Laboratory of Sports Medicine, Department of Physical Education and Sport Science, Aristotle University of Thessaloniki, Thessaloniki, Greece.

Journal of Strength and Conditioning Research, 2005, 9(), 79 84 2005 National Strength & Conditioning Association COMPARATIVE STUDY OF FIELD AND LABORATORY TESTS FOR THE EVALUATION OF AEROBIC CAPACITY IN SOCCER PLAYERS THOMAS I. METAXAS, NIKOLAOS A. KOUTLIANOS, EVANGELIA J. KOUIDI, AND ASTERIOS P. DELIGIANNIS Laboratory of Sports Medicine, Department of Physical Education and Sport Science, Aristotle University of Thessaloniki, Thessaloniki, Greece. ABSTRACT. Metaxas, T.I., N.A. Koutlianos, E.J. Kouidi, and A.P. Deligiannis. Comparative study of field and laboratory tests for the evaluation of aerobic capacity in soccer players. J. Strength Cond. Res. 9():79 84. 2005. The purpose of this study was to evaluate the maximal oxygen uptake (V O2 max) values in soccer players as assessed by field and laboratory tests. Thirty-five elite young soccer players were studied (mean age 8..0 years, training duration 8..5 years) in the middle of the playing season. All subjects performed 2 maximal field tests: the Yo-Yo endurance test (T ) for the estimation of V O2 max according to normogram values, and the Yo-Yo intermittent endurance test (T 2 ) using portable telemetric ergospirometry; as well as 2 maximal exercise tests on the treadmill with continuous (T ) and intermittent (T 4 ) protocols. The estimated V O2 max values of the T test (56. ml kg min ) were 0.5%,.4%, and.% (p 5) lower than those of the T 2 (62.96 ml kg min ), T (6.59 ml kg min ) and T 4 (64.98 ml kg min ) tests, respectively. Significant differences were also found between the intermittent exercise protocols T and T (p 0) and the continuous exercise protocols T 2 and T 4 (p 0). There was a high degree of cross correlation between the V O2 max values of the ergospirometric tests (T 2 versus T, r 0.47, p 05; T 2 versus T 4, r 0.59, p 0; T versus T 4 r 0.79, p 0). It is necessary to use ergospirometry to accurately estimate aerobic capacity in soccer players. Nevertheless, the Yo-Yo field tests should be used by coaches because they are easy and helpful tools in the training program setting and for player followup during the playing season. KEY WORDS. treadmill exercise tests, Yo-Yo field tests, estimation, V O2 max INTRODUCTION Modern soccer demands both strong physical conditioning and technical characteristics (2). The improvement of aerobic and anaerobic capacity, muscle strength, speed, and neuromuscular coordination are the main targets of soccer training (46). Soccer players have to adapt to the requirements of the game in order to compete at the highest level. Therefore, aerobic capacity is an important skill for the evaluation of performance in soccer players (, 42, 44). It is well known that physical conditioning and aerobic capacity in particular, depend on important elements: maximal oxygen consumption (V O2 max), anaerobic threshold, and work economy (24, 8). Soccer includes high-intensity, intermittent boats of exercise, which stresses the anaerobic glycolysis metabolic pathway (, 2, 7, 25, 49). The contribution of anaerobic glycolysis to total metabolism in soccer has been examined in various studies mainly by determining the blood lactate concentration (7, 22, 50). The responses to intermittent activity are characterized by higher levels of physiological strain compared to that of continuous exercise performed at the same mean work rate (4, ). The aerobic capacity of a soccer player is customarily evaluated via various field and laboratory tests. The majority of these tests are executed with protocols based on continuous types of exercise. However, a test is more reliable and effective when it is specific to the exercise patterns (27). Therefore, many European soccer teams use the Yo-Yo field test in daily practice (, 4). The physiological responses to intermittent exercise have also been compared with those of continuous exercise at the same average workload in an attempt to evaluate differences between exercise patterns (2, 6, 8). The comparison of the 2 exercise patterns is interesting because both intermittent and continuous exercises are used in soccer training to facilitate physiological adaptations and to improve performance (4). Intermittent exercise is performed at the same average work rate as continuous exercise and has been associated with increased physiological strain (6). Other researchers have failed to note differences in the energy cost between intermittent and continuous exercises (2, 8). To date, no substantial information is available regarding the relative physiological cost of a single bout of soccer-specific intermittent exercise compared with that of a continuous one at the same average intensity using field and laboratory methods. Therefore, the purpose of this study was to compare 2 field tests (Yo-Yo continuous and intermittent endurance tests) with 2 laboratory treadmill exercise tests (continuous and intermittent) in the evaluation of maximal oxygen uptake in elite soccer players. The proposed hypothesis of the study is that there are no significant differences between the various exercise tests regarding maximal oxygen uptake determination. METHODS Experimental Approach to the Problem All soccer players performed 4 maximal exercise tests; 2 in the field and 2 in the laboratory for an evaluation of their aerobic capacity in the middle of the regular training season. (Table ). The field tests were executed on a grass field. The tests were carried out randomly, and the time interval between them was at least 2 days. All tests 79

80 METAXAS, KOUTLIANOS, KOUIDI ET AL. TABLE. Experimental study design. Field tests T T 2 Laboratory tests T T 4 Yo-Yo endurance test Yo-Yo intermittent endurance test Continuous incremental protocol Intermittent exercise protocol Indirect method of aerobic performance valuation Portable direct ergospirometry Direct ergospirometry Direct ergospirometry and measurements were performed in the morning within a 5-day period. The test-retest reliability for all ergospirometric tests was 0.98. The temperature was 20 C 22 C, and the relative humidity was no more than 50% in the field as well as in the laboratory during the tests. Wind velocity on the field did not exceed 0.5 m s. Subjects Thirty-five elite soccer players participated in this study. Their mean age was 8..0 years, and they were all in training systematically for more than 6 years (mean training period 8..5 years). All players were members of the Greek National Soccer Team U-20 (under 20 years old). All subjects were nonsmokers and none used any ergogenic aid or medication known to affect cardiorespiratory function during the study. The protocol of this study was in accordance with the guidelines of the Ethical Committee of Aristotle University of Thessaloniki and the revised declaration of Helsinki. All participants gave their written consent. Anthropometric Measurements All soccer players underwent a physical examination, anthropometric tests including body fat assessment with skinfold measurements (4-fold method) by specific caliper (Lafayette Instrument Co., Lafayette, Indiana), and resting electrocardiogram (Excel 06, Cardiette, Trento, Italy). Field Tests The Yo-Yo endurance test level (T ) was used to evaluate the ability to run continuously for a long period of time. According to this test, the basic aim for the soccer player is to perform as many 20-meter distances as possible within the given time limit (4). When the participant stops, the last speed and the number of performed 20- meter distances at this speed are recorded, including that of the last run. V O2 max value was estimated according to the relative normogram of the running distance and associated oxygen consumption. The Yo-Yo intermittent endurance test level (T 2 ) was also performed to evaluate the players ability to repeatedly perform during intervals over a prolonged period of time. Due to the lack of any indicative normograms for the estimation of V O2 uptake in this test, a portable telemetric device was used (Cosmed K2 Co., Rome, Italy). This telemetric device forms a direct measurement method of consumed O 2 from the working muscles. Heart rate was recorded before, during, and after the tests using a Polar Sport Tester (Polar, Kempele, Finland). The players performed repeated 20-meter shuttle runs interspersed with a short recovery during which the players jogged until exhaustion (4). In this test, players had a 5-second rest between each shuttle. The subjects began to run forward 20 meters at the time of the first signal. The running speed had to be adjusted so that runners TABLE 2. Continuous treadmill protocol. Stage 2 4 5 6 7 Speed (km/h) 8.0 4.0 6.0 8.0 2 Grade (%).0.0.0 TABLE. Intermittent treadmill protocol. Stage 2 4 5 6 7 8 9 0 2 4 5 6 7 8 9 Speed (km/h) Grade (%) 6.0 4.0 6.0 8.0 4.0 4.0 5.0 Time (min) Time (min) could reach the 20-meter marker exactly at the time of the next signal. Runners turned at the second marker and then returned to the first marker, which they had to reach by the time of the next signal. When the start marker was reached the subjects continued forward at a slower speed, ran around the cone that was positioned farther back from the start marker, and waited for the next signal. The actual time of this jog was exactly 5 seconds. The goal of the participants was to perform as many 2 20-meter intervals as possible within the given time limit. When each participant stopped, the last speed and the number of performed 2 20-meter intervals at this speed were recorded. The last 2 20-meter interval was included. Laboratory Tests All soccer players performed 2 additional maximal exercise tests on a treadmill (TM 400 Trackmaster, Carrollton, TX) within week after the end of the field tests. The first test used a continuous (T, Table 2) protocol, and the second test used an intermittent (T 4, Table ) protocol. Maximal oxygen uptake was measured during both

PERFORMANCE TESTING IN SOCCER 8 TABLE 4. Anthropometric characteristics of subjects. Variables Height (m) Weight (kg) Body surface area (m 2 ) Lean body mass (kg) Body fat (%) (Mean SD).77 6 70.4 5.6.86 9 59.86 4.2 4.80 2.62 tests via a breath-by-breath gas analyzing system (Quark b 2, Cosmed Co., Rome, Italy). The following parameters were evaluated: resting heart rate (HR), systolic and diastolic blood pressures (sbp and dbp), maximum heart rate (HRmax), maximum systolic and diastolic blood pressures (sbpmax and dbpmax), exercise duration, maximal oxygen uptake (V O2 max), maximum ventilation (VEmax), the VE:V O2 ratio, and respiratory frequency (Rf). The V O2 max was considered to be the highest V O2 measured, when a plateau in O 2 consumption was reported despite an increase in workload. Time to the point of exhaustion was recorded as the test result. Blood Analysis Duplicated blood samples (0 l each) were drawn from an ear lobe. The mean concentration of the 2 blood lactate values was determined at rest and in the 4th minute of recovery of each test using a lactate analyzer (Photometer LP 400, Dr. Lange). The reproducibility values for the Dr. Lange lactate analysis kit was 99.5%. Statistical Analyses All results are reported as mean values and standard deviations ( SD). A one-way analysis of variance for repeated measurements was used to determine differences among 4 exercise tests. Linear regression analysis was used to estimate the relationship between 2 variables. For statistical analysis, SPSS.5 for Windows was used (Statistical Package for the Social Sciences, Chicago, IL). In all cases the level of statistical significance was set at p 5. RESULTS The anthropometric data and physical characteristics of all subjects are presented in Table 4. Time to the point of exhaustion was significantly higher in tests T 2 and T 4, which had intermittent workloads, compared with those of T and T (p 5) (Table 5). However, the highest exercise time was noted in the treadmill intermittent exercise test. The T exercise test had a significantly greater duration by 6.% (p 5) compared with that of T ; moreover, the T 4 had a higher exhaustion time by 0.4% (p 5) compared with that of T 2. In the T test, soccer players appeared to have the lowest value of V O2 max in comparison to the other tests (p 5). More specifically, the V O2 max in T was found to be lower by 0.5% (p 5),.4% (p 5), and.% (p 5) compared with those of the T 2,T, and T 4 tests, respectively. Furthermore, a significant difference was also found in the V O2 max values between the intermittent exercise tests T and T (p 0) and the continuous tests T 2 and T 4 (p 0). In T the V O2 max was higher by 2.2% (p 5) compared with that of T 4. The V O2 max values of the ergospirometric tests showed a high degree of cross correlation (T 2 versus T, r 0.47, p 05; T 2 versus T 4, r 0.59, p 0; T versus T 4, r 0.79, p 0) (Figures, 2, and ). Maximum blood lactate levels in the T 2 test were higher by.6% (p 5) compared with those of the T test. Moreover, the maximum blood lactate levels in the T 4 test were higher by 4.5% (p 5) and by 8.8% (p 5) than those of the T and T tests, respectively. There was no significant difference in maximum blood lactate levels between the T 2 and T 4 tests. Finally, there was no statistically significant difference in the HRmax, VE, V O2 :HR, Rf, and VE:V O2 between the 4 tests. DISCUSSION The major finding of the present study is that the V O2 max measurement in soccer players using treadmill exercise TABLE 5. Results of ergospirometric tests (mean SD). Resting heart rate (b min ) Systolic blood pressure (mm Hg) Diastolic blood pressure (mm Hg) Lactate (mmol l ) Time to exhaustion (min) Maximum heart rate (b min ) Maximum systolic blood pressure (mm Hg) Maximum diastolic blood pressure (mm Hg) Ventilation (L min ) Ventilation V O2 Respiratory frequency (b min ) V O2 :heartrate (ml b min ) V O2 max (ml kg min ) Lactate max (mmol l ) Estimated Yo-Yo endurance test 62. 7. 20.2 7.6 78.5 6.0.78 0..28 0.8 97. 6.5 74.0 9. 72.2 6.4 56. 2.7 9.9.74 Field tests Yo-Yo intermittent endurance test 62.8 8. 2. 6.7 79.4 6.4.50 0.9 5.68 2.87 96. 9.4 7 8.2 74.4 6.2 5. 9.5 0.8 5.7 55. 0. 22.6 2.8 62.96.78.28.75 Continue protocol exercise test 6. 6.9 8. 0.4 77. 6.2.66 0.45 4.2.96 96.7 6.7 76.6 2.9 70.9 7. 42.8 27.6 29.5 5.8 55.9.8 26.8 4.9 6.59 4.64 0. Laboratory tests Intermittent protocol exercise test 62.7 7. 7.9 7. 77.0 5.4.8 0.45 7. 2.62 94.9 6. 7.7 0.5 70.9 5.6 45.9 2 2. 6.0 59.8.4 27.9.8 64.98 4.79 0.80.90 p 5 versus T p 5 versus T 2 p 5 versus T p 5 versus T 4

82 METAXAS, KOUTLIANOS, KOUIDI ET AL. FIGURE. Linear regression analysis of V O2 max between T 2 and T (r 0.47, p 05). FIGURE. Linear regression analysis of V O2 max between T and T 4 (r 0.79, p 0). FIGURE 2. Linear regression analysis of V O2 max between T 2 and T 4 (r 0.59, p 0). protocols is preferred over the Yo-Yo field tests. It seems that the Yo-Yo field tests with or without the telemetric method underestimated the measured maximal oxygen consumption values in contradiction to the performed hypothesis. In addition, these methods for measuring V O2 max should not be rejected because they can be easily performed and are helpful in planning soccer training. The anthropometric data of our soccer players were similar to the corresponding results from previous studies in soccer (, 4, 9, 7, 45, 48). However, the anthropometric characteristics from the teams of various countries and different leagues presented a wide range of results, especially in body weight (4). Anthropometric studies in soccer players have shown that height and body weight are important factors in the performance of these athletes (4, 4). It was also found that anthropometric characteristics are different depending on the position that each player on the team plays, as well as on the level of competition (professional vs. amateur) (0, 40, 52). It is a known fact that a high percentage of body fat reduces athletic performance. Some researchers found values of body fat at 9% 6% in high-level soccer players (44), whereas low vales (%.5%) were observed in professional soccer players (28, 40). The range of percentage of body fat in our soccer players can be attributed to differences in training age. The aerobic capacity of our soccer players, which was indirectly estimated in the continuous field test (Yo-Yo endurance test), was in agreement with the values being reported for young elite soccer players that were obtained with the same method (4, 6, 7,, 4). Similar values of V O2 max were also observed by Metaxas et al. (200) in elite adult soccer players who performed the same Yo-Yo test (29). Furthermore, the Yo-Yo intermittent test has been proven to be a valid measure of physical performance in soccer with a high reproducibility (27). Nevertheless, the same authors observed that the Yo-Yo intermittent test performance was significantly correlated with the time to exhaustion and V O2 max values at the treadmill test, which led to the assumption that the treadmill exercise tests do not underestimate aerobic performance in soccer. The results of this study are also in agreement with others, supporting the notion that a portable telemetric ergospirometer is a reliable method for determining an athlete s aerobic capacity in the field (0,, 26, 9). It seems that this method can contribute effectively to creating the best training plan and lead to a higher level of sport performance in modern soccer. The lower values of V O2 max in the Yo-Yo continuous test compared with those in the Yo-Yo intermittent test are possibly due to various factors such as the turn in the shuttle run, as well as the unstable running rate. Another possible cause is the effect of the acceleration immediately after the signal and turn. Regardless of these limiting factors on the achievement of V O2 max values, it should be noted that the attainment of the estimated maximal heart rate ensured that players reached their exhaustion levels. Moreover, encouragement and competition between players during the test contributed to this finding (). Regarding the V O2 max determination in the labora-

PERFORMANCE TESTING IN SOCCER 8 tory, higher values were observed in both continuous and intermittent tests in comparison to the corresponding field tests. This difference can be explained partly by the different running conditions, considering that in the field the turns in the shuttle run are a suspending factor in the attainment of maximum V O2. It is also supported by intermittent workload protocols that demonstrate higher values of V O2 max in comparison with constant protocols (5, 5, 5, 6). Indeed, our soccer players reached their highest values of V O2 max during the intermittent test in the laboratory. These findings are the result of using the constant protocols, which cause higher local muscle exhaustion, whereas in the intermittent protocol, the recovery phase contributes to the production of higher muscle workload (, 7, 40, 52). The greater muscle exhaustion in the tests in which the constant workload protocol was used is due to the higher absolute time of exercise compared with the interval tests, in which the breaks for recovery are deleted. However, during the field and laboratory tests there was no significant difference in maximal heart rate. This ensures that all athletes achieved a maximum effort in all trials. The high level of blood lactate in all participants is also a criterion for the attainment of V O2 max and demonstrates the use of anaerobic energy production during exercise (24). The greatest values, which were noted during the interval attempts, are the results of high energy anaerobic adaptations of high-intensity efforts (7, 20, 42, 5). At the end of the Yo-Yo endurance test our soccer players recorded a significantly lower blood lactate concentration in comparison with the relative values measured both in the Yo-Yo intermittent recovery test and the laboratory tests. This is due to the lowest maximal workload that the soccer players strived for at the last stage. The explanation for this lies in the running velocity at the last stage between tests in which the constant workload protocol was applied (0, 47). Reduced maximal velocity must be due to maximal local muscle fatigue caused by the shuttle run during the field tests in comparison with the tests conducted in the laboratory, which had both a constant and intermittent progressively increasing workload (). Blood lactate concentration seems to be independent of a soccer player s aerobic capacity, because no significant correlation was found between the maximum blood lactate values and V O2 max or the distance covered by athletes in the T test. A comparative study by Metaxas et al. (2000) on the cardiorespiratory adaptations of Greek professional and amateur soccer players showed that professionals had higher levels of adaptation (0). This could be due to the type and intensity of exercises applied in the different training models (2, 42). It was suggested that the level of aerobic capacity in Greek soccer players was significantly higher than in Portuguese (2), British (8), English (4), and Asian (9) players of the same age. This difference may also be the result of the aerobic targeted training problems that are applied by Greek coaches even to adolescent soccer players. PRACTICAL APPLICATIONS In conclusion, the determination of V O2 max in soccer players using treadmill exercise tests seems to be a more accurate method in comparison with field Yo-Yo tests. Thus, the evaluation of aerobic capacity with laboratory tests constituted an appropriate process of cardiorespiratory index analysis in soccer players. However, determining the V O2 max via telemetric method in the field is also useful for the evaluation of cardiorespiratory efficiency. Finally, the Yo-Yo field tests should be considered as an easy and helpful tool for trainers and should be applied for players follow-up during the playing season. REFERENCES. APOR, P. Successful formulae for fitness training. In: Science and Football. T. Reilly, A. Lees, K. Davids, and W.J. Murphy, eds. London: E & FN Spon, 988. pp. 95 07. 2. ÅSTRAND, P.O., AND K. RODAHL. Textbook of Work Physiology. New York: McGraw-Hill, 977.. BANGSBO, J. Physiology of training. In: Science and Soccer. T. Reilly, ed. London: E & FN Spon, 996. pp. 5 64. 4. BANGSBO, J. Fitness Training in Football. A Scientific Approach. Bagsvaerd, Denmark: HO Storm, 996. pp. 8 99. 5. BANGSBO, J., AND F. LINDQUIST. Comparison of various exercise tests with endurance performance during soccer in professional players. Int. J. Sports Med. :25 2. 992. 6. BANGSBO, J., L. NøRREGAARD, AND F. THORSø. The effect of carbohydrate diet on intermittent exercise performance. Int. J. Sports Med. :52 57. 992. 7. BANGSBO, J., L. NøRREGAARD, AND F. THORSø. Activity profile of competition soccer. Can. J. Sport Sci. 6:0 6. 99. 8. BAXTER-JONES, A.D., P. HELMS, N. MAFFULLI, J.C. BAINES- PREECE, AND M. PREECE. Growth and development of male gymnasts, swimmers, soccer and tennis players: A longitudinal study. Ann. Hum. Biol. 22:8 94. 995. 9. CHIN, M.K., R.C. SO, Y.W. YUAN, R.C. LI, AND A.S. WOND. Cardiorespiratory fitness and isokinetic muscle strength of Asian junior soccer players. J. Sports Med. Phys. Fitness 4:250 257. 994. 0. CRANDALL, C., S. TAYLOR, AND P. RAVEN. Evaluation of the Cosmed K2 portable telemetric oxygen uptake analyzer. Med. Sci. Sports Exerc. 26:08. 994.. DAL MONTE, A., AND S. LUPO. Specific ergometry in the functional assessment of top class sportsmen. J. Sports Med. Phys. Fitness 29:2 28. 989. 2. DRUST, B., N.T. CABLE, AND T. REILLY. Investigation of the effects of the pre-cooling on the physiological responses to soccer specific intermittent exercise. Eur. J. Appl. Physiol. 8: 7. 2000.. DRUST, B., T. REILLY, AND N.T. CABLE. Physiological responses to laboratory-based soccer specific intermittent and continuous exercise. J. Sports Sci. 8:885 892. 2000. 4. DUNBAR, G.M.J., AND K. POWER. Fitness profiles of English professional and semi-professional soccer players using a battery or field tests. In: Science and Football III. T. Reilly, J. Bangsbo, and M. Hughes, eds. London: E & FN Spon, 997. pp. 27. 5. DUNCAN, G.E., E.T. HOWLEY, AND B.N. JOHNSON. Applicability of Vo 2 max criteria: Discontinuous versus continuous protocols. Med. Sci. Sports Exerc. 29:27 278. 997. 6. EDWARDS, R.H., L.G. EKELUND, R.C. HARRIS, C.M. HESSER, AND E. HULTMAN. Cardiorespiratory and metabolic costs of continuous and intermittent exercise in man. J. Physiol. 24:48 497. 97. 7. EKBLOM, B. Applied physiology of soccer. Sports Med. :50 60. 986. 8. ESSEN, B. Glycogen depletion of different fibre types in human skeletal muscle during intermittent and continuous exercise. Acta Physiol. Scand. 0:446 455. 978. 9. FAINA, M., C. GALLOZZI, S. LUPO, R. COLLI, R. SASSI, AND C. MARINI. Definition of the physiological profile of the soccer player. In: Science and Football. T. Reilly, A. Lees, K. Davids, and W.J. Murphy, eds. London: E & FN Spon, 988. pp. 58 6.

84 METAXAS, KOUTLIANOS, KOUIDI ET AL. 20. FOHRENBACH, R., A. MADER, W. THIELE, AND W. HOLLMANN. Test procedures and metabolically oriented intensity distribution in a sub-maximal load structure in sprint training. Unpublished translation, Beliconen. 986. 2. GARGANTA, J., J. MAIA, AND J. PINTO. Somatotype, body composition and physical performance capacities of elite young soccer players. In: Science and Soccer II. T. Reilly, J. Clarys, and A. Stibbe, eds. London: E & FN Spon, 99. pp. 292 294. 22. HELGERUD, J., L.C. ENGEN, U. WISLøFF, AND J. HOFF. Aerobic endurance training improves soccer performance. Med. Sci. Sports Exerc. :925 9. 200. 2. HOFF, J., U. WISLøFF, L.C. ENGEN, O.J. KEMI, AND J. HELGE- RUD. Soccer specific aerobic endurance training. Br. J. Sports Med. 6:28 22. 2002. 24. HOWLEY, E., D. BASSET, AND H. WELCH. Criteria for maximal oxygen uptake: Review and commentary. Med. Sci. Sports Exerc. 27:292 0. 995. 25. JONES, A.D.G., AND P. HELMS. Cardiorespiratory fitness in young British soccer players. In: Science and Football II. T. Reilly, J. Clarys, and A. Stibbe, eds. London: E & FN Spon, 99. pp. 298 0. 26. KAWAKAMI, Y., D. NOZAKI, A. MATSUO, AND T. FUKUNAGA. Reliability of measurement of oxygen uptake by a portable telemetric system. Eur. J. Appl. Physiol. Occup. Physiol. 65:409 44. 992. 27. KRUSTRUP, P., M. MOHR, T. AMSTRUP, T. RYSGAARD, J. JOHAN- SEN, A.STEENSBERG, P.K. PEDERSEN, AND J. BANGSBO. The Yo- Yo intermittent recovery test: Physiological response, reliability and validity. Med. Sci. Sports Exerc. 5:697 705. 200. 28. MATKOVIC, B.R., S. JANKOVIC, AND S. HEIMER. Physiological profile of top Croatian soccer players. In: Science and Football II. T. Reilly, J. Clarys, and A. Stibbe, eds. London: E & FN Spon, 99. pp. 7 9. 29. METAXAS, T., N. KOUTLIANOS, T. SEDELIDES, AND E. KOUIDI. Evaluation of the level of aerobic capacity in pubertal soccer players in the field test. Health Sport Perform. 4:6 22. 200. 0. METAXAS, T., T. SEDELIDES, C., DIPLA, N. KOUTLIANOS, A. ZAF- EIRIDIS, E. KOUIDI, AND A. DELIGIANNIS. Cardiorespiratory adaptations in professional and amateur soccer players. Hung. Rev. Sports Med. 4:85 94. 2000.. MOFFATT, R., L. CHITWOOD, AND K. BIGGERSTAFF. The influence of verbal encouragement during assessment of maximal oxygen uptake. J. Sports Med. Phys. Fitness 4:45 49. 994. 2. MONTANARI, G., AND L. VECCHIET. Physiological aspects. In: Textbook of Sports Medicine Applied to Football. L. Vecchiet, A. Calligaris, G. Montanari, and A. Resina, eds. Firenze: Menarini International, 992. pp. 5 8.. NICOLAS, C.W., F.E. NUTTALL, AND C. WILLIAMS. The Longhborough intermittent shuttle run test: A field test that simulates the activity pattern of soccer. J. Sports Sci. 8:97 04. 2000. 4. NICOLAS, C.W., K. TSINTZAS, L. BOOBIS, AND C. WILLIAMS. Carbohydrate-electrolyte ingestion during intermittent high-intensity running. Med. Sci. Sports Exerc. :280 286. 999. 5. NOWACKI, P.E., AND M. PREUHS. The influence of a special endurance training on the aerobic and anaerobic capacity of soccer players tested by the soccer treadmill methods. In: Science and Football II. T. Reilly, J. Clarys, and A. Stibbe, eds. London: E & FN Spon, 99. pp. 86 9. 6. NOWACKI, P.E., D.Y. CAI, C. BUHL, AND V. KRUMMELBEIN. Biological performance in German soccer players (professionals and juniors) tested by special ergometry and treadmill methods. In: Science and Football. T. Reilly, A. Lees, K. Davids, and W.J. Murphy, eds. London: E & FN Spon, 988. pp. 45 47. 7. ODETOYINBO, K., AND R. RAMSBOTTOM. Aerobic and anaerobic field testing of soccer players. In: Science and Football III. T. Reilly, J. Bangsbo, and M. Hughes, eds. London: E & FN Spon, 997. pp. 2 26. 8. PATE, R.R., AND A. KRISKA. Physiological basis of the sex difference in cardiorespiratory endurance. Sports Med. :87 98. 984. 9. PEEL, C., AND C. UTSEY. Oxygen consumption using the K2 telemetry system and a metabolic cart. Med. Sci. Sports Exerc. 25:96 400. 99. 40. PUGA, N., J. RAMOS, J. AGOSTINHO, I. LOMBA, O. COSTA, AND F. DE FREITAS. Physical profile of a first division Portuguese professional soccer team. In: Science and Football II. T. Reilly, J. Clarys, and A. Stibbe, eds. London: E & FN Spon, 99. pp. 40 42. 4. REILLY, T., J. BANGSBO, AND A. FRANKS. Anthropometric and physiological predispositions for elite soccer. J. Sports Sci. 8: 669 68. 2000. 42. REILLY, T. Energetics of high-intensity exercise (soccer) with particular reference to fatigue. J. Sports Sci. 5:257 26. 997. 4. REILLY, T. Fitness assessment, anthropometry. In: Science and Soccer. T. Reilly, ed. London: E & FN Spon, 996. pp. 25 29. 44. REILLY, T., AND N. SECHER. Physiology of sports: An overview. In: Physiology of Sports. T. Reilly, N. Secher, P. Snell, and C. Williams, eds. London: E & FN Spon, 990. pp. 465 485. 45. REILLY, T.What Research Tells the Coach about Soccer. Washington: American Alliance For Health, Physical Education, Recreation and Dance, 979. 46. SENDELIDES, T., E. KOUIDI, T. METAXAS, N. KOUTLIANOS, AND A. DELIGIANNIS. Cardiorespiratory adaptations in soccer players. Hung. Rev. Sports Med. 44:4 5. 200. 47. SHEPHARD, R.J. Biology and medicine of soccer: an update. J. Sports Sci. 7:757 786. 999. 48. TIRYAKI, G., F. TUNCEL, F.YAMANER, S.A. AGAOGLU, H.GU- MUBDAD, AND M.F. ACAR. Comparison of the physiological characteristics of the first, second and third league Turkish soccer players. In: Science and Football III. T. Reilly, J. Bangsbo, and M. Hughes, eds. London: E & FN Spon, 997. pp. 2 6. 49. TUMILTY, D. The physiology of soccer. National Sports Research Centre. Canberra, Australia: Australian Sports Commission, 99. pp. 24. 50. WELTMAN, A.The Blood Lactate Response to Exercise. Champaign, IL: Human Kinetics, 995. 5. WILLIFORD, H.N., M. SCHARFF-OLSON, W.J. DUEY, S. PUGH, AND J.M. BARKSDALE. Physiological status and prediction of cardiovascular fitness in highly trained youth soccer athletes. J. Strength Cond. Res. :0 5. 999. 52. WISLøFF, U., J. HELGERUD, AND J. HOFF. Strength and endurance of elite soccer players. Med. Sci. Sports Exerc. 0:462 467. 998. Address correspondence to Dr. Thomas Metaxas, tommet@ phed.auth.gr.