Original research
Greater chance of high core temperatures with modified pacing strategy during team sport in the heat

https://doi.org/10.1016/j.jsams.2013.02.013Get rights and content

Abstract

Objectives

To measure the activity profile, hydration status and core temperature of elite team sport athletes during matches in hot and cool conditions.

Design

Thirty-five professional Australian footballers (age 25.9 ± 3.5 yrs; height 188.4 ± 7.8 cm; body mass 90.6 ± 8.8 kg), gave informed consent to participate in this study. Core temperature (Tc), hydration and running performance were compared in eight hot and eight cool matches classified via a rating of the risk of heat illness from the Wet Bulb Globe Temperature (WBGT).

Methods

Core temperature was measured via an ingestible sensor before matches and after each quarter and player movement was recorded by 5 Hz GPS and expressed per period of the match (rotation), for distance; high-intensity running (HIR, 4.17–10.00 m s−1), sprinting (>4.17 m s−1) and maximal accelerations (2.78–10.00 m s−2). All data was compared for hot and cool matches and the magnitude of effects was analysed with the effect size (ES) statistic.

Results

Core temperature was elevated from rest at all time-points during matches (37.3–39.4 °C), with small additional elevations after the first and third quarters in hot matches (ES: 0.39 ± 0.40 and 0.37 ± 0.42 respectively). In hot matches 12 players had Tc > 40 °C but only one in cool matches. Total distance was reduced in the latter parts of each half (−6.5%, −0.49 ± 0.58; and −6.7%, −0.57 ± 0.59), yet the high intensity tasks of sprinting and accelerating were preserved.

Conclusions

Players tolerated core temperatures up to 40.5 °C during hot matches but reduced the volume of running undertaken, thus preserving the ability to undertake high intensity activities.

Introduction

The effect of heat load on physical performance is controversial.1, 2 The rate of body heat storage may induce a feed-forward mechanism thus regulating exercise intensity;3 but others refute this by using revised thermometric calculations of that same data.1 The self-paced nature of exercise in team sport makes calculation of heat storage difficult.2 It is unlikely that a critical core temperature (Tc)4 limits exercise performance in the field;5 yet there is increasing evidence that during self-paced exercise in the heat the pacing strategy of athletes is altered in an anticipatory manner.6 Little is known of the pacing strategies utilised in team sport under environmental challenges, and whether team-sport athlete activity is changed with a high environmental challenge. In fact, it is unclear due to the unpredictable nature of football7 if players can effectively pace during competition.

Australian football (AF) is a unique contact sport with a very high player activity profile,8, 9, 10 with a long competitive season11 requiring players to compete in a wide range of environmental conditions.12 For example, players regularly cover up to 16 km, and accelerate maximally up to 150 times in each game,8 thus ensuring a large metabolic cost and related heat production.13 By comparison, elite soccer players cover distances of only approximately ∼10 km,14 or 63% of those covered by elite AF players during matches. Importantly, some soccer players may reach Tc exceeding 40 °C during matches.15 Given the likely greater metabolic demand of AF,10, 13 players may potentially experience even larger increases in Tc than in soccer. Only one study has quantified Tc in AF players,12 but only in a small sample of pre-season matches with a relatively low distance travelled by athletes,8, 9 and thus with a lower heat production. Furthermore, that study did not compare to matches played in cool conditions.

It is possible that modest levels of dehydration reduce the performance of team-sport athletes,16 as Tc rises by between 0.15 and 0.20 °C for each 1% of body mass lost during an activity.17 This Tc rise, especially when coupled with dehydration, may increase the perception of effort of players,16 possibly leading to a reduction in effort.18 However, if the “pacing model” is true,6 players may have already altered their pacing to allow maintenance of high intensity efforts. Thus, AF matches, with the highest distance travelled per minute of the field sports,10 played in a variety of environmental conditions provide an excellent model to examine both the physiological strain on players, and the activity or potential pacing response of players to matches. The aims of this study were therefore to compare the activity profile, hydration status and Tc of AF players during matches played in hot and cool environmental conditions, with specific reference to pacing strategy.

Section snippets

Methods

Thirty-five elite Australian footballers (age 25.9 ± 3.5 yrs; height 188.4 ± 7.8 cm; body mass 90.6 ± 8.8 kg at commencement of the study, [mean ± standard deviation (SD)] gave informed consent to participate in this study, which was approved by the Victoria University Human Research Ethics committee. Participants were all registered players of an Australian Football League Club. Eight players were sampled in each match, with 4 sampled in each of the 16 matches, 3 for >5 hot and cool matches, 2 for 3–5

Results

The risk of heat illness is shown in Table 1. Players spent considerably less (15.0%) time playing in hot than in cool matches (hot = 97.26 ± 15.39; cool = 115.44 ± 14.66 min, 1.15 ± 0.66). Players rated exertion lower in hot matches (−3%, −0.43 ± 0.39). Consequently, player load derived from the session RPE method was lower in hot matches (−19%, −1.07 ± 0.41).

Matches played under hot conditions had a higher ambient temperature across all time points when compared to cool matches (Fig. 1A). For hot matches,

Discussion

This study confirms individual player Tc regularly exceeding 40 °C in matches, and the likelihood of this was doubled when matches were played in hot versus cool conditions. The Tc of elite Australian footballers rose to >39 °C during matches, irrespective of environmental conditions. Further, players reduced the amount of low-intensity activity undertaken in the latter part of each half in hot matches which may act to preserve the ability to undertake high intensity activity.

Core temperatures

Conclusion

Elite team sport athletes consistently generate Tc > 39 °C during matches regardless of the ambient environmental conditions. During matches played in the heat, the current athletes modulated match running by reducing the volume of running undertaken, which preserves the ability to undertake the high intensity activities.

Practical implications

  • Elite team sport athletes experience and tolerate high core temperatures during competition.

  • Team sport athletes can modulate their activity in high or extreme environmental conditions whilst preserving high intensity activities during matches.

Funding

This research was funded by an Australian Football League research grant.

Acknowledgments

The authors acknowledge the players and coaches of the Western Bulldogs Football Club, and especially Mr Cameron Falloon for facilitating data collection. There are no competing interests affecting this manuscript. This investigation was funded by an Australian Football League Research Grant.

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    1

    Current address: Department of Comparative Biosciences School of Veterinary Medicine, University of Wisconsin-Madison, United States.

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