Long-term heat acclimation improves heart rate recovery when completed once, but not twice daily: British Association of Sport and Exercise Science Annual Student Conference, Plymouth

Heart rate recovery (HRR) is a simple indicator of aerobic fitness, which can be used to assess physiological changes and quantify intervention efficacy. HRR is also used as an indirect marker of autonomic function, overtraining syndrome and a prognostic predictor of mortality. The aim of this study was to assess HRR and aerobic fitness before and after once (OD) and twice daily (TD) heat acclimation (HA), relative to matched temperate exercise (TE). After ethical approval was granted, forty recreationally active male participants completed a cycling maximal oxygen uptake (V̇O2max) test pre- and post-HA. HRR was measured 60 s following volitional exhaustion. Participants were matched for V̇O2max and biophysical characteristics, before completing ten sessions of once a day HA (ODHA: 23 ± 6 years, 77.2 ± 10.0 kg, 178 ± 8 cm) or twice a day HA (TDHA: 25 ± 7 years, 75.3 ± 9.5 kg, 179 ± 4 cm), or, ten sessions of once a day TE (ODTE: 22 ± 1 years, 77.3 ± 8.6 kg, 174 ± 4 cm) or twice a day TE (TDTE: 22 ± 1 years, 75.2 ± 7.8 kg, 178 ± 7 cm). Twice daily exercise was completed on non-consecutive days. Exercise involved cycling for 60 min at 65% V̇O2max in hot (40°C, 20% relative humidity [RH]) or temperate (20°C, 40% RH) conditions. Total work done was matched between-groups. HRR significantly improved following ODHA (+17 ± 11 beats.min-1, P<0.001) and ODTE (+13 ± 10 beats.min-1, P=0.001). No HRR improvements were observed following TDHA (+1 ± 11 beats.min-1, P=0.110) nor TDTE (-6 ± 8 beats.min-1, P=0.110), however, following TDTE HRR was attenuated (P=0.001) compared to TDHA (mean difference 7 ± 14 beats.min-1). There were no improvements (P>0.05) in V̇O2max following ODHA (+0.18 ± 0.13 L.min-1 [4.8%]), TDHA (+0.16 ± 0.16 L.min-1 [4.5%]), ODTE (+0.10 ± 0.09 L.min-1 [2.6%]) or TDTE (+0.05 ± 0.08 L.min-1 [1.4%]). There were also modest correlations (all P>0.05) between plasma volume changes (∆PV) and HRR for ODHA (r=0.4) and TDHA groups (r=0.7), and correlations between ∆HR at rest and HRR in ODHA (r=-0.4). Ten sessions of once daily moderate-intensity exercise in hot and temperate conditions improved HRR. Although no changes in HRR were observed after twice daily exercise, TDHA prevented the attenuation displayed in HRR after TDTE, possibly due to a greater PV expansion. HRR correlated with ∆PV in HA groups, indicating a possible recovery/therapeutic effect of HA on cardiovascular health.