Figure 2 shows total distance covered, percentage of total distance covered for each quarter, relative time spent in each speed zone, and individual player Tcore responses for each quarter. Players covered a total distance of 9.38 ± 1.47 km, of which 71% was spent completing LIA and MIA (<14 km·h−1), with the remainder of activity either HIR (18%) or VHIR (11%). The MIA distance covered was significantly reduced (p = 0.05) in the fourth quarter compared with the first quarter. There were no other significant differences (p > 0.05) between each of the 4 quarters for either the distance or velocity in any other respective motion category. However, ES analyses indicated a moderate ES for a reduction in MIA distance covered in the third quarter (d = 0.62), a moderate ES for a reduction in the total distance covered in the fourth quarter (d = 0.72), and a moderate ES for a reduction in HIR in the fourth quarter (d = 0.61). Finally, there were moderate ESs between the first and fourth quarters, with a faster mean HIR velocity (d = 0.61). Specifically, as evidenced in Figure 2, the reductions in total distance covered from the first to second, third, and fourth quarters were 2, 10, and 18%, respectively, whereas the reductions in total distance from the second to third and fourth quarters were 8 and 14%, respectively.
Correlation analysis between Tcore and GPS data indicated significant correlations (p < 0.05) for the relationships between the first-quarter rise in Tcore and HIR velocity (r = 0.72), first-quarter Tcore and MIA velocity (r = 0.68), second-quarter Tcore and LIA velocity (r = −0.90), second-quarter Tcore and MIA velocity (r = 0.88), fourth-quarter rise in Tcore and VHIR distance (r = 0.70), and fourth-quarter Tcore and MIA velocity (r = 0.73).
The [La−] values sampled after each quarter reached a plateau after the first quarter at 8.6 ± 0.4 mmol·L−1 (Figure 3). Pregame USG was 1.009 ± 0.004, and body mass change was 2.13 ± 0.86 kg or 2.5 ± 1.0% body mass (85.67 ± 3.19 vs. 83.54 ± 3.55 for pre- and postgame mass, respectively). Session-RPE data indicated that the games were rated as an 8.6 ± 1.4 (very very hard) out of a possible 10, and thermal comfort was rated as a 7.0 ± 0.6 (very hot) out of a possible 8. Finally, no significant differences (p > 0.05) were evident between pre- and postgame repeated vertical jump efforts for height jumped (38.9 ± 3.0 vs. 39.6 ± 3.3 cm for pre- and postgame, respectively).
This study investigated the association between Tcore responses and exercise intensity during field-based team sport competition. The present results indicate that reductions in MIA were associated with the maintenance of Tcore, generally below 39.5°C. Further, although HIR distance covered decreased over the duration of the game, the mean velocities attained within HIR and VHIR zones were not reduced. Accordingly, it seems that during competitive, free-paced exercise in warm conditions, players may prevent the excessive rise in Tcore predominantly by a reduction in MIA, possibly so as to minimize disruptions to HIR to maintain game performance.
Previous studies that have examined the thermoregulatory responses to soccer match play have reported mean peak Tcore values below 39.5°C (6,15). In the warmer environmental conditions encountered in the present study, the mean peak Tcore also remained below 39.5°C, although it demonstrated a rise in Tcore of more than 2°C. Notably, however, some players in previous studies and in the present research did reach Tcore values approaching 40.0°C, which are known to place athletes at an increased risk of heat illness (however, no signs or symptoms of heat-related illness were evident in the present study).
Several factors may account for the moderate Tcore values recorded. First, the nature of an ARF match consisting of 4 × 25-minute quarters with unlimited player interchange results in prolonged (5-20 minutes) recovery periods that may allow sufficient respite from exercise and minimize excessive rises in Tcore. Second, the self-paced nature of competitive intermittent-sprint games allows players to continually adjust exercise intensities or pace themselves to minimize any excessive rise in Tcore. Finally, the improved effectiveness of convective cooling in an external environment compared with a laboratory, in addition to evaporative cooling mechanisms, may provide further protection against a critically high Tcore (24).
The relationship of Tcore with constant-load exercise performance within laboratory settings is well established, and it has been shown that both a high Tcore and faster rises in Tcore are associated with a reduction in intensity or cessation of exercise (8,23). Although the relationship of Tcore and intermittent-sprint activity in laboratory conditions have not been examined as widely, previous studies have shown that high ambient temperatures are associated with higher and faster Tcore responses and reduced exercise performance (16,18). The current study attempted to extend this work into a more ecologically valid competition environment by examining the relationship of Tcore and (free-paced) intermittent-sprint exercise performance in an elite, competitive (ARF) match. Similar to the data of Morris et al. (18) from intermittent-sprint exercise under controlled laboratory conditions, the current study indicates that there was an association between higher work intensities and a larger rise in Tcore in the first (25-minute) quarter of a game. Previous investigators have reported larger and faster increments in Tcore to be an important factor involved in performance reduction (8,11,19). In the present study, a reduction in total and MIA distance was evident as the game continued, along with small reductions in HIR distance. Additionally, mean HIR velocity showed a small increase in the final quarter compared with the first. The relationship between MIA and Tcore suggests that a greater increase in Tcore may result in the reduction of MIA during later stages of the game. The pattern of a plateau in Tcore after the first quarter, in association with a reduction in MIA and HIR distance while maintaining VHIR activity, may indicate the use of pacing strategies to ensure control of Tcore within tolerable limits.
Although the exact mechanisms limiting exercise performance associated with a critically high Tcore or an increased rate of Tcore are not yet fully understood (23), the classical models of thermoregulation suggest that there are critical temperatures that result in overloading the cardiovascular and metabolic systems (1). More recently, however, reductions in CNS drive and down-regulation of neural recruitment and muscle activation have been proposed as more likely mechanisms (12,19). On the basis of the present results, it is difficult to suggest the exact mechanisms underlying the progressive decrements in physical performance in the respective velocity zones as the game progressed. A range of factors such as glycogen depletion, hypohydration, hyperthermia, accumulation of potassium in muscle interstitium, and increased perception of effort may be associated with reductions in work during prolonged, intermittent-sprint sports (2,10). Nonetheless, an important finding of this study is that the rise in Tcore was associated with a decline in the distances covered-in particular, a reduction in MIA-while HIR and VHIR velocities (and postgame VJ efforts) were maintained. This may suggest that as VHIR is critical for game performance, to control the rise in internal thermal load, there is a selective reduction in noncritical work (jogging and below-threshold running), as observed in the reduction in MIA.
A discussion of performance decrement in team sport exercise should entail a multifactorial approach, and, as such, there are a variety of other factors to consider. In this study, moderate to high blood [La−] concentrations, as an indicator of the extent of anaerobic glycolysis and possible anaerobic metabolic perturbations, were observed throughout the ARF matches, similar to levels previously reported in other team sports that demand prolonged, intermittent sprinting such as soccer (2) and rugby league (3). These results, combined with GPS data showing that players completed 11.3 ± 3.5% of total match distance through VHIR, suggest that there is a high glycolytic demand in ARF. Indeed, the [La−] values found in this study most likely reflect the balanced response to many high-intensity efforts during the game and indicate that the rate of glycolysis is high for periods during a game. In agreement with previous studies that have not observed a relationship between metabolic markers of fatigue and reductions in intermittent-sprint exercise in hot, controlled environments (4,26) and during soccer match play (10), we also did not observe a relationship between [La−] and intermittent-sprint performance.
Hypohydration has been suggested to be a contributor to performance reductions during both continuous and intermittent-sprint exercise (7,13). In this study, we observed a 2.13 ± 0.86 kg (2.5%) decrement in body mass after the 100 minutes of match play. Further, all players were similarly hyperhydrated before game commencement, as evidenced by the low pregame urine USG values, because of standardized fluid ingestion protocols. Although previous studies have shown performance reductions in intermittent-sprint performance in laboratory studies with 2.0 ± 0.7% body mass loss, we did not observe reductions in VHIR or repeated VJ performance in this study. Based on previous research, it is likely that the extent of dehydration was not sufficient to blunt peak power in a single set of repeated VJ efforts (9). Therefore, in the absence of explicit markers associated with in-competition performance reduction (or reduced work performed), similar results from laboratory conditions for intermittent-sprint and continuous exercise have previously indicated that sufficient increases in Tcore result in down-regulation of central activation of muscle recruitment (12,27). As such, it may be that the reduction in both mean velocity and distance covered in the fourth quarter, particularly in MIA, was a result of Tcore reaching sufficiently high temperatures for a reduction in muscle recruitment (4). As VHIR is integral to game performance, and as both games involved highly competitive fourth quarters, the selective reduction in MIA while maintaining VHIR may have been a result of this proposed reduction in activity to control the internal thermal environment. Given the context of in-game data collection, it must be noted that changes in the game environment and player interchanges may also result in a reduced requirement of physical activity in the closing periods of a game regardless of Tcore (although both games in which the current data were collected involved physically demanding final quarters).
In conclusion, this is the first study to report the association of changes in game activity patterns and Tcore in warm conditions during free-paced, elite team sport competition. Unlike laboratory data, the free-paced nature of the game allows regulation of exercise intensity to ensure a plateau in Tcore, and, accordingly, Tcore rarely increased above 39.5°C. Additionally, this regulation of exercise intensity seems to be at the expense of MIA rather than HIR or VHIR, as is reported in laboratory data. Although factors related to the cellular metabolic environment and hydration status are also likely to contribute, the association between MIA and Tcore, while maintaining VHIR and, to a lesser extent, HIR, may suggest the use of pacing strategies to control the rise in the internal heat load while continuing to engage in regular, intense efforts during team sport exercise in warm conditions.
Intermittent-sprint exercise in the heat, as performed by many team sports during summer competition or preseason training, can result in increased internal body temperature. The present research highlights that athletes may adopt pacing strategies to limit the continued rise in core body temperature based on a reduction of work during nonessential activities. As the demands of competition dictate continued engagement in high-intensity exercise, reductions in the volume and intensity of low-intensity activity are more evident than those of high-intensity activity, which is further associated with the prevention of Tcore rising excessively. Accordingly, coaches and practitioners may adjust expected game demands in the heat or, alternatively, attempt to counter these adopted pacing strategies to maintain or increase the workload performed, particularly toward the end of a match.
This study was funded by a CSU Faculty of Education Research Grant. The authors would like to thank Joel Hocking for his valuable assistance with the data collection. Also, the authors would like to thank the staff and players at the Essendon Football Club for their assistance.
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Keywords:© 2009 National Strength and Conditioning Association
core temperature; motion analysis; team sport performance; heat