Effect of sauna-based heat acclimation on plasma volume and heart rate variability
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We investigated the effect of post-exercise sauna bathing on plasma volume (PV) expansion and whether such responses can be tracked by changes in heart rate (HR)-based measures.
Seven, well-trained male cyclists were monitored for 35 consecutive days (17 days baseline training, 10 days training plus sauna, 8 days training). Sauna exposure consisted of 30 min (87 °C, 11 % relative humidity) immediately following normal training. Capillary blood samples were collected while resting seated to assess PV changes. HR (HRwake) and vagal-related HR variability (natural logarithm of square root mean squared differences of successive R–R intervals, ln rMSSDwake) were assessed daily upon waking. A sub-maximal cycle test (5 min at 125 W) was performed on days 1, 8, 15, 22, 25, 29, and 35 and HR recovery (HRR60s) and ln rMSSDpostex were assessed post-exercise. Effects were examined using magnitude-based inferences.
Compared with baseline, sauna resulted in: (1) peak PV expansion after four exposures with a likely large increase [+17.8 % (90 % confidence limits, 7.4; 29.2)]; (2) reduction of HRwake by a trivial-to-moderate amount [−10.2 % (−15.9; −4.0)]; (3) trivial-to-small changes for ln rMSSDwake [4.3 % (1.9; 6.8)] and ln rMSSDpostex [−2.4 % (−9.1; 4.9)]; and (4) a likely moderate decrease in HRR60s [−15.6 % (−30.9; 3.0)]. Correlations between individual changes in PV and HR measures were all unclear.
Sauna bathing following normal training largely expanded PV in well-trained cyclists after just four exposures. The utility of HR and HRV indices for tracking changes in PV was uncertain. Future studies will clarify mechanisms and performance benefits of post-training sauna bathing.
KeywordsHeat exposure Blood volume Cardiac parasympathetic activity Post-exercise Cyclists
Coefficient of variation
Heart rate during the 5-min sub-maximal (125 W) exercise test
Heart rate recovery
Heart rate recovery at 60 s post-exercise
Heart rate variability
Heart rate upon waking
- ln rMSSDpostex
Natural logarithm of the rMSSD following sub-maximal exercise
- ln rMSSDwake
Natural logarithm of the rMSSD upon waking
Square root mean of the sum of the squared differences between adjacent normal R–R intervals
Smallest worthwhile change
Yo-Yo Intermittent recovery level 2 test
The authors would like to thank the participants for their substantial commitment and assistance with the study. This study was funded by the Centre of Excellence for Applied Sport Science Research at the Queensland Academy of Sport. Aaron Halliday was supported by a scholarship from the Centre of Excellence for Applied Sport Science Research at Queensland Academy of Sport. Dr Jamie Stanley is supported by research grant funding from the Centre of Excellence for Applied Sport Science Research at Queensland Academy of Sport.
Conflict of interest
The remaining authors declare that they have no conflict of interest.
The current experiment was conducted in Australia with all activities complying with the current laws of Australia.
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