Soccer-specific protocol
The soccer-specific intermittent protocol (see Fig. 1) was designed by Drust et al. (2000). The treadmill speed follows the observation by Van Gool et al. (1988, as cited in Drust et al., 2000) for specific movements; the speed for walking, jogging, cruising, and sprinting was 6 km/h, 12 km/h, 15 km/h, and 21 km/h respectively. Duration of each bout was determined by matching the percent total time of each movement with the data collected by Reilly and Thomas (1976) (see Fig. 2). Static recovery periods were also included, in which participants remained stationary on the treadmill. The protocol was then extended to 90 min, matching the duration of a real game. Details of the protocol such as total number of bouts for each activity are shown in Table 2.
Table 2. The number of bouts for each activity during the course of 90 min and the duration of each bout.
Results
Heart rate
Heart rate responses were reported in Fig. 3. Heart rate under neutral and heated condition was 165.5 ± 4.4 beats/min and 173.1 ± 8.5 beats/min respectively. In both conditions, heart rate increased over time. Since there is a linear relationship between heart rate and oxygen consumption with increasing rates of work (Wilmore and Costill, 1999), the mean oxygen uptake (35.6 ± 0.8 ml/kg/min) was used to estimate the percentage of maximal heart rate (HR-max); under the neutral condition, it was estimated to be 76% of HR-max. In the heated condition, the mean oxygen uptake (35.6 ± 0.4 ml/kg/min) gave an estimation of a 76% HR-max.
Fig. 3. Heart rate response under neutral and heated conditions (mean ± s).
Rate of perceived exertion (RPE) and thermal comfort
Mean RPE score in neutral and heated condition were 13.5 ± 1.0 and 14.4 ± 1.3 respectively. RPE increased over time, though there was a drop between 45 to 60 min in the neutral environment (Fig. 4). Average thermal comfort score was 6 ± 0.5 in the neutral condition and 7.3 ± 0.2 in the heated condition. In both environments, the score increased over time at the same rate (Fig. 5).
Fig. 4. Rate of perceived exertion under neutral and heated conditions (mean ± s).
Fig. 5. Thermal comfort responses under neutral and heated conditions (mean ± s).
Core temperature
Core temperature responses were presented in Fig. 6. It had little differences under the two conditions in the first half (neutral = 38 ± 0.3 ˚C; heated = 38 ± 0.4˚C). However, the difference rose to 0.6˚C in the second half as the heated measurement reached 38.7 ± 0.3˚C. Core temperature was found to increase as the experiment progressed.
Fig. 6. Core temperature responses under neutral and heated conditions (mean ± s).
Oxygen uptake (VO2)
Oxygen uptake decreased over time in the neutral environment and increased when heated (Fig. 7). In the first half, the neutral condition had a higher VO2 value, 36.1 ± 0.6 ml/kg/min compared to 35.4 ± 0.5 ml/kg/min in the heated conditoin. In the second half, however, VO2 in the neutral environment decreased to 35.1 ± 0.7 ml/kg/min while VO2 in the heated environment slightly increased to 35.8 ± 0.3 ml/kg/min. It was noticed that in each point of time when measurement was taken, the variability between subjects was large.
Fig. 7. Oxygen uptake under neutral and heated conditions (mean ± s).
Blood lactate
Blood lactate responses were shown in Fig. 8. Blood lactate value was higher in the heated condition (3.7 ± 2.4 mmol/L) than in the neutral condition (3.0 ± 1.9 mmol/L). This difference was greatest at the end of the experiment. Blood lactate also increased over time, with a major rise from 0 to 45 min. A summary of responses of all variables in both conditions over time was presented in Table 3.
Fig. 8. Blood lactate concentration under neutral and heated conditions.
Table 3. Values for the first and second half under neutral and heated conditions (mean ± s).
Discussion
Heart rate
When comparing to past researches (Table 4.), the mean heart rate under neutral condition (165.5 ± 4.4 beats/min) was similar to the findings of Drust et al. (2000), Seliger (1986b, as cited in Bangsbo, 1993), and Van Gool et al. (1988, as cited in Bangsbo, 1993). The higher value obtained by Agnevik (1970, as cited in Bangsbo, 1993) was probably due to the fact that only one subject was used in his study. The average 76% of HR-max was similar to the 75% found by Reilly and Thomas (1979). As the treadmill test did not include change of directions, jumping, tackling, and movements with the ball, the HR value was found to be 15-20 beats/min lower than in a real game situation (Bangsbo, 1993). Exercise increases the demands on the cardiovascular system. As the intensity of exercise increased, more blood supply was needed. Since cardiac output is governed by heart rate and stoke volume, heart increased with the duration of exercise. Under the heated condition, HR (173.1 ± 8.5 beats/min) was significantly higher than that in the neutral environment. This could be explained by the increased demand of blood flow to carry oxygen to different muscles in the body. As well, increased blood flow to the skin was needed to dissipate excess heat in the regulation of body temperature (Wilmore and Costill, 1999).
Past studies have proven a relationship between VO2 and HR (Reilly, 1996; Bangsbo, 1993), meaning that VO2 could be estimated from HR. According to the HR = 0.032 VO2 +76.7 relationship found by Esposito et al. (2004) in field test, the VO2 in this study should be 2775ml/min; dividing this number by the mean mass of the subjects gave a value of 34.8 ml/kg/min. This value was very similar to the measured value (35.6 ± 0.8 ml/kg/min) obtained in this study. However, one must note that the HR-VO2 relationship was obtained from continuous submaximal exercise; hence, its validity for soccer could be questioned due to the intermittent pattern (Bangsbo, 1993).
Table 4. Mean heart rate collected from past researches.
Rate of perceived exertion (RPE) and thermal comfort
Both RPE and thermal comfort increased gradually as the test progressed. RPE recorded at the end of the 90 minutes under neutral environment was 15.2 ± 1.3, which was similar to the ratings collected by Drust et al. (2000) (15 ± 2). The correlation of RPE and HR reported by MacKinnon (1999) was also observed in this experiment. The RPE scale gave a quantitative identification of the subjects’ feeling of fatigue and the sensation of effort. Under the heated condition, RPE increased significantly during the 2nd half, showing that the subject perceived greater fatigue and effort. It was important to note that the RPE value could be largely affected depending on the intensity of activity immediately prior to the point of recording. The lack of activities such as tackling and dribbling was another limitation of the experiment as Reilly and Ball (1984, as cited in Reilly, 2003) suggested that RPE rises when dribbling in parallel with the rise in metabolism. The validity of this variable could also be affected as the value reported was purely objective. This also applied to thermal comfort because some subjects might be better in heat tolerance. In both conditions, thermal comfort increased steadily throughout the experiment, which could be explained by the increased in core temperature. Corresponding to the increase in body and environmental temperature, thermal comfort under heated condition (7.3 ± 0.2) was 1.3 greater than in neutral condition.
Core temperature
During prolonged exercise, rectal temperature was found to be linearly related to work-rate (Saltin and Hermansen, 1966, as cited in Bangsbo, 1994). Agreeing with the findings of Drust et al. (2000), core temperature was higher in the second half. It was interesting to find that the core temperature in the heated condition was significantly higher than the neutral condition only in the second half. This was because the under the heated condition, the temperature was close to acute heat level, almost exceeding the level (25-30˚C) for the cardiovascular system to function in thermoregulation (Germann and Standfield, 2002). Homeostasis was disturbed as the body faced hyperthermia. As the subjects were approaching to the end of the 90 minutes, they suffered from heat exhaustion, which saw the core temperature rising higher. As well, increased sweating under the heat meant a greater fluid loss, reducing the blood volume available to prevent the buildup of heat (Wilmore and Costill, 1999). Since subjects were not allowed to re-hydrate, the body temperature was further elevated. As the treadmill test was limited to certain movements, the high energy turnover in a real football game was underestimated, and so was heat production. Thus, rectal temperature collected by Ekblom (1986, as cited in Bangsbo, 1994) after matches (39.5˚C) was higher than in the present study. However, error could occur as rectal temperature was taken outside the surface of the body. A more accurate approach would be directly measuring the muscle temperature of the subjects.
Oxygen uptake (VO2)
In this study, the average VO2 throughout the game was 35.6 ± 0.8 ml/kg/min, similar to Seliger’s (1968a, as cited in Reilly, 2003) finding of 35.5 ml/kg/min and Ogushi et al.’s (1993, as cited in Reilly, 2003) 35-38ml/kg/min.The average value was 61% of the VO2max, significantly less than the 75% reported by Bangsbo (1993) and the 75-80% reported by Reilly (1990, as cited in Reilly, 1996). Bangso (1993) suggested that treadmill testing only gives a 70% of the actual VO2 value during a match. This was explained by the lack of movements such as direction changing, passing, tackling, jumping, which would increase muscle activity and energy cost (Reilly, 2003); Kawakami et al. (1992, as cited in Reilly, 2003) found that the highest VO2 value was obtained in dribbling. Other reasons include the lack of frequent change in stride rate and stride length in a real game (Reilly, 2003) and the nonexistence of all-out effort (Reilly, 2003). As well, better trained athletes tend to have a greater VO2 value (Reilly, 2003). Nonetheless, the intermittent protocol, compared to continuous exercise, still better represent a real game situation as VO2 is significantly higher and increased further in the long-term (Bangsbo, 1994). As oppose to the neutral condition, VO2 increased over time in the heated environment because of the higher energy demand due to increased sweat production and respiration (Wilmore and Costill, 1999).
Blood lactate
Concentration of lactate was used to indicate anaerobic energy production in this study. The average value collected in the neutral environment was 3.8 ± 1.8 mmol/L at the end of the first half and 4.4 ± 1.2 mmol/L at the end of the 90 minutes. These values were generally lower when compare to past studies (see Table 5). This could be explained as blood lactate increases during dribbling as oppose to normal run (Reilly and Ball, 1984, as cited in Reilly, 2003). Past studies also found blood lactate to be lower in the second half as athletes tend to cover a shorter distance and run with less intensity (Bangsbo, 2003). This was not observed in the present study because the running distance and speed was controlled. Agnevik (1970, as cited in Bangsbo, 1993) found a significantly greater value of 10 mmol/L. This showed that variability of blood lactate measurement could be large. Blood lactate measured mainly depended on the activity the subjects were engaging in 5 minutes prior to blood sampling (Reilly, 2003). During high intensity movement in soccer, it could reach above 11mmol/L (Ekblom, 1986, as cited in Reilly, 2003). Gerisch et al. (1988, as cited in Reilly, 2003) also found higher blood lactate concentration when players are involved in man marking activities. In addition,it also varies according to the playing level of the subjects as elite athletes were found to have a higher value (Ekblom, 1986, as cited in Bangsbo, 2003). However, one must be aware of the fact that blood lactate does not represents all lactate produced in the body as lactate is metabolized with the active muscles (Brooks, 1987, as cited in Bangsbo, 2003). Some lactate would also be absorbed by tissues such as the heart, liver, kidney, and other inactive muscles (Brooks; 1987, as cited in Bangsbo, 2003). Therefore, blood lactate shows an underestimation of total lactate production. A more accurate approach would be measuring muscle lactate via biopsy, which could be difficult to achieve due to ethical reasons. The higher lactate concentration under heated environment could be explained. Increase in temperature put greater stress on the cardiovascular system; as blood flow was increased to carry oxygen to muscles, the use of muscle glycogen also increased, which raised lactic acid production (Wilmore and Costill, 1994). The increased dependence on CHO metabolism also encouraged glycogen utilization (Jentjens et al., 2002). In addition, the differences between the two conditions increased over time, which matched the result from Jentjens et al. (2002).
Table 5. Blood lactate concentrations from past researches (mean ± s).
Conclusion
It could be concluded that the soccer-specific intermittent treadmill protocol, comparing to continuous exercise, provides a more accurate representation of the work-rate pattern. It allows one to better study and quantify the physiological responses during soccer. However, specific game activities such as accelerations, change of direction, jumping, tackling, and dribbling were absent; this contributes to the under estimation of the total energy expenditure in reality. The intermittent and dynamic nature of soccer related activities remains the most difficult aspect for laboratory protocols to model.
References
Bangsbo, J. (1993). The Physiology of Soccer. Copenhagen: HO + Storm.
Bangsbo, J. (1994). Physiological demands. In Football (Soccer) (edited by B. Ekblom), pp. 43-58. London: Blackwell Scientific Publications.
Bangsbo, J. (1997). The Physiology of Intermittent Activity in Football. In Science and Football III (edited by T. Reilly, J. Bangsbo, and M. Hughes), pp. 114-124. London: E & FN Spon.
Drust, B., Reilly, T., and Cable, N.T. (2000). Physiological responses to laboratory-based soccer-specific intermittent and continuous exercise. Journal of Sports Sciences, 18, 885-892.
Esposito, E., Impellizzeri, F.M., Margonato, V., Vanni, R., Pizzini, G., and Veicsteinas, A. (2004). Validity of heart rate as an indicator of aerobic demand during soccer activities in amateur soccer players. European Journal of Applied Physiology, 93, 167-172.
Germann, W.J. and Standfield, C.L. (2002). Principles of Human Physiology. San Francisco: Benjamin Cummings.
Holmyard, D.J., Cheetham, M.E., Lakomy, H.K.A., and Williams, C. (1988). Effects of recovery duration on performance during multiple treadmill sprints. In Science and Football (edited by T. Reilly, A. Lees, K. Davids, and W. Murphy), pp. 134-141. London: E & FN Spon.
Jentjens, R.L.P.G., Wagenmakers, A.J.M., and Jeukendrup, A.E. (2002). Heat stress increases muscle glycogen use but reduces the oxidation of ingested carbohydrates during exercise. Journal of Applied Physiology, 92(4), 1562-1572.
MacKinnon, S.N. (1999). Relating heart rate and rate of perceived exertion in two simulated occupational tasks. Ergonomics, 42(5), 761-766.
Meyhew, S.R. and Wenger, H.A. (1985). Time motion analysis of professional soccer. Journal of Human movement Studies, 11, 49-52.
Nevill, M.E., Garrett, A., Maxwell, N., Parsons, K.C., and Nowitz, A. (1995). Thermal strain of intermittent and continuous exercise at 10˚C and 35˚C. Journal of Physiology, 483P, 124-125.
Reilly, T. and Thomas, V. (1976). A motion-analysis of work-rate in different positional roles in professional football match-play. Journal of Human Movement, 2, 87-97.
Reilly, T. and Thomas, V. (1979). Estimated energy expenditures of professional association footballer. Ergonomics, 22, 541-548.
Reilly, T. (1996). Motion analysis and physiological demands. In Science and Soccer (edited by T. Reilly), pp. 65-82.London: E & FN Spon.
Reilly, T., Bangsbo, J., Franks, A. (2000). Anthropometric and physiological predispositions for elite soccer. Journal of Sports Sciences, 18, 669-683.
Reilly, T. (2003). Motion analysis and physiological demands. In Science and Soccer Second Edition (edited by T. Reilly and A.M. Williams), pp. 59-72. London: Routledge.
Wilmore, J.H. and Costill, D.L. (1999). Physiology of Sport and Exercise. Champaign: Human Kinetics.