Physiological and performance adaptations to an in-season soccer camp in the heat: Associations with heart rate and heart rate variability M. Buchheit 1 , S. C. Voss 1 , L. Nybo 2 , M. Mohr 2 , S. Racinais 3 1 Sport Science Department, ASPIRE, Academy for Sports Excellence, Doha, Qatar, 2 Department of Exercise and Sport Sciences, Section of Human Physiology, University of Copenhagen, Copenhagen, Denmark, 3 ASPETAR – Qatar Orthopaedic and Sports Medicine Hospital, Research and Education Centre, Doha, Qatar Corresponding author: Martin Buchheit, PhD, ASPIRE, Academy for Sports Excellence, PO Box 22287, Doha, Qatar. Tel: (+974) 4413 6103, Fax: (+974) 4413 6060, E-mail: martin.buchheit@aspire.qa Accepted for publication 27 June 2011 The aim of the present study was to examine the associa- tions between adaptive responses to an in-season soccer training camp in the heat and changes in submaximal exercising heart rate (HRex, 5-min run at 9 km/h), postex- ercise HR recovery (HRR) and HR variability (HRV). Fifteen well-trained but non-heat-acclimatized male adult players performed a training week in Qatar (34.6  1.9°C wet bulb globe temperature). HRex, HRR, HRV (i.e. the standard deviation of instantaneous beat-to-beat R–R interval variability measured from Poincaré plots SD1, a vagal-related index), creatine kinase (CK) activity, plasma volume (PV) changes, and post-5-min run rate of per- ceived exertion (RPE) were collected at six occasions in temperate environmental conditions (22°C). Players also performed the yo-yo intermittent recovery test level 1 (Yo-Yo IR1) in the same environmental conditions (22°C), both at the beginning and at the end of the training week. Throughout the intervention, HRex and HRV showed decreasing (P < 0.001) and increasing (P < 0.001) trends, respectively, while HRR remained unaffected (P = 0.84). Changes in HRex [-0.52, 90% confidence limits (-0.64; -0.38), P < 0.001] and SD1 [0.35 (0.19; 0.49), P < 0.001] were correlated with those in PV. There was no change in RPE (P = 0.92), while CK varied according to training contents (P < 0.001), without association with HR-derived measures. Yo-Yo IR1 performance increased by 7  9% (P = 0.009), which was correlated with changes in HRex [-0.64 (-0.84; -0.28), P = 0.01]. In conclusion, we found that an in-season soccer training camp in the heat can significantly improve PV and soccer-specific physical per- formance; both of which are associated with changes in HRex during a 5-min submaximal run. Soccer matches are often played in challenging condi- tions where the temperature can exceed 30°C, with or without a high relative humidity. With the recent announcement of the 2022 soccer World cup to be held in Qatar, there is likely to be an increased interest in methods for improving training and soccer performance in the heat. While the literature on actual match (running) performance is not extensive, players compet- ing in hot conditions generally cover less distance and show exacerbated match-related fatigue than under cooler conditions (Mohr et al., 2010; Ozgunen et al., 2010). Experimental studies have shown that, in addition to the various interventions that can prevent excessive dehydration and/or increase in core temperature (e.g. precooling, ice pack applications, and ice-slush inges- tion; for review, see Quod et al., 2006; Maughan et al., 2010), exercise-heat acclimation is likely the most effec- tive strategy to limit the reduction in match running performance in hot conditions (Sunderland et al., 2008). As such, training in the heat for a few days/weeks prior to soccer competitions in hot environments is highly recommended (Grantham et al., 2010; Maughan et al., 2010). The physiological adjustments to heat acclima- tion are well established (e.g. Ladell, 1951; Hellon et al., 1956) and include decreased central temperature, reduc- tion of the temperature threshold for sweating (Ladell, 1951), plasma volume (PV) expansion, enhanced myo- cardial efficiency and improved cardiovascular adjust- ments (Wyndham et al., 1976), reduced oxygen uptake at a given power output, and muscle glycogen sparing (Young et al., 1985). Therefore, it is reasonable to pos- tulate that exercise-heat acclimation can have an ergo- genic effect, even in temperate conditions (i.e. 22°C). While this has recently been demonstrated in well- trained cyclists (Lorenzo et al., 2010), it is still unknown if training in the heat has ergogenic benefit for soccer players when playing in temperate conditions. If this was to be effective, training in the heat could also be recom- mended as a specific training intervention to improve players’ aerobic fitness level before or during the com- petitive season. Scand J Med Sci Sports 2011: ••: ••–•• doi: 10.1111/j.1600-0838.2011.01378.x © 2011 John Wiley & Sons A/S 1