263   Correlation between direct and indirect VO2max measurements in indoor soccer players

ABSTRACT

The importance of the morphofunctional qualities to greater efficiency in sports has increased the interest in the improvement on the physical fitness levels of athletes. However, not many studies on the physiological variables of indoor soccer are available in the world scientific literature. Thus, the objective of this work was to verify the existence of correlation between the direct and indirect VO _2max measurement tests in indoor soccer players. Thirteen indoor soccer players with age of 18.6 ± 1.9 years, height of 177.1 ± 3.5 cm, weight of 68.5 ± 9.5 kg and body mass index (BMI) of 21.7 ± 2.3 kg/m ² were analyzed. For the direct VO _2max measurement, the computerized ergospirometric system was used ( VO-2000 , Aerosport, Medgraphics, St. Paul, Minnesota) and for the indirect VO _2max measurement, the 3,200 m field test was performed. The statistical analysis was elaborated through the t-Student test for paired samples and through the Pearson correlation coefficient. The O _2max values ​​obtained in the direct measurement test presented no significant differences in relation to the indirect measurement (62.8 ± 10.1 vs. 58.5 ± 8.5 ml/kg/min, respectively). When the VO _2max values ​​obtained in both tests were correlated, a strong correlation (r = 0.72) was observed. In short, the indirect measurement tests presented good acceptance among indoor soccer players due to the high correlation with the direct measurement tests, the low cost and the attainment of important information that will be useful in the training prescription and follow-up.

Key words: Aerobic capacity. VO _2max prediction tests.

INTRODUCTION

The importance of the morphofunctional qualities to greater efficiency in sports has increased the interest in the improvement on the physical fitness levels of athletes. However, not many studies on the physiological variables of indoor soccer are available in the world scientific literature.

Just like field soccer, the indoor soccer is also a sportive modality characterized by varied-extension and random-periodicity intermittent efforts. The current indoor soccer requires high-intensity and short-duration efforts, which distinguishes this sportive modality from other high-level ones ^(1,2) . The agility of the events and actions during an indoor soccer game requires the athlete to be prepared to react as quickly and effectively as possible to the most different stimuli ⁽³⁾ .

The specific work on the energy supply aerobic system aims at increasing the individual cardiovascular resistance, which is extremely important in the physical preparation of indoor soccer players. The determination of this parameter is based on the calculation of the maximum oxygen intake ( O _2max ) that, for professional soccer players, ranges from 55 to 65 ml/kg/min ⁽⁴⁾ . The development of the aerobic capacity is one of the factors that determine the performance of these athletes during a game.

With regard to the O _2max measurement, there are two methods that present advantages and disadvantages: the direct and indirect methods. The direct O _2max measurement is performed by submitting the individual to an ergometric test with progressive loads and analyzing the exhaled oxygen and carbon dioxide fractions during effort as well as the pulmonary ventilation. This measurement provides a more reliable result; however, this method presents high cost in relation to the indirect measurement and requires sophisticated equipment and qualified labor to conduct the tests; longer time spent with each appraised, and also requires higher motivation from the individual, since it is generally performed in laboratory environment ⁽⁵⁾ . With regard to the indirect O _2max measurement, the so-called field tests may be used, where the calculation of this variable is performed through equations based on preestablished time or distance. In this case, several individuals may be evaluated with low cost at once, and the test conditions, in some cases, are the closest from situations of practice and specificity of the sporting modality. However, its accuracy is often questioned, since many of these equations are specific for predetermined groups (e.g.: white sedentary men, black athlete women, etc) ⁽⁶⁾ .

Considering the different metabolic systems and energetic demands required in the indoor soccer and the lack of data on the physiology of this sport, the objective of the present study was to directly and indirectly correlate O _2max measurements in indoor soccer players.

MATERIAL AND METHODS

Sample

Thirteen male indoor soccer players with age of 18.6 ± 1.9 years, height of 177.1 ± 3.5 cm, weight of 68.5 ± 9.5 kg and body mass index (BMI) of 21.7 ± 2.3 kg/m ² participated in the present study. After being informed of the objectives of the study and of its possible benefits and risks, the subjects signed a consent term (resolution 196/96 of the National Health Council), previously approved by the Agamenon Magalhães Hospital Ethics Committee.

Experimental procedure

Direct

The _2max measurement

The experiments were conducted between 2:00 pm and 5:00 pm with room temperature kept between 22 and 24ºC, barometric pressure of 760 mmHg and air relative humidity of 55%. In the day before the test, the participants were asked to remain at least 24 h without performing any type of exhausting exercise.

Initially, with the individual on the treadmill ( Inbramed Millennium , model ATL, Porto Alegre, RS, Brazil), the blood pressure (BP) was measured using the auscultatory method as well as the heart rate (HR), through electrodes ( Micromed ) placed at the manubrium, right and left iliac crest (derivation CM ₅ ). The electrodes were connected to an electromyography equipment ( Micromed ^® ) and the HR values ​​were visualized through the Elite software (Micromed Biotechnology, Brasilia, Brazil).

Later, the individual was connected to a metabolic gas analyzer ( VO-2000 , Aerosport, Medgraphics, St. Paul, Minnesota) through which the gas samples were collected and measured every 10 seconds during the test. The participants were submitted to ramp protocol with initial velocity of 5.0 km/h (0% of inclination), progressive increments, final velocity of 17 km/h (5% of inclination) and duration equal to 10 minutes or until voluntary exhaustion.

The criteria considered for the O _2max calculation were at least two from those mentioned below:

Establishment of a flat on the oxygen intake curve in relation to the load;

Gas exchange ratio > 1.0;

Physical exhaustion.

Indirect O _2max measurement

For the indirect O _2max measurement, the 3,200 m field test was used through the formula:

The _2max (ml.kg.min) = 118.4-4.774 (T) ⁽⁷⁾ ,

where: T = time in minutes and decimal fraction of 3,200 m.

The test was performed on a 1,000 m track between 4:00 pm and 5:00 pm on different days and with minimum interval of two days and maximum interval of five days after treadmill test. In the day before tests, the athletes were oriented not to perform exhaustive physical activities, not to drink alcoholic beverages and not to smoke.

The volunteers were divided into groups of five individuals and the time obtained during the circuit was recorded through chronometer.

Statistical analysis

In order to verify the presence of correlation between direct and indirect O _2max measurement tests of indoor soccer athletes, the Pearson (r) correlation coefficient was used, and to check the presence of significant differences between the O _2max average values ​​in the direct and indirect measurements, the Student's test for paired samples was used. The results were expressed as mean ± standard deviation (A ± SD). The value p < 0.05 was considered as statistical significance level.

RESULTS

Figure 1 shows a strong correlation (r = 0.72) between O _2max values ​​obtained in the direct and indirect measurement tests of indoor soccer athletes.

Infigure 2 , no significant differences were found between O _2max values ​​in the direct and indirect measurements.