Physiological and performance adaptations to an in‐season soccer camp in the heat: Associations with heart rate and heart rate variability

The aim of the present study was to examine the associations 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), postexercise 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 perceived 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 performance; both of which are associated with changes in HRex during a 5‐min submaximal run.