Taekwondo is a combat sport wherein leg techniques

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1 IMPACT FORCE AND TIME ANALYSIS INFLUENCED BY EXECUTION DISTANCE IN A ROUNDHOUSE KICK TO THE HEAD IN TAEKWONDO ISAAC ESTEVAN, 1 OCTAVIO ÁLVAREZ, 2 CORAL FALCO, 1 JAVIER MOLINA-GARCÍA, 3 AND ISABEL CASTILLO 4 1 Department of Physical Activity and Sport Sciences, Catholic University of Valencia, Valencia, Spain; 2 Cheste Sport Medicine Center, Valencia Sports Council, Valencian International University, Valencia, Spain; 3 Department of Music, Plastic and Body Expression University of Valencia, Valencia, Spain; and 4 Department of Social Psychology, University of Valencia, Valencia, Spain ABSTRACT Estevan, I, Álvarez, O, Falco, C, Molina-García, J, and Castillo, I. Impact force and time analysis influenced by execution distance in a roundhouse kick to head in Taekwondo. J Strength Cond Res 25(10): , 2011 The execution distance is a tactic factor that affects mechanical performance and execution technique in taekwondo. This study analyzes roundhouse kick to head by comparing maximum impact force, execution time, and impact time in 3 distances according to athletes competition level. It also analyzes relationship between impact force and weight in each group. It examines wher execution distance affects maximum impact force, execution time, and impact time, in each level group or 2 different competition levels. Participants were 27 male taekwondo players (13 medallists and 14 nonmedallists). The medallists executed roundhouse kick to head with greater impact force and in a shorter execution time than did nonmedallists when y kicked from any distance different to ir combat distance. However, results showed that execution distance is influential in execution time and impact time in nonmedallist group. It is considered appropriate to orientate high-level competitors to train for offensive actions from any distance similar to long execution distance because it offers equally effectiveness and a greater security against opponent. Also, practitioners should focus ir training to improve time performance because it is more affected by distance than impact force. KEY WORDS biomechanics, performance, martial arts, level, tactic, technique Address correspondence to Isaac Estevan, isaac.estevan@ucv.es. 25(10)/ Journal of Strength and Conditioning Research Ó 2011 National Strength and Conditioning Association INTRODUCTION Taekwondo is a combat sport wherein leg techniques are most used in combats (9). The literature stated that athletes must adapt to ir optimal execution distance because it is crucial in terms of scoring point and taekwondo performance matches (10). Execution or combat distance must be determined individually (9) according to anthropometric characteristics of subject (e.g., leg length that is considered by athletes as ir combat distance) (7,8,10). In a sparring match, athletes have to kick from shorter and longer distances than ir execution distance, which affects ir execution technique. In this way, literature on combat sports suggests that changes in execution distance affects mechanical parameters of kick (7,10). Among key mechanical parameters related to scoring points, we find impact force and time (7,13). Execution time is considered a main factor for attacker to be able to score points (10). Kicks directed to head showed an execution time that ranged from to seconds depending on level of taekwondo players and kick performed (1,22,23). Roundhouse kick to head implies a short execution time and a high chance to score (10). When a point is scored, it does not interrupt combat. Therefore, it is important that competitor, after making 1 move, is capable of being in a position to execute a new technique. A variable that allows analysis of correct positioning to coordinate different actions is impact time (20). Impact time may depend on measuring device (18), and it is an important factor for coordination of 2 or more kicks (20). To score a point, it must be a powerful strike to areas established by rules (24). The variable traditionally used to measure forcefulness has been striking force (5,17). There are differences in units of measurement given in studies that analyze impact force of kicks, which complicates comparison and standardization of results (17,21). Likewise, among results obtained in different studies, we find units presented in newtons (7), in kg (force) (3), in g (force acceleration) (11), and in Nm (5). Noneless, re is a consensus about VOLUME 25 NUMBER 10 OCTOBER Copyright National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.

2 Mechanical Parameters in Taekwondo high peak impact force when a roundhouse kick is done (Figure 1). When strike force requires to be enhanced, athletes usually use ir higher technique level (7) or ir mass (13). Along se lines, studies found a positive correlation between weight and striking (18,19), where weight predicted 32.6% of variation in maximum impact force in noncompetitors group (7). Researchers who have analyzed hit performance in combat sports have included participants with experience in sport compared by competitive level (5,10,14,15,17). In general, y have found mechanical differences as a lower execution time and a higher maximum impact force for competitors than for noncompetitors (7). Likewise, hyposes of this study were as follows: first, level influences mechanical performance in roundhouse kick to head in every execution distance. Second, weight predicts maximum impact force in lower level athletes for all execution distances. Finally, execution distance affects mechanical parameters in each group. Consequently, to advance knowledge in mechanical influence in 1 key tactical point, that is, execution distance in taekwondo, overall purpose of this study was threefold: first, to compare maximum impact force, execution time, and impact time in roundhouse kick to head in relation to execution distance according to athletes competition level (medallists and nonmedallists). Second, to explore relationship between maximum impact force and weight in each group; and finally, to examine wher execution distance affects maximum impact force, execution time, and impact time, in each of 2 different level groups. METHODS Experimental Approach to Problem The roundhouse kick to head is one of actions with highest percentage of knock-out (16) and a kind of kick Figure 1. Measure of a force curve (N) of a roundhouse kick to head. whose performance is best adapted from any distance (10). Because of change of World Taekwondo Federation competition rules in 2010, kicks to head score 3 points, whereas kicks to chest score just 1 point. Nowadays, in combats, kicks to head are much more often used than before changing of rules. To contribute scientific knowledge relating to variables relevant to kick performance in combat sports, distance, time and force, present study based on Falco et al s. (7) procedure was carried out. To measure force exerted by body on a load, a force platform was placed with 5 piezoresistant sensors (Flexiforce_ A201 model by Tekscan Company Inc., South Boston, MA, USA) in a pentagonal structure on dummy. This structure was chosen taking into consideration that force of kick would be distributed more homogeneously on area of hit. The 5 force platform sensors were calibrated following manufacturer s recommendations (sensor-by-sensor first and finally 5 sensors toger). The calibrating process allowed us to adjust sensitivity based on a known load that helped achieve best resolution. The sensor s system sensitivity was 22.2 N. The Cronbach alpha internal consistency reliability was Every participant kicked twice for each of 3 different execution distances (6 trials per athlete). Figure 2 presents experimental setup with 3 execution distances and dummy on force platform. Visual Basic 6.0 was used to develop software capable of analyzing data captured by system. The variables of study were as follows: execution distance defined as horizontal distance between competitor s front foot and objective to be hit. Execution time is defined as time that passes from instant when striking foot leaves floor until it reaches objective and achieves maximum impact force (7). Impact time is defined as length of time striking foot is in contact with objective to be struck. Impact force is defined as its maximum level on striking objective (oriented to measure forces perpendicular to surface of target). Weight and height were measured on a calibrated digital scale (SECA, Vogel & Halke, GmbH & Co, Hamburg, Germany). Subjects Healthy and trained 27 male Spanish taekwondo athletes, selected through a convenience model, participated in study; ir average 6 SD age was years, with weight between 52.2 and 98 kg ( ), and height between 1.58 and 1.93 m ( ). They were divided into 2 groups according to results obtained in official competitions: medallist group (n = 13) composed of medal winners in official national and international events and nonmedallist group (n = 14) consisting of taekwondo athletes who have not won medals in official competitions. All athletes have competition experience of at least 4 years and train for at least 3 hwk Journal of Strength and Conditioning Research

3 Journal of Strength and Conditioning Research Figure 2. System setup with 3 distances: short (ED 1 ), medium (ED 2 ), and long (ED 3 ). 1. force platform; 2. contact platform; 3. Pc; 4. microcontroller; and 5. mannequin. Procedure Before data collection, all athletes completed institutional written informed consents. The institutional review board approved all study procedures (Ethical Committee of University of Valencia). A protocol was designed and followed by subjects: (a) The individual s leg length was used to determine 3 kicking distances. The leg length was identified as medium distance (ED 2 : m), long distance was identified as product of ED 2 and 1.33 (ED 3 : m), and short distance equaled ED 2 multiplied by 0.67 (ED 1 ) ( m). The target area was leveled to TABLE 1. Comparative analysis between medallist and nonmedallist groups in maximum impact force, execution time, and impact time as a function of execution distance.* ED Mean 6 SD Medallist (n = 13) Nonmedallist (n = 14) MIF (N) ED 1 1, , ED 2 1, , ED 3 1, , ET (s) ED ED k ED k IT (s) ED ED k ED k *ED = execution distance; ED 1 = short; ED 2 = medium; ED 3 = long; MIF = maximum impact force; ET = execution time; IT = impact time. p, Significant differences between medallists and nonmedallists. No significant differences among execution distances (p, 0.05). kno significant differences among execution distances (p, 0.05). subject s chin height. (b) After a 20-minute warm-up, athlete was placed in front of dummy with supporting leg at trial distance and kicking leg on contact platform. Each subject was asked to freely kick breastplate of dummy, as strong and fast as you can when you are ready. (c) The order of trials kicks were medium (ED 2 ), long (ED 3 ), and short (ED 1 ) distances, in sequential order, for all subjects. Subjects were given seconds of rest between kicks in same distance and 2 minutes between distances. Statistical Analyses The preliminary analysis (Kolmogorov Smirnov) showed a normal distribution of all considered variables. A mixedmodel analysis of variance (ANOVA) (a = 0.05) was used to compare maximum impact force, execution time, and impact time according to competition level and to examine effects of execution distance on maximum impact force, execution time, and impact time in both groups separately (medallists and nonmedallists). A Greenhouse Geisser correction was used to provide a more conservative analysis, where necessary, to compensate for violations of sphericity assumption. Pairwise comparisons were performed using Bonferroni statistics to analyze significant effects of ANOVAs. Cohen s d score was quantified to analyze effect size (4). Cohen considered that a d. 0.8 signified a large effect. The intraclass correlation coefficients (ICCs) for maximum impact force, execution time, and impact time were determined. The 95% confidence intervals (CIs) were determined for each ICC (25). Pearson s correlation coefficient was calculated for each level according to execution distance to determine relationship between weight and maximum impact force. Also, a linear regression was used to assess effect of weight on impact force. Statistical analyses were carried out by SPSS 15.0 computer package. RESULTS Statistical descriptions (mean and SD) are shown in Table 1. The maximum impact force, execution time and impact time had an ICC of R = 0.89 (95% CI, ), R = 0.79 (95% CI, ), and R = 0.33 (95% CI, to 0.66), respectively. The mixed-model ANOVA 3 (execution distance [short, medium and long]) 3 2(competitive VOLUME 25 NUMBER 10 OCTOBER

4 Mechanical Parameters in Taekwondo level [medallists and nonmedallists]) using Bonferroni posttest to get pairwise comparisons according to level, revealed that medallists kicked from ED 1 and ED 3 with a greater maximum impact force than did nonmedallists (p, 0.05). Moreover, medallists kick in shorter execution time from ED 1 and ED 3 than did nonmedallists (p, 0.05). short distance equaled ED 2 multiplied by 0.67 d = 0.86 in ED 1 and d = 1.04 in ED 3. For execution time was d = 1.12 and d = 1.70 in ED 1 and ED 3, respectively. No differences in impact time were found according to level. To determine relationship between weight and maximum impact force, Pearson s correlation coefficient was calculated. For nonmedallists, significant positive correlations (p, 0.01) were found between weight and maximum impact force in ED 1 (r = 0.77), ED 2 (r = 0.64), and ED 3 (r = 0.64). For medallist group, re were no significant correlations between weight and maximum impact force in any of distances examined. A linear regression was used to assess effect of weight on impact force where, in nonmedallist group, weight explains 58.8% of variation in maximum impact force (p, 0.01) in ED 1 (b = 0.77), 40,1% in ED 2 (b = 0.63), and 40,6% in ED 3 (b = 0.64). Furrmore, Bonferroni posttest was used to examine effects of execution distance in both groups separately and revealed that for medallists, execution time was significantly shorter (p, 0.01) from ED 1 and ED 3 without significant differences between ED 1 and ED 2 or ED 2 and ED 3. The normalized effect size was d = For nonmedallist group, execution time was significantly shorter from ED 1 and ED 3 (p, 0.01) and from ED 2 and ED 3 (p, 0.01), without significant differences between ED 1 and ED 2. Also, in nonmedallist group, impact time was significantly shorter (p, 0.01) from ED 1 than from ED 3. The normalized effect size for execution time was d = 5.74 and d = 2.79 in comparisons between ED 1 and ED 3 and between ED 2 and ED 3, respectively. For impact time, it was d = DISCUSSION One of purposes of this study was to examine and compare mechanical variables in roundhouse kick to head in relation to execution distance (short, medium, long) according to athletes competitive level (medallists and nonmedallists). To this end, we used a data-collection system which provided specific information on impact time, execution time, and maximum impact force in combat sports, thus responding to demands of research that or authors have stated (17). We have also confirmed difficulty of comparing our results on a descriptive level with previous studies (3 5,11,15) because of diversity of units of measurement used (14). Neverless, our datacollection system enables us to compare and relate our results to those studies in which SI units are used (12). In results obtained on maximum impact force, execution, and impact time according to competitive level, no differences were observed in impact time in any of execution distances between groups. Medallists and nonmedallists spend a similar time on impact action. This is in contrast with findings of Pearson (18), who stated that impact time depends on level of subjects and elastic properties of target. Unfortunately, both measurement devices are different, so no comments regarding elastic properties can be made. Noneless, in our study, no differences were found between medallists group and nonmedallists group in impact time that is an important factor for coordination of 2 or more kicks (20). In this line, athletes of this study would have same disposition to coordinate different actions to score points. After competitive level comparisons, Falco et al. (7), using same procedure as in this study, found that those athletes of a higher level executed kick with a greater maximum impact force and in a shorter execution time from all distances, in comparison to lower level athletes. In our research, we have observed similar results from short and long distances, but not from distance considered to be ir combat distance. Taking that result into account, it could be explained that medallists are capable of carrying out execution from different distances and hitting with less execution time and greater maximum impact force than nonmedallists. According to Cohen s d score, differences we found can be estimated as large differences (4). So, although not all mechanical variables analyzed are influenced by level in every execution distance, competitive level seems to be a factor that considerably affects mechanical performance in roundhouse kick to head. In sum, competitive level is a factor that must be taken into account when performance analysis or research is developed. Anor purpose of this study was to explore relationship between maximum impact force and weight in each competitive level group. Pearson (18) and Pedzich et al. (19) found a positive correlation between weight and impact forces. In our research, we found that nonmedallists (lower level athletes) base ir powerful impact force largely on ir weight, but medallists do not. This may be because higher level athletes achieve greater impact forces based on principle of kinetic chain (7), or something similar, based on a better technical execution (12,19). However, lower level athletes would use ir weight to generate this force so as to compensate for ir technical deficiencies, which would also explain differences found in execution time. In regard to analysis on basis of execution distance and considering each group individually, in medallists group, maximum impact force did not show significant differences, which means that medallists are capable of kicking with similar impact force to score points from all execution distances. That is, maximum impact force does not seem to depend on execution distance when medallists kick. In contrast, in time variables ( execution and impact), medallists kick in short distance 2854 Journal of Strength and Conditioning Research

5 Journal of Strength and Conditioning Research (ED 1 ) lower than in long distance (ED 3 ). It seems that execution distance affects time variables. Those a priori logical results (more distance should involve more time to cover) are supported by Cohen s d score, and a d. 0.8 means a large effect (4). Moreover, we observe that medallists group is capable of striking with similar maximum impact force and execution time from medium and long distances. It seems that in a sparring match in which athletes are getting used to maintaining a distance similar to medium or combat distance (ED 2 ), in medallists group, distance does not compromise efficiency of action; refore, high-level competitors could be advised to train for offensive actions from long distances (according to Matsushigue et al. [12], it should be around 2 m; and in our study, it should be similar to long distance), as action is equally effective and athletes may acquire greater security against opponent. Following analysis taking into account execution distance influence, for nonmedallists group, in short distance, impact time was lower than in long distance. This result could be because as execution distance increased more than combat distance (ED 2 ), nonmedallists needed to lean forward toward objective so as not to lose ir right position or posture. Noneless, although Cohen s d enlightens us about a large effect size, ICC value for impact time is small, which means poor reliability in results of this variable (25). Also, in execution time, nonmedallists kick in short and medium execution distances lower than in longer execution distance. The nonmedallists group is not capable of executing kick in same time when distance increases, which may imply a less efficient execution. Recent studies (10) found that target distance mainly affects reach control function, so increase of distance could influence time performance. It seems that impact force is less influenced by distance than time variables are, which is one of key points to score a goal (2,10). Taekwondo training should be more oriented to improving time performance (execution and impact time) than impact force because lower and higher level athletes impact force is not compromised by execution distance. There are few research studies (6) that analyze force and time data as a function of execution distance and take into account competition level in roundhouse kick to head. In this study, we observe that medallists perform better (higher impact force and lower execution time) in or distances y are not getting used to kick (e.g., short and long distances) than nonmedallists. Moreover, medallists kick with similar impact force and impact time from 3 execution distances, whereas nonmedallists cannot do this and also spend more execution time when distance increases. Thus, by sharing comments of Nien et al. (15), we may state that a higher competitive level assumes better execution (15) and efficiency (6) of roundhouse kick to head. Therefore, medallists group performs this kick with a greater force and in a shorter time than nonmedallists group and so can potentially obtain better mechanical and sporting results. PRACTICAL APPLICATIONS The findings of this study suggest several practical applications. We suggest that coaches who analyze biomechanical performance or execution technique should analyze time and force variables. Using just time variables or just force variables would not be right method to know how athletes performance is. Moreover, if coaches would like to analyze data comparing athletes performance, y should take into account level as a variable because level affects mechanical results. From a training and performance perspective, taking into account fact that in medallists group distance does not compromise efficiency of action, it is considered appropriate that coaches orient high-level competitors to train for offensive actions from long distances (similar to long distance). The action is equally effective; opponent may not be used to defending himself or herself from this kind of distance, and athletes may acquire a greater security against opponent. From a coach s perspective, re are large differences according to level in execution distances in which athletes are not used to kicking (e.g., short and long distance). So nonmedallists or lower level athletes should train from se short and long distances because y would learn to adapt mselves to distance, which is an important point in taekwondo sparring matches. Moreover, as per coach s aim, athletes usually focus ir training to enable m to hit stronger or to improve impact force. Neverless, taekwondo players should focus ir training to improve factors related to time performance because time variables are more influenced by execution distance and time is also one of key points in taekwondo sparring matches. ACKNOWLEDGMENTS This research has been supported by Sports Service of University of Valencia. We would like to thank Ms. Graciela Tomás García, sworn translator of English, who has helped us in revising translation to English language of this document. REFERENCES 1. Balius, X, Angulo, R, and Kinzler, S. Cinema tica y Dina mica de las Cinco Te cnicas ma s Frecuentes. Taekwondo. Vol 13. Madrid, Spain: Comité Olímpico Español, Bolander, RP, Neto, OP, and Bir, CA. The effects of height and distance on force production and acceleration in martial arts strikes. J Sci Med Sport 8: 47 52, Chiu, PH, Wang, HH, and Chen, YC. Designing a measurement system for taekwondo training. In: Proceedings of 25th International Symposium on Biomechanics in Sports. Menzel, HJ, and Chagas, MH, eds. Ouro Preto, Brazil: Federal University of Minas Gerais, pp Cohen, A. Statistical Power Analysis for Behavioural Sciences (2nd ed.). Mahwah, NJ: Lawrence Erlbaum Associates, VOLUME 25 NUMBER 10 OCTOBER

6 Mechanical Parameters in Taekwondo 5. Conkel, BS, Braucht, J, Wilson, W, Pieter, W, and Fleck, SJ. Isokinetic torque, kick velocity and force in taekwondo. Med Sci Sports Exerc 20: S5, Estevan, I, Molina-García, J, Falcó, C, and Álvarez, O. Comparación de la eficiencia de la patada al pecho y a la cara en taekwondo, según la distancia de ejecución. Rev Int Cienc Deporte 21: , Falco, C, Alvarez, O, Castillo, I, Estevan, I, Martos, J, Mugarra, F, and Iradi, A. Influence of distance in a roundhouse kick s execution time and impact force in Taekwondo. J Biomech 42: , Hristovski, R, Davids, K, Araújo, D, and Button, C. How boxers decide to punch a target: Emergent behaviour in nonlinear dynamical movement systems. J Sci Med Sport CSSI 1: 60 73, Jakubiak, N and Saunders, DH. The feasibility and efficacy of elastic resistance training for improving velocity of Olympic taekwondo turning kick. J Strength Cond Res 22: , Kim,JW,Kwon,MS,Yenuga,SS,andKwon,YH.Theeffectsof target distance on pivot hip, trunk, pelvis, and kicking leg kinematics in Taekwondo round house kick. Sports Biomech 9: , Lee, CL and Huang, C. Biomechanical analysis of Back kicks attack movement in taekwondo. In: Proceedings of 24th International Symposium on Biomechanics in Sports. Schwameder, H, Strutzenberger, G, Fastenbauer, V, Lindinger, S and Müller, E eds. Salzburg, Austria: University of Salzburg, pp Matsushigue, KA, Hartmann, K, and Franchini, E. Taekwondo: Physiological responses and match analysis. J Strength Cond Res 23: , McGill, SM, Chaimberg, JD, Frost, DM, and Fenwick, CMJ. Evidence of a double peak in muscle activation to enhance speed and force: An example with elite mixed martial arts fighters. J Strength Cond Res 24: , Neto, O, Magini, M, and Saba, M. The role of effective mass and hand speed in performance of kung fu athletes compared with non-practitioners. J Appl Biomech 23: , Nien, YH, Chuang, LR, and Chung, PG. The design of force and action time measuring device for martial arts. In: The Engineering of Sport. Vol. 5. Hubbard, M, Mehta, RD and Pallis, JM eds. Davis, CA: International Sports Engineering Association, pp Olivé, R. Estudio de la cadera del practicante de taekwondo. Unpublished PhD sis, Universidad Autónoma de Barcelona, Barcelona, Spain, O Sullivan, D, Chung, C, Lee, K, Kim, E, Kang, S, Kim, T, and Shin, I. Measurement and comparison of Taekwondo and Yongmudo turning kick impact force for two target heights. J Sci Med Sport 8: 13 16, Pearson, J. Kinematics and kinetics of taekwon-do turning kick. Unpublished PhD sis, University of Otago, Pedzich, W, Mastalerz, A, and Urbanik, C. The comparison of dynamics of selected leg strokes in taekwondo WTF. Acta Bioeng Biomech 8: 1 9, Roosen, A and Pain, MTG. Impact timing and stretch in relation to foot velocity in a taekwondo kicking combination. J Biomech 49: S562, Smith, MS, Dyson, RJ, Hale, T, and Janaway, L. Development of a boxing dynamometer and its punch force discrimination efficacy. J Sports Sci 18: , Tsai, YJ, Gu, GH, Lee, CJ, Huang, CF, and Tsai, CL. The biomechanical analysis of Taekwondo front-leg axe-kick. In: Proceedings of 23th International Symposium on Biomechanics in Sports. Wang, Q ed. Beijing, China: China Institute of Sport Science, pp Tsai, YJ, Huang, CF, and Gu, GH. The kinematic analysis of spinwhip kick of taekwondo in elite athletes. J Biomech 40: S2, Vieten, M, Scholz, M, Kilani, H, and Kohloeffel, M. Reaction time in taekwondo. In: Proceedings of 25th International Symposium on Biomechanics in Sports. Menzel, HJ and Chagas, MH eds. Ouro Preto, Brazil: Federal University of Minas Gerais, pp Weir, J. Quantifying test retest reliability using intraclass correlation coefficient and SEM. J Strength Cond Res 19: , Journal of Strength and Conditioning Research