Abstract
Recent research on whole-body cryotherapy has hypothesized a major responsibility of head cooling in the physiological changes classically reported after a cryostimulation session. The aim of this experiment was to verify this hypothesis by studying the influence of exposing the head to cold during whole-body cryostimulation sessions, on the thermal response and the autonomic nervous system (ANS). Over five consecutive days, two groups of 10 participants performed one whole-body cryostimulation session daily, in one of two different systems; one exposing the whole-body to cold (whole-body cryostimulation, WBC), and the other exposing the whole-body except the head (partial-body cryostimulation, PBC).10 participants constituted a control group (CON) not receiving any cryostimulation. In order to isolate the head-cooling effect on recorded variables, it was ensured that the WBC and PBC systems induced the same decrease in skin temperature for all body regions (mean decrease over the 5 exposures: -8.6°C±1.3°C and -8.3±0.7°C for WBC and PBC, respectively), which persisted up to 20-min after the sessions (P20). The WBC sessions caused an almost certain decrease in tympanic temperature from Pre to P20 (-0.28 ±0.11°C), while it only decreased at P20 (-0.14±0.05°C) after PBC sessions. Heart rate almost certainly decreased after PBC (-8.6%) and WBC (-12.3%) sessions. Resting vagal-related heart rate variability indices (the root-mean square difference of successive normal R-R intervals, RMSSD, and high frequency band, HF) were very likely to almost certainly increased after PBC (RMSSD:+49.1%, HF: +123.3%) and WBC (RMSSD: +38.8%, HF:+70.3%). Plasma norepinephrine concentration was likely increased in similar proportions after PBC and WBC, but only after the first session. Both cryostimulation techniques stimulated the ANS with a predominance of parasympathetic tone activation from the first to the fifth session and in slightly greater proportion with WBC than PBC. The main result of this study indicates that the head exposure to cold during whole-body cryostimulation may not be the main factor responsible for the effects of cryostimulation on the ANS.
Highlights
Over the last decade, there has been increasing interest in the study of the influence of the autonomic nervous system (ANS) on the modulation of cardiac activity
The mean decrease in Tskin for the whole body recorded immediately after (Post) the first cryostimulation session (-8.3±0.7°C and -8.6°C±1.3°C for PBC and WBC, respectively) was very likely larger than that recorded after the fifth cryostimulation session (-7.6±0.8°C and -7.7±0.9°C for PBC and WBC, respectively)
In recently published work from our team, we showed that the stimulation of the autonomic nervous system after a single whole-body cryostimulation session was more pronounced by using a cryochamber system compared to an open tank [13]
Summary
There has been increasing interest in the study of the influence of the autonomic nervous system (ANS) on the modulation of cardiac activity. Some studies suggest that myocardial exposure to high levels of norepinephrine may result in β receptor-mediated cytotoxic effects and apoptosis as well as α receptor-mediated hypertrophic effects [4,5], dramatically increasing the risk of mortality [1]. The prevention of such effects by increasing vagal activity has become a priority for researchers
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