and were instructed to swim as fast as possible, keeping to the stroke drill technique. All trials were performed during a specific testing session. Each participant performed every trial with a rest interval of more than 5 minutes (see Table 2 for stroke drill definitions). Table 2. Stroke drills applied in the study. A No-breathing formal freestyle swimming (reference technique) B Crawl catch-up stroke, kicking some seconds after each stroke C One arm front crawl with the resting arm extended in front, breathing on the arm-moving side D One arm front crawl with the resting arm close to the body, breathing on the no-moving side E Controlled two-arm freestyle, swimmer perform arm-pull and arm-recovery simultaneously finishing each half cycle with one arm resting close to the body and the second one resting extended in front, kicking some seconds after each stroke. During each trial the swimmers` time and intra-cycle velocity were recorded synchronized with underwater video recording. With this information, peak and mean velocity for each stroke (and its phases) were obtained. The behaviour of the swimmers during the experiment was recorded with two underwater cameras, which were attached by a metal device to the lateral edge of the pool. Both cameras were placed 50cm below the water surface and were positioned 10 and 15m from the starting wall of the pool, respectively. The swimmers swam in lane 3, so that the cameras were able to record at least 15m of their performance from a lateral view. An electronic timer connected to a touch pad was used to measure the swimmer’s times. Velocity was measured by a linear encoder, attached to the swimmers` waist using a belt. The associated software enabled us to receive the data and videos in the computer in real time, ready to be processed through two USB 2.0 ports. Velocity data were sampled at 200Hz and the video signal synchronized at 50 Hz. After selecting the stroke-phase sections of the videos, and before further processing, data were filtered using a Butterworth filter, with a cut-off frequency of 6 Hz. For every participant, three strokes were selected and represented, in order to compare the velocity variations among them. A complete underwater stroke phase was considered from the beginning of the entry of the hand into the water until it was completely out. All the underwater stroke durations (100%) were normalized to a percentage scale, making it easier to compare strokes of different duration. A statistical analysis was performed using Microsoft Excel 2007 and Statistica/Mac Plus 5.1. Then, the raw data was checked for normal distribution characteristics (p=0.96 for velocity). And finally, an analysis of variance with repeated measures was applied to determine variability between exercises using Scheffé post-hoc tests for intra-group comparisons. 45
BiomechanicsandmedicineinswimmingXi results Mean values of 25m-velocity and 25m-performance for every swimming drill were obtained (see Table 3). The recorded times and mean velocities were slower in all the swimming drills compared with the nobreathing freestyle (p
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Pahlen Norge AS Pahlen Norge AS Cha
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