Soccer is one of the most popular sports around the world, with >240 million players worldwide (16,34). The aim of physical conditioning training of soccer may be to maximize the capacity of an athlete to give responses to changing situations of the environment (8). One of these responses is change of direction (COD) speed. For several authors, the performance of soccer must be determined by several factors, that is, COD (8,21).
The COD performance is believed to be influenced by a variety of physical factors (21). First, the external structure of the movement (acceleration-brake-turn-acceleration capacity), which must be related with the nature of muscle contraction and second, the dominance that must be determined in the first stages of maturation, and it influences the preference and time speed to turn.
Regarding muscle contraction factors, during COD movements, a series of concentric contractions are preceded by muscle stretching that are known as stretch-shortening cycles (SSCs). So, the assessment of SSC is the main concern to estimate the capacity to perform linear and COD sprints with high efficacy (12,35). However, the direct measurement of SSC is difficult, time consuming, and costly. Traditionally, indirect tests have been used to assess the components of SSC. Some of the most popular tests involve the countermovement jump (CMJ) and the drop jump (DJ) for elastic and reactive strength components, respectively (1,2). The relationship between CMJ-DJ and sprint and power skills has been reported in several studies (6,7) and specifically in COD tests (4,11,14,20,21,25,33,36).
The association between CMJ and DJ with COD performances has been studied in bilateral tests (3,19,20,33,36), without clear consensus about the results obtained. Some studies showed a negative and significant relationship between the COD time and CMJ height (4,10,20,21,25,29,33,36) and DJ height (14,36). In contrast, others did not find any relationship (23). Because CMJ and DJ are positively correlated, an enhanced statistical analysis must be performed to explore the predictors of COD. A few studies using stepwise analysis tried to find the best predictor of COD performance (4,9,14,21,25,33,36).
Although the COD performance must be determined by dominance, where we can turn faster to 1 side than to the opposite, its prediction would be improved by measuring the unilateral strength (single-leg test). There are few studies that have assessed elastic (CMJ test) and reactive (DJ test) strength components using unilateral tests (14,25,26,36) and its association with linear and COD sprints (14,25,36).
However, currently, there is a new tendency that reveals that COD sprints enhance systemized strength training, leading us to obtain very important advantages (17,18,21,22,24,25,27,28,31,36) because training principles of specificity and efficacy are fulfilled.
However, there is a lack of knowledge in the literature, wherein stepwise analyses are used to obtain predictors of COD performance either for dominance or for no dominance turn side in amateur soccer players. The purpose of this study was, first, to analyze the correlation between COD and jumping test (CMJ and DJ) on male amateur soccer players (all of them had right dominance); second, to assess the effect of single-leg-jump dominance on COD performance, and finally, to explore what are the best jump test as predictors of COD using stepwise regression analysis.
Experimental Approach to the Problem
To determine the relationship between elastic and reactive strength component and COD sprint performances, we carried out jump tests (CMJ and DJ) and 10-m COD sprints with 2 different turn types (90° and 180°). To explore the influence of the strength of the dominance leg, we performed the 1-leg-CMJ with each limb (dominant and nondominant). Two regular heights cited traditionally in training studies with similar samples were used in the DJ test (15 and 30 cm). The assessments were performed in 2 days. The jump tests were performed during the first day, and after 2 days, without heavy physical activity, the performance of COD tests was measured.
Forty-five men were selected for the study, 42 completed all the tests, and 3 were injured after the COD test, and they were excluded from the analysis. All the participants were undergraduate physical education students (age: 20.11 ± 3.68 years), healthy (weight: 73.41 ± 8.43 kg, body mass index: 23.17 ± 2.59 kg·m−2, and body fat percentage: 17.10 ± 8.76%) and considered physically active according to the recommendations established by the World Health Organization. Also, they were amateur soccer players who were training 3 d·wk−1 and playing 1 match every weekend. All the subjects had right leg dominance. Each student was properly informed about the purpose and risks of the study. The students gave written and signed informed consent to participate as volunteers. The Ethics Committee of the University of Malaga approved the study design and procedures.
The Ergojump (Boscosystem, Ergotest/Muscle-Lab™ V 8.0 software) platform was used to record the height of the CMJ and the DJ test. Bosco jump protocols (7) were followed to standardize the test jumps as subrogate of lower limbs strength.
The COD tests were carried out on a synthetic (Taraflex®, Spain), plain, and indoor floor. The time spent in the sprints was measured with 6 photoelectric cells connected to a telemetry power system using a stopwatch with a precision up to the millisecond (Byomedic®, Spain).
All the jump and sprint tests were carried out in the afternoon, and the environmental temperature and humidity were 17°C and 60%, respectively. Before measurements, the participants were asked to carry out a 25-minute warm-up, with an increasing intensity, to prepare the body for performance tests and to prevent injuries (15).
The standards established by Bosco et al. (7) were followed for the jump test session (2-leg CMJ, 1-leg CMJ with right [CMJR] and left [CMJL] lower limb and DJ with a 15- and 30-cm height). Briefly, 3 attempts were performed and the best one was selected for the statistical analysis; the recovery time among attempts was 5 minutes.
Two COD sprints of 90° and 180° were carried out as described below: (a) COD sprint with a turn of 180° (COD180°): it was a 10-m run. Five meters straight ahead and a turn of 180° to come back to the start line (Figure 1A). We recorded the partial times between the start and the first 5 m (180°T1), the second 5 m (180°T2) and total time (180°Total). (b) COD sprint of 90° (COD90°): Two 10-m runs were carried out. Five meters straight ahead and a turn of 90° to go to the finish line, one sprint to the left (COD90°L) and a second one to the right (COD90°R, Figure 1B). Two partial times were recorded: start line to 5 m (90°RT1 and 90°LT1) and from the turning point to the finish line (90°T2 and 90°LT2); we also recorded the total time (90°Total and 90°LTotal). The best result of the 3 attempts was selected for the statistical analysis.
All variables are shown as mean and SD. The Kolmogorov-Smirnov test was used to confirm the normality. The reliability was calculated by Cronbach's alpha intraclass coefficient. The relationship between jumps and COD sprint tests was analyzed using Pearson coefficient correlation. Stepwise regression analysis was used to estimate the best predictor model of the COD. Coefficients of determination (R 2) were used to represent the goodness of the predictor models with the CMJ or DJ test as independent variable. The level of significance was set at p ≤ 0.05 for all the tests. SPSS 17.0 software for Windows (SPSS, Inc., Chicago, IL, USA) was used for the statistical analysis. A specific statistical analysis was performed to calculate the sample size minimum needed for multiple regressions analysis. Considering a statistical power of 80%, a type 1 error or alpha of 0.05 and effect size of 0.82 (this is the value equivalent to a R 2 = 0.45, which was the maximum prediction coefficient found in the literature for similar studies), we would need a minimum sample size of 18 subjects.
The assessments of jumps and COD tests for all variables are shown in Table 1. The results are means of the best attempt in each test for the total sample (Table 1). All the tests were performed thrice (attempts) to analyze their reliability (internal consistency). Cronbach's alpha intraclass coefficient correlation showed a good reliability for all the heights (jumps) and times (COD) (Table 2).
Our results showed strong associations between COD performance (time in seconds) and CMJR performance (height in centimeters), either in individual (Figure 2) or by Pearson's correlation coefficient analysis (90°LT2, r = 0.644 and 90°LTotal, r = 0.642, p < 0.001; Table 3). Also, we found negative and significant associations between several COD variables (time to complete each sprint and partial times) and jump test variables (height in centimeters) as can be observed in Table 3. The CMJL showed only weak correlations with COD90° (90°LT2, r = 0.333 and 90°LTotal, r = 0.329, p < 0.05; Table 3).
Figure 3 shows the differences between COD90°L and COD90°R performance (90°LT1 − 90°RT1 difference = 0.14; p <0.001; 90°LT2 − 90°RT2 difference = 0.31; p < 0.001; 90°LTotal − 90°RTotal difference = 0.43; p < 0.001). The fastest time was obtained in COD90°L (partial and total times, p < 0.001 for all).
Subsequently, a bivariate correlation analysis was carried out between the results obtained from the variables in the strength tests (CMJ and DJ) and the results from COD sprints of 180° and of 90° to the right and to the left. These results are presented in Table 3.
Lastly, data about linear regressions (Figure 4 and Table 4), which determine the relationship between variables, were obtained to check the linearity established between the correlated variables of the strength executed in the CMJR and CMJ and the partial time 90°LT2 (r 2 = 0.414; p < 0.001; r 2 = 0.483; p < 0.001, respectively) and total time 90°LTotal (r 2 = 0.413; p < 0.001; r 2 = 0.462; p < 0.001, respectively).
The main finding of this study was that the performance of COD90°L (total and partial times) on amateur soccer players was determined mainly by 1-leg CMJR. The traditional 2-leg CMJ test was also a significant predictor for the same COD90°L. Also, a novel contribution was that only right-dominant subjects were selected. This criterion should be a main concern because the dominance is an important characteristic to select the preference side to turn in COD, which must determine the success of several motor skills in the field of team sports such as soccer (8). Two aims were covered with this approximation: first, we could confirm that COD90°R was not predicted by jump tests in right-leg dominant soccer players, even though it had significant correlations with several tests (Table 3). Second, we can be sure that all players turn faster to the left side than to the right side (p < 0.01), and so there are higher statistical power and a lower probability to accept the alternative hypothesis erroneously (positive false or type 1 error).
A significant negative correlation was found between all COD and jump tests (except for CMJL, Table 3), which may mean that the greater the explosive strength (CMJ, elastic and DJ, reactive) performance, the lower is the time spent to execute COD sprints (90° and 180°, Table 3). Our results are in accordance with those of other studies wherein negative and significant correlations were observed between COD time and height of 2-leg CMJ jump tests (4,21,29,33,36). Even though there are few investigations with 1-leg jump tests (14,25,36), their results were not similar to our Pearson's correlation coefficients. This discrepancy could be explained by the differences between the subjects included in their studies, wherein several athletes from different modalities were included in the same analysis or that the subjects did not have right leg dominance.
Regarding DJ, there are controversies in the literature and their conclusions must be related with the type of sprint used (COD or linear). Significant correlations between the DJ test and linear sprints were obtained in several studies (5,21,30,32,36), which was higher than our data (Table 3). This discrepancy should be related to the distance of the test (10 against 35 m), so the higher the distance of the sprint, the higher the importance of the reactive strength of the ankle (32). In contrast, the association between COD and DJ15 is close to that of Young's study (36) that found significant relationships between COD and DJ15 (r = −0.59; p < 0.001), which was only slightly higher from our results (r = −0.48; p < 0.001).
However, the correlation analysis informs us only about the individual association between variables. So the capacity to predict the performance of COD sprints must be determined using multiple regression analysis, because the DJ, 2-leg CMJ, and the 1-leg CMJ correlated to each other.
Studies analyzing the usefulness of jump tests to predict short sprint performance showed that coefficients of determination (R 2) between CMJ and time vary from 16 to 65% (4,9,12,32,33). There are only 4 studies in the literature that used regression analysis to estimate the predictors of COD (4,21,25,33). Two studies were similar in terms of sample and tests with our study. Their results showed that jump performance explains the COD time up to 25% (21) and 24% (25). The conclusions of these studies were lower than our conclusions in terms of R 2 data for CMJ tests (41 and 46%, for 1 and 1–2 legs, respectively). However, they used only COD180°, and they did not include student soccer players (just students). Vescovi and McGuigan (33) used high-level soccer players and obtained a determination coefficient (R 2 = 0.49) between 10-m linear part of the test and CMJ height that is similar to ours. In our study, we obtained significant R 2 results but by using COD90°L (dominant turn side) only. So the discrepancies between Vescovi's data and our data should be related to the characteristic of the test and the sample, because Vescovi did not analyze COD90° and used a sample comprising right and left dominant subjects (33).
Regarding DJ tests, although we obtained significant correlations between COD time and height of DJ, after stepwise regression analysis, DJ performance (15 or 30 cm) did not enter in any regression model. An important reason could justify that DJ was not a predictor of COD, so the DJ was performed mainly using ankle movement; taking into consideration that all extensor muscles of lower limbs are involved in COD execution, our results would be logical. A second possible explanation could be related to the assessment protocol; here, the drop height is the most important concern to obtain the best performances. We selected a DJ of a 15- and 30-cm height in our study because this was most commonly used in several studies with a similar sample of active subjects (21,30). We followed proposals such as in the work of Diallo et al. (13), who obtained beneficial results with training in which the height did not exceed 40 cm. Finally, our sample did not regularly train with DJ exercises, which may influence the best performances in this type of executions. Our data are in contrast to those of Young et al. (36) who claimed that reactive strength (DJ) is the best predictor of the COD sprint. Also, Djevalikian (14) suggested that even though there are no significant values between the agility and jump tests, a highly significant correlation between the DJ and the plyometrics must support a causality relationship. In our point of view, this partial analysis should be completed with a stepwise analysis wherein all jump tests must be included. Moreover, as commented in the previous paragraphs in Young's study, a heterogeneous sample was used, so professional athletes such as basketball players were included because they may have more experience in DJ exercises and the DJ test must be the most important determinant of COD performances.
As pointed out earlier, the use of the 1-leg jump test is scarce in studies of explosive strength performance with athletes, though, it may make sense because several sport skills are performed using 1 leg, so the success in these techniques must be determined by explosive strength (as assessed by the CMJ) of 1-leg actions. Our data confirm this hypothesis, because dominant CMJR showed the highest correlations coefficients with COD90°L, in accordance with another study wherein similar jump (squat jump) and COD tests were used for the first time (27). Also, we found that CMJR was the main predictor of the COD90°L performance. To our knowledge, there is only 1 study (36), which had analyzed the COD performance using the jump unilateral test (CMJ) in professional athletes, although they did not find significant determination coefficients. Our data confirm for the first time that the 1-leg CMJ is one of the most important predictors of COD90°L (dominant turn side) in amateur athletes and specifically in soccer players.
The analysis of partial times is the most important contribution of this article. Because T2 must be determined by the capacity to brake and accelerate in a short time, it should be normal that CMJR could be the best predictor of this partial time. The results reveal significant correlations in both the partial and total times and the jump tests, that is, the greater the height (explosive strength) of the CMJ tests (elastic strength component) and DJ tests (reactive strength component), the lower is the time spent by the player when executing a COD sprint, either the 90°LT2 or 180°T2 (Table 3).
Our data confirm that the greater the jump strength recorded in the CMJ tests, the lower is the time spent in performing a COD90°L (right leg support). Moreover, for the first time, we showed that the 1-leg CMJ was the most important variable to better estimate partial times of COD than total time was, although the latter was significantly predicted by the CMJ and CMJR tests. To sum up, the first linear regression analysis is derived from the equation for partial time (equation 1) and for total time (equation 2).
In conclusion, our results corroborated for COD90° that the outcomes established by other researchers who have pointed out that the greater the muscle strength, the lower is the spent time in certain sprint executions. Also, we suggest that COD performance can be estimated from 1-leg CMJ executions, although only for amateur soccer players and those with right leg dominance.
Our results are specific for our sample type; nonetheless, to complete the knowledge about COD assessment, further investigation are required involving other groups of expert athletes and non-expert athletes in sports in which COD is crucial for achieving a high level of performance.
The results of this study provide further evidence to suggest that elastic and reactive strength components, assessed by means of Bosco tests (CMJ and DJ), are correlated to the execution speed in COD180°, COD90°R, and COD90°L sprints in amateur soccer players. However, the accurate assessment of COD requires costly and time-consuming protocols, and jump tests need fewer instruments and easy application. So, by using jump tests, we can estimate a specific variable (COD) of soccer performance.
During COD, a strong sudden braking is performed, which is similar to CMJ executions, so the jump height (CMJ) depends on the take-off speed, the latter being dependent on the athlete's ability to brake and accelerate suddenly or SSC (6). Traditionally, the 2-leg CMJ test has been used; here, we confirm that the 1-leg CMJ test is more successful in estimating COD90°, which highlights the importance of the specificity of jump tests to predict the performance in motor skills with COD and to prescribe more rational exercises to improve COD performance.
The authors would like to express their gratitude to the Human Movement Laboratory and the Department of Didactics of Music, Plastic and Body Language Expression of the University of Malaga.
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Keywords:© 2012 National Strength and Conditioning Association
COD; performance; soccer; speed; countermovement jump (CMJ); drop jump (DJ)