Elite North American soccer performance in thermally challenging environments: An explorative approach to tracking outcomes

Aims: The physiologic challenges related to performances in hot conditions calls for dedicated consideration when planning athlete training, although complete amelioration of the effects of heat may not be possible. We aimed to quantify within-subject correlations between different measures of environmental temperature and performance changes over multiple elite soccer competitions. Methods: Thirty-seven elite male soccer players (age: 26 +/- 3.4 years, height: 171 +/- 2 cm, body mass: 78 +/- 7.1 kg) competed in North America over four seasons (range: 3 to 98 matches). Players games and reported differential- RPE immediately post game. Temperatures at kick-off, week average temperature, the difference between game-day and week average (Diff(Temp)), and heat index at kick-off were obtained. Within-player correlations were calculated using general linear models to quantify associations between fluctuations in temperature measures and physical and perceived outputs. Results: Correlations between total distance and the various temperature measures were trivial to small (range: -0.08 to 0.13, p= <0.001-0.02). Small negative correlations were found between all temperature measures except DiffTemp and high-speed running (HSR) (range: -0.17 to -0.14, p= <0.001). Most correlations between differential-RPE and temperature measures were trivial to small and not significant (r = 0.06 to 0.18 p = 0.03-0.92) although breathlessness-RPE and heat index showed a small significant association (P = 0.018) Conclusion: Decrements in HSR appear to be associated with increased environmental temperature, however, these associations are small in magnitude.