# 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. # Results All data of all subjects were taken into account in the presentation of the results. 

## Skin and tympanic temperatures Baseline skin temperature (Tskin) for all body regions was similar between groups before the cryostimulation, without difference between the first and fifth cryostimulation session ([Fig 2](#)). Each day, an *almost certain* very large reduction in mean skin temperature of the whole body was recorded within the 20-min after the PBC and WBC exposures ([Fig 2](#)). However, no difference was recorded between cryotherapy modalities from Post to P20. In both modalities, the mean Tskin *almost certainly* remained lower than baseline values up to P20 ([Fig 2](#)). In both modalities, a *very likely* large reduction in Tskin of the head was recorded in Post and persisted up to P5, in larger proportion for WBC than PBC ([Fig 2](#)). In both cryotherapy modalities, 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). <figure> 10.1371/journal.pone.0124776.g002 <p><img src="" /></p> <figcaption>Changes in the mean skin temperature of the whole body (except the head) and the head alone.<br /> Values were recorded before (Pre), immediately after (Post) and for 20-min (P1 to P20) after the first and the fifth whole-body cryostimulation (WBC), and partial-body cryostimulation (PBC) sessions, and in the control (CON) condition. Within-group change (Post conditions vs. Pre): * likely; \*\* very likely; \*\*\* almost certain. Between-group (vs. CON) difference in the change: # likely; 

## very likely; 

### almost certain. Between-group (PBC vs. WBC) difference in the change: ¤ likely; ¤¤ very likely; ¤¤¤ almost certain.</figcaption> </figure> The tympanic temperature (Ttymp) *almost certainly* decreased after the WBC sessions by -0.28±0.18°C on day 1 and -0.34±0.25°C on day 5 and *almost certainly* remained lower than baseline values up to P20 ([Fig 3](#)). There was a trivial difference in the decrease in Ttymp between the first and the last WBC sessions. A small to moderate reduction in Ttymp was recorded after PBC sessions, occurring *almost certainly* in P20 (-0.14±0.05°C). The reduction in Ttymp over the week was *very likely* larger after WBC than PBC in Post (-0.31±0.15°C vs. -0.07±0.12°C for WBC and PBC, respectively) and P5 (-0.26±0.15°C vs. -0.04±0.10°C for WBC and PBC, respectively). <figure> 10.1371/journal.pone.0124776.g003 <p><img src="" /></p> <figcaption>Changes in tympanic temperature.<br /> Values were recorded before (Pre), immediately (Post), 5-min (P5), and 20-min (P20) after the first and the fifth whole-body cryostimulation (WBC), and partial-body cryostimulation (PBC) sessions, and in the control (CON) condition. Within-group change (Post conditions vs. Pre): * likely; \*\* very likely; \*\*\* almost certain. Between-group (vs. CON) difference in the change: # likely; 

## very likely; 

### almost certain. Between-group (PBC vs. WBC) difference in the change: ¤ likely; ¤¤ very likely; ¤¤¤ almost certain.</figcaption> </figure> 

## Cardiovascular parameters No marked changes in systolic and diastolic BP were recorded after the cryostimulation sessions in PBC and WBC modalities. Only a *likely* small decrease in systolic BP was recorded after the third PBC session and a *likely* small increase in diastolic BP after the two first WBC sessions, but these increases disappeared for the next sessions ([Fig 4](#)). <figure> 10.1371/journal.pone.0124776.g004 <p><img src="" /></p> <figcaption>Changes (% of pre values) in systolic and diastolic blood pressures from pre to post whole-body cryostimulation (WBC), and partial-body cryostimulation (PBC) sessions, and in the control (CON) condition.<br /> The shaded area represents trivial changes. Values were recorded daily on five consecutive days before and after each cryostimulation sessions. Between-group (PBC or WBC vs. CON) difference in the change: * likely; \*\* very likely; \*\*\* almost certain. Between-group (PBC vs. WBC) difference in the change: # likely; 

## very likely; 

### almost certain.</figcaption> </figure> In addition, HR *almost certainly* decreased in moderate to very large proportions after each PBC and WBC sessions, without significant differences over the week ([Fig 5](#)). The mean decrease obtained over the five cryostimulation sessions was -5.02±1.07 bpm after PBC and -7.7±2.3 bpm after WBC, and these changes were *almost certainly* different from the CON group. The decrease in HR was *likely* to *almost certainly* greater after WBC than PBC sessions in day 2, 3 and 5. <figure> 10.1371/journal.pone.0124776.g005 <p><img src="" /></p> <figcaption>Changes (% of pre values) in heart rate and HRV indices of parasympathetic activity from pre to post whole-body cryostimulation (WBC), and partial-body cryostimulation (PBC) sessions, and in the control (CON) condition.<br /> The shaded area represents trivial changes. Values were recorded daily on five consecutive days before and after each cryostimulation session. Between-group (PBC and WBC vs. CON) difference in the change: * likely; \*\* very likely; \*\*\* almost certain. Between-group (PBC vs. WBC) difference in the change: # likely; 

## very likely; 

### almost certain.</figcaption> </figure> HRV indices of parasympathetic activity, namely RMSSD and HF were *likely* to *very likely* increased in moderate to large proportions after each WBC and PBC session, without marked differences between the modalities or between the first and the fifth session ([Fig 5](#)). The mean Pre-Post increases in RMSSD over the week were +49.1±15.4% and +38.8±14.7% in the PBC and WBC modalities, respectively, while HF increased by 122.3±40.0% and 70.3±29.5% in the PBC and WBC modalities, respectively. 

## Plasma catecholamine concentrations Cold-induced changes in plasma catecholamine concentrations are depicted in [Fig 6](#). The first cryostimulation session *likely* induced a moderate to large increase in plasma norepinephrine concentration without any significant difference between PBC and WBC. These changes in plasma norepinephrine concentrations compared with baseline values were associated with a *likely* moderate increase in plasma epinephrine for WBC only. No further significant changes in plasma catecholamine concentrations were recorded for the last WBC and PBC sessions. <figure> 10.1371/journal.pone.0124776.g006 <p><img src="" /></p> <figcaption>Changes (% of pre values) in plasma concentration in catecholamines from pre to post whole-body cryostimulation (WBC), and partial-body cryostimulation (PBC) sessions, and in the control (CON) condition.<br /> The shaded area represents trivial changes. Values were recorded on the first (1<sup>st</sup> day) and fifth (fifth day) session. Between-group (PBC or WBC vs. CON) difference in the change: * likely; \*\* very likely; \*\*\* almost certain.</figcaption> </figure>


Extracted trial result:
Outcome: Tskin of the head (continuous)
Intervention: whole-body to cold (whole-body cryostimulation, WBC)
Comparator: whole-body except the head (partial-body cryostimulation, PBC)
Intervention group size: 10
Comparator group size: 10
Notes: Numerical values are only in the figures.