This study is part of two larger projects, which were conducted between 2016 and 2022 in the experimental facilities of the Division of Environmental Physiology (Solna, Sweden), examining the effects of prolonged repeated hypergravity exposures on the human cardiovascular system (see Eiken et al. 2022; Keramidas et al. 2023). The study was approved by the Human Ethics Committee of Stockholm (Ref. no.: 2016/1889-31/4 and 2019/06542) and conformed to the Declaration of Helsinki. Prior to participation, written informed consent was obtained from all subjects.

Twenty healthy male flight-cadets of the Swedish Air Force volunteered to participate [mean (range) age: 24 (21–27) years, weight: 80 (70–94) kg, height: 180 (171–186) cm]; they were recruited to the study before attending any flight training. All subjects performed the hand cold-pressor test, and nine of them also conducted the foot cold-pressor test. An a priori power analysis was not performed for these specific datasets, because the current work dealt with a secondary question within the larger projects (see Eiken et al. 2022).

Subjects underwent, in a 7.25-m radius human-use centrifuge (ASEA, Sweden), a 5-weeks G-training regimen, comprising three 40-min sessions per week. Subjects were seated upright in the tangentially pivoted centrifuge gondola, wherein the seat back reclines 28° from the vertical. They remained relax throughout each G exposure; they were thus not allowed to perform anti-G straining maneuvers (i.e., Valsalva, isometric contraction/tensing of skeletal muscles), did not wear anti-G suits, and were not exposed to positive pressure breathing. During each session, the G load was oscillated, at 1-min intervals and at 0.5 G/sec transition rate, between idle speed (1.4 G) and a G load corresponding to ~ 85% of the individual, rapid onset-rate G tolerance: the mean (range) G load was 2.8 (2.6–3.4) G in the 1st week, 3.0 (2.6–3.6) G in the 2nd week, 3.1 (2.6–3.6) G in the 3rd week, 3.2 (2.8–4.0) G in the 4th week, and 3.2 (2.9–4.1) G in the 5th week.

Before and after the 5-weeks G-training regimen, all subjects performed a hand cold-pressor test, and nine of them also a foot cold-pressor test. Both tests were performed with the subjects in an upright sitting position. Each test commenced with a 10-min baseline phase. Thereafter, subjects immersed their right hand or foot for 4 min in 4 °C water; they were instructed to remain relax, breathe normally, and avoid any Valsalva-like maneuver throughout. The hand and foot cold-pressor tests were separated by a ~ 15-min interval; their order, which remained constant in the two testing periods, was alternated among subjects: four and five of them performed first the hand and foot cold-pressor test, respectively. For the individual subject, the time of the day that the tests were conducted were the same in the two testing periods. The temperature in the laboratory was maintained at ~ 24 °C. Beat-to-beat systolic (SAP), diastolic (DAP) and mean (MAP) arterial pressures were measured continuously using a volume-clamp technique (Finometer, Finapres Medical Systems BV, Amsterdam, the Netherlands). The pressure cuff was placed around the middle phalanx of the third finger of the left hand, and the reference pressure transducer was positioned at the level of the heart. Before each test, a brachial cuff was attached on the same arm, and the calibration process was performed according to the manufacturer’s instructions. Heart rate (HR) was derived from the arterial-pressure curves as the inverse of the inter-beat interval. Cardiac stroke volume was estimated by a three-element model of arterial input impedance from the arterial-pressure waveform (Modelflow, Finometer; Wesseling et al. 1993). Cardiac output (CO) was estimated by multiplying HR by stroke volume, and TPR was calculated by dividing MAP by CO. Subjects were asked, every minute, to provide ratings of the immersed-limb pain (from 0—no pain to 10—maximal pain).

Baseline values were calculated as the average of the final 5 min of the 10-min baseline phase. Data from the cold-water immersion phase were calculated as the average of the entire 4-min period. Data are presented as absolute values, and as relative (%) changes to baseline. Normality of distribution for all datasets was assessed with the D’Agostino–Pearson test. All data were analyzed with two-way [test phase (baseline × cold stress) × testing period (before × after G training)] repeated-measures analysis of variance (ANOVA). Sphericity was assessed using Mauchly’s test, and the Greenhouse–Geiser ɛ correction was applied when necessary. When ANOVA revealed a significant F value, the Bonferroni correction was used to adjust for multiple post hoc comparisons. Differences in pain perception and the relative changes in cardiovascular responses to cold were assessed with a Wilcoxon test and a paired two-tailed t test, respectively. Statistical analyses were conducted using Prism 10.0 (GraphPad Software Inc., San Diego, CA, USA). Unless otherwise stated, data are presented as mean values with standard deviation. The α level of significance was set a priori at 0.05.