The complexity of soccer, especially from a physiological point of view, can be fully appreciated when one considers the main actions performed by players during a game. Di Salvo et al. (2007) reported that during a match, soccer players experienced short and intense periods of great effort characterized by periods of anaerobic metabolism; however, aerobic metabolism predominated during the recovery periods.
The value of VO2max is the index most widely used for measuring aerobic fitness of soccer players, and can be determined in both laboratory and field tests. However, since soccer involves intermittent and sudden changes in a player’s direction, the nature of tests used to determine VO2max in the laboratory conditions can be quite different from the actions observed during actual play (Drust et al., 2007).
For this reason, several field tests have been suggested to verify aerobic fitness of soccer players. Among the most widely used field tests, we studied those which required continuous activities such as the Shuttle Run Test (SRT) suggested by Léger and Lambert (1982), and also tests requiring intermittent exercise such as the Carminatti’s test (TCar) proposed by Carminatti et al. (2004).
Reports concerning correlations between the results of the SRT and TCar tests and the results of laboratory tests, as well as the variables measured in the two types of tests (field and laboratory) are scarce and controversial (Aziz et al., 2005; Denadai et al., 2002; Silva et al., 2011; St Clair Gibson et al., 1998). Thus, we performed the present study to correlate and compare the results of variables measured in indirect and field tests (SRT and TCar) with the results of the same variables evaluated in laboratory tests in a group of youth soccer players.
Material and Methods
This study included 27 healthy youth male soccer players (age 16.77 ± 0.75 years) who were members of a first division team playing at the regional level of Brazilian soccer. All study participants had been systematically training for at least one year. The study protocol was approved by the Ethics Committee for Research at the Catholic University Center Salesian Auxilium (protocol - 452/2012). All participants and their parents were fully informed about the study’s procedures and potential risks, and they provided a signed informed consent form.
Determination of body mass and body height
Each participant’s height (H) was measured using a wall stadiometer (SUNNY®) with accuracy of 0.1 cm, and body mass (BM) was determined using a digital scale (TANITA TBF - 305®) with accuracy of 0.1 kg.
Determinations of VO2max and Vamax
Values of VO2max and Vamax were determined using an incremental and intermittent testing protocol involving the use of a motorized treadmill (Imbramed Millenium Super ATL) maintained at a 1% incline. After a warm-up, the test was initiated at a speed of 10 km⋅h−1, which was increased by 1 km⋅h−1 every 3 min (corresponding to one stage) until volitional exhaustion.
Values of VO2max were determined using a gas analyzer (METALYSER 3B - CORTEX®) which allowed data to be collected on a breath by breath basis. Values for the final 30 s of each stage were averaged. The highest VO2 value obtained during the last 30 s of each stage was recorded as VO2max. The velocity associated with VO2max (Vamax) was the lowest exercise intensity at which VO2max was obtained. If the intensity of VO2max was not sustained for at least 1 min, the intensity at the previous stage was recorded as Vamax (Silva et al., 2011).
Shuttle Run Test (SRT)
The actual SRT was preceded by a standardized warm-up, stretching and familiarization with the protocol. The test was conducted on a soccer field which had two cones placed 20 m apart. The player was required to run back and forth continuously in the 20 m path. The player was prompted to change running speeds by an electronic device which emitted sounds at regular intervals and recorded the player’s running speed. The initial running speed of 8.5 km⋅h−1 was increased by 0.5 km⋅h−1 every minute until the player voluntarily claimed exhaustion.
The player’s speed during the final stage was used to calculate peak velocity (VPeak) using the following equation (Denadai et al., 2002):
where a = the player’s velocity during the last completed stage and b = the time achieved in the last completed stage.
The result obtained from the above calculation was used in the following equation to predict a value of VO2max in units of ml⋅kg−1⋅min−1 (Léger et al., 1988):
where X = peak velocity (VPeak) in km⋅h−1and A = age in years.
Carminatti’s test (TCar)
The TCar was preceded by familiarization with the protocol, a standardized warm-up and stretching. The TCar is a maximal incremental test of a staggered intermittent type and has multiple stages of 90 s duration. The player performs 5 repetitions of shuttle run exercises consisting of 12 s of running (variable distances) interspersed with 6 s of walking (~ 5 m). The pace is dictated by an audio signal issued at regular intervals of 6 s. The signal determines the running speed to be attained during shifts between parallel lines marked on the ground and also marked by cones. The initial test speed of 9 km⋅h−1 (initial distance of 15 m) is increased by 0.6 km⋅h−1 at each stage until the participant voluntarily indicates exhaustion while attempting to complete successive stages 1 m greater than the initial distance (Carminatti et al., 2004).
All testing procedures were performed in a two-week period during which the participants attended the laboratory, the athletic track and the soccer field of Physical Education Unisalesiano Lins four different times.
At first, the players attended the laboratory, where anthropometric measurements were recorded and an intermittent progressive treadmill test to voluntary exhaustion was performed. Additionally, each participant’s height, body mass, aerobic power, VO2max and speed associated with VO2max (Vamax) were evaluated. The second and third rounds of testing were conducted outside the laboratory on a soccer field and performed in random order. During these visits, the volunteers completed the Shuttle Run Test (SRT) and Carminatti’s Test (TCar).
To avoid fatigue, the visits were scheduled with a 2 day interval between following tests. Additionally, the players were advised not to engage in strenuous physical activities or ingest alcoholic or caffeinated beverages at least 24 hours before each testing period. They were also required to get a good night’s sleep and to consume a light meal at least 2 h prior to testing.
Prior to all physical tests, the participants performed a standardized warm-up and stretching which lasted ~ 10 min. The first 5 min were used for the warm-up at 50-60% of the participant’s maximum heart rate (HRmax) as determined by the Karvonen formula (Camarda et al., 2008). The second 5 min consisted of stretching the main muscle groups to be utilized in the test.
Data were analyzed using descriptive statistics and results are expressed as mean ± standard deviation (SD). The normal distribution of data was tested using the Shapiro-Wilk test. Normally distributed data were analyzed as follows: a) the Student’s t test was used to compare values obtained for VO2max as determined directly using the treadmill test with VO2max values determined indirectly with the Shuttle Run Test, b) analysis of variance (one-way ANOVA) with the Tukey’s post-hoc test was used to compare peak velocities determined using the treadmill and Shuttle Run Tests (SRT) with those determined using the Caminatti’s Test (TCar), c) the Pearson’s product moment coefficient was used to verify possible correlations between values determined using indirect tests with those measured directly using laboratory tests. All statistical analyses were performed using SPSS Version 19 and p-values < 0.05 were considered statistically significant.
The variables examined in this study, including their means, SD and coefficients of variation are shown in Table 1.
The results of VO2max are presented in Table 1, and indicate significant differences between values obtained in the SRT and those in the treadmill test.
The values for speeds and maximal heart rate achieved in the direct and indirect tests are shown in Figures 2 and 3, respectively. There were significant differences between the values obtained from treadmill tests and those obtained from the TCar with respect to peak velocity in the SRT. There were also significant differences between the values obtained in all the tests with respect to the maximal heart rate.
The main objective of this study was to determine correlations between values of different variables obtained using indirect field tests specific to soccer and corresponding values obtained directly in laboratory conditions. We found differences in the values of VO2max as determined directly and indirectly (SRT), as well as lower peak velocities in the SRT compared to peak velocities in the treadmill test and TCAR. We also found moderate to high correlations between values determined in the SRT and TCar, respectively, with corresponding values determined directly in the treadmill test. Reilly et al. (2000) reported that VO2max was one of the variables most often used to physiologically characterize soccer players and this variable showed values ranging between 55 and 68 ml⋅kg−1⋅min−1. This variation is primarily due to inherent differences in the tactical positions taken during a match (Reilly et al., 2000) and the fitness level of the player (Wells et al., 2012).
The mean VO2max values found in this study are in line with those previously reported in the literature. Rampinini et al. (2010) found that professional soccer players with different tactical roles had similar VO2max values (mean value ~ 58.5 ml⋅kg−1⋅min−1). Additionally, Fornaziero et al. (2009) reported similar values for this variable in semi-professional players (59 ml⋅kg−1⋅min−1) and players in other basic categories of Brazilian soccer (56 ml⋅kg−1⋅min−1). Thus, the results of this study confirm the results of previous studies conducted with both semi-professional and well-trained professional soccer players.
Although some studies have found no differences in values of VO2max as determined in the SRT and in laboratory conditions (Aziz et al., 2005; Edwards et al., 2003), Grant et al. (1995) reported the underestimation of VO2max values when determined using predictive formulas. According to Lamb and Rogers (2007), the predictive formulas developed for measuring VO2max using the SRT are influenced by intrinsic factors such as the sample’s size, degree of heterogeneity and other dependent variables used in the regression model. In the current study, we used the predictive formula developed by Léger et al. (1988), which was intended for use in populations of students, healthy adults, academic practitioners and individuals practicing sports such as basketball.
Ahmaidi et al. (1992) reported that differences in VO2max as determined in different tests may be related to biomechanical factors; for instance, correct body movements are more efficient regarding energy expenditure. Making more efficient movements increases the recruitment of muscle fibers by reducing the recruitment of type II fibers and increasing the recruitment of type I fibers (Mauger and Sculthorpe, 2012). The participants of the present study were much more accustomed to performing field tests specific to soccer compared to treadmill tests. Thus, the fact that they were more familiar with tests similar to the SRT may have led them to perform movements which required less energy expenditure and maximal heart rate. Furthermore, characteristics of the SRT, such as its sudden changes in speed, direction and acceleration may have led to greater voluntary anaerobic participation (Grant et al., 1995).
A previous study by Silva et al. (2011) may help explain the difference in values obtained for VPeak in the SRT and when using the TCar and Vamax (Treadmill). Those authors evaluated soccer players similar to the participants of this study and found no significant differences between values of VPeak in the TCar and Vamax as determined in a treadmill test. Moreover, Ahmaidi et al. (1992) and Galloti and Carminatti (2008) reported lower values for VPeak in the SRT compared to Vamax determined on a treadmill, as well as for VPeak evaluated in the TCar. Jones and Carter (2000) stated that the value of VPeak in continuous tests was associated with Vamax as determined on a treadmill; however, the authors noted that this physiological index was influenced by anaerobic capacity, muscle strength and the neuromuscular ability to run at high speeds. Thus, it is likely that low values of VO2max and VPeak in the SRT are associated with a higher level of effort needed to complete an incremental test without pausing, coupled with low levels of power and anaerobic capacity; all leading to a lower level of tolerance towards exercise (Galloti and Carminatti, 2008; Grant et al., 1995).
In general, the SRT appears to be highly predictive for VO2max (Lamb and Rogers, 2007). Although several studies have found a low correlation between the values of VO2max determined in the SRT and those determined in laboratory tests (O’Gorman et al., 2000; St Clair Gibson et al., 1998), most studies have shown the opposite (McNaughton et al., 1998; Wilkinson et al., 1999). However, according to Drust and Svensson (2005), the estimated error introduced when determining VO2max using a prediction formula may lead to inaccurate results. Similarly, VPeak in the TCar correlates well with values of aerobic variables such as VO2max and Vamax determined with a treadmill test (Silva et al., 2011). However, in the present study, the values of variables examined in the SRT and TCar showed strong and moderate correlations, respectively.
Castagna et al. (2006) suggested that a possible explanation for the low correlation between results of VO2max and field tests designed for soccer was that the outcomes of field tests may be associated with factors other than aerobic power, such as the ability to accelerate, decelerate and quickly change directions. According to Pasquarelli et al. (2010), these abilities to some extent seem to be influenced by player’s muscle power of the lower limbs, as training these abilities can positively influence player’s acceleration capacity, which increases performance in progressive and multistage high intensity field tests such as the SRT and TCar test.
Although the values of VO2max in our sample lie within normal values for young soccer players, the values of Vamax are below those reported in the literature (Silva et al., 2011). According to Silva et al. (2009), Vamax is the index that best describes the relationship between maximum aerobic power and running economy, as individuals with similar VO2max values may have different Vamax values. This difference is explained by running economy that can be influenced by strength and anaerobic power of the athlete (Higino et al., 2011) and also has a significant effect on the outcome of our study.
Our results show that VO2max determined by the formula proposed by Léger et al. (1988) underestimates values determined directly in laboratory conditions. Among the peak velocities determined in the SRT and TCar, only values of VPeak in the TCar showed no significant differences with respect to Vamax determined on a treadmill. However, both variables determined in the SRT and TCar showed low correlations with those presented in the literature. This leads us to believe that factors other than maximum aerobic power determine the outcomes of field trials such as the SRT and TCar performed with youth soccer players.
Thus, as practical implications for the use of the SRT and TCar tests, it is suggested that these tests should be used with great caution with regard to the evaluation of aerobic fitness of youth soccer players. The fact that athletes are more familiar with tests similar to the SRT may lead them to perform movements requiring less energy expenditure and maximal heart rate. Furthermore, characteristics of the SRT, such as sudden changes in speed, direction and acceleration, may also induce greater voluntary anaerobic participation (Grant et al., 1995), associated with a higher level of effort needed to complete an incremental test without pausing, coupled with low levels of power and anaerobic capacity; all leading to a lower level of tolerance towards exercise (Galloti and Carminatti, 2008; Grant et al., 1995).
Although the SRT and TCar tests are specific to soccer, they should not be considered good tools for assessing aerobic capacity of youth soccer players as they are influenced by factors other than cardiorespiratory fitness.
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