Centrifugal acceleration to 3G_z is related to increased release of stress hormones and decreased mood in men and women

Abstract

It has been suggested that the central and peripheral neural processes (CPNP) are affected by gravitational changes. Based on the previous experiments during parabolic flights, central and peripheral changes may not only be due to the changed gravitational forces but also due to neuroendocrine reactions related to the psycho-physiological consequences of gravitational changes. The present study focuses on the interaction of neuroendocrine changes and the physical and mental states after acceleration to three-time terrestrial gravity (3G_z).

Eleven participants (29.4 ± 5.1 [SD] years (male (n = 8): 30 ± 5.1 years; female (n = 3): 27.7 ± 2.1 years) underwent a 15 min acceleration to 3G_z in a human centrifuge. Before and after the acceleration to 3G_z circulating stress hormone concentrations (cortisol, adrenocorticotropic hormone (ACTH), prolactin, epinephrine, norepinephrine) and perceived physical and mental states were recorded. A second control group of 11 participants underwent the same testing procedure in a laboratory session. Serum cortisol concentration during exposure to the centrifugal acceleration increased by 70%, plasma concentration of ACTH increased threefold, prolactin twofold, epinephrine by 70% and norepinephrine by 45%, whereas the perceived physical well-being decreased.

These findings demonstrate that psycho-physiological changes have to be regarded as a relevant factor for the changes in CPNP during phases of hypergravity exposure.

• Hypergravity
• centrifuge
• neuroendocrine stress hormones
• mood
• physical state

Introduction

Research within altered gravity conditions has received considerable attention in both public and scientific sectors. Advances in aircraft design technologies and in the understanding of biophysiological adaptations to changed gravity conditions have increased during the last two decades. Whereas research in microgravity is related to living and performing in space, research under hypergravity conditions is mainly linked to military jet-piloting but could also be useful for space flights as a short-arm centrifugation is proposed as a counter-measure for weightlessness.

In both the areas of microgravity and hypergravity, physiological phenomena have been reported, ranging from changes in circulatory and cardiovascular responses, muscle metabolism and central and peripheral neural processes (CPNP) (Nicogossian et al. 1994; Girgenrath et al. 2005; Gobel et al. 2006). Until very recently gravity conditions were held responsible for the observed impairment of CPNP (Chen et al. 1999; Miyoshi et al. 2003; Davey et al. 2004). However, research neither on hypergravity nor on microgravity considered the effects of stress or arousal on CPNP until Fowler and Manzey (2000) explicitly drew attention to the notion that it is essential to identify stressors and their impact on CPNP in these conditions. This has led to numerous studies of changes in mood during long-term exposure to microgravity (Lorenz et al. 1996; Manzey and Lorenz 1998; Manzey et al. 1998) or of changes in mood related to increased neuroendocrine stress levels during confinement (Chouker et al. 2001, 2002). Short-term changes of gravity levels, especially hypergravity, however have been factored out so far, although the impact of temporary increased gravity levels on neural functions has been reported, e.g. for the safety of flights in high performance aircraft (Gobel et al. 2006).

Psychosomatic stress is associated with adaptation responses of various physiological systems, in particular increased activity in the hypothalamus-pituitary-adrenal (HPA) axis (i.e. adrenocorticotropic hormone (ACTH) and cortisol) and in the sympathetic-adrenal-medullary systems (SAM) (e.g. epinephrine and heart rate) (Charmandari et al. 2005). Several types of stressors such as hypoglycaemia, surgery, diet and body hyperthermia result in an increased prolactin secretion. Although hypothalamic control of prolactin secretion is dominated by tonic inhibitory mechanisms via dopamine, a functional role of prolactin-releasing factors appears to be necessary for acute secretory responses (Yen 1991).

Although increasing arousal is known to affect CPNP (DeMoja et al. 1987; Jones and Hardy 1988; Hackfort et al. 2000) the increased release of stress hormones does not imply by itself an influence on CPNP. Rather it seems necessary to record also the perceived state of mood that is associated with changes in hormonal concentrations. It was deemed essential to differentiate between physical and emotional stress and to find a scale that was sensitive to short-term changes of mood. These characteristics are found in a handheld-based item-scale (MoodMeter^®) developed especially for use on young athletes (Kleinert 2006).

This study aimed (1) to see whether there are increased circulating concentrations of stress hormones during hypergravity and (2) if these are connected to changes in perceived physical, emotional and/or psychological state.

Materials and methods

Access to a human centrifuge designed for experiments in hypergravity was provided by the German Space Agency (DLR) in Cologne, Germany. The centrifugal cabin (radius of 5 m) swings out during rotation so that the resultant of the centrifugal and gravitational acceleration is always orthogonal to the gondola floor (G_z acceleration). The centrifuge was able to accelerate up to 9G, but due to medical and safety restrictions, in the present study acceleration was limited to three-time terrestrial gravity (3G_z). 3G_z was chosen because the experiments of one of the authors (Girgenrath et al. 2005; Gobel et al. 2006) have been conducted at the same increase in gravity and results therefore could explain earlier findings on motor control. After approval from the local institutional ethics committee, 11 participants aged 29.4 ± 5.1 (SD) years (male (n = 8): 30 ± 5.1 years; female (n = 3): 27.7 ± 2.1 years) with no prior experience in hypergravity testing were subjected to centrifugal acceleration. All participants underwent a clinical check and gave written informed consent. None of the participants reported taking any medication during the study. In a pre-experiment briefing, the participants were informed that a number of blood collections would be made, were informed about the acceleration procedure and possible consequences (nausea, dizziness) and were introduced to the use of the MoodMeter^®. During the 3G_z phase, the participants were seated and secured with a safety belt. For safety reasons they were monitored by ECG, video and audio loops, and were instructed about the use of a panic button. If heart rate showed abnormal behaviour or participants showed signs of impending unconsciousness the centrifuge was stopped without delay, which happened once in the course of this study. Data from the affected participant were excluded from further analysis.

Blood collection and recording of the mood of participants by the MoodMeter^® occurred three times during the study: first, before the centrifuge started (PRE), second immediately after deceleration to 1G (POST) and approximately 1 h after the POST measurement (POST 1HR). Constant acceleration at 3G_z lasted 15 min.

A control group of 11 participants aged 21.7 ± 2.3 years (male (n = 7): 22.6 ± 2.4 years; female (n = 4): 20.3 ± 0.5 years) was asked to perform exactly the same procedure under stress-free conditions in a laboratory setting. Blood collection and recording of mood in the control group occurred in a time frame similar to that of the centrifuge group. Limited access to the centrifuge unfortunately made it impractical to make measurements on this control group in the centrifugal cabin at 1G_z. But as the cabin was illuminated during the 15 min of 3G_z and the subjects had permanent voice and video contact with the operators, it is considered unlikely that an increase in stress hormones is explained simply by being enclosed in this cabin. Furthermore, the first PRE measurement was made after the subjects sat in the cabin for approximately 8–10 min, without acceleration, and these hormone concentrations were not elevated compared with the first measurements in the control groups. Moreover, a previous study showed no effects on the motor performance in an isometric force production task when subjects were seated for a longer time in the cabin before acceleration to 3G_z (Gobel et al. 2006), or in two groups performing either in the cabin or the laboratory (unpublished observation), whereas significant changes were obtained under 3G_z.

Blood collection

Stress hormones were determined in blood samples taken through an indwelling venous catheter, which was inserted into the left arm of each participant by a physician 2 h prior to acceleration and was kept open with 0.9% saline. The following samples were drawn in pre-chilled vacutainers^®: 3 ml blood into an EDTA-containing tube for ACTH analysis, 10 ml for cortisol and prolactin analysis, and 10 ml blood into an EGTA-GSH containing tube for catecholamine analysis. Blood samples were centrifuged at 5000 rpm for 10 min after collection in a cooled centrifuge. Plasma was divided into fractions, first stored at − 20°C for 8 h, and afterwards frozen at − 80°C until analysis. The control values for hormones as stated on the package inserts of the test kits were: prolactin male 4.04–15.2 ng/ml, female 4.79–23.3 ng/ml; cortisol 6.2–19.4 μg/dl; ACTH 1.6–13.9 pmol/l; epinephrine < 84 ng/l; norepinephrine < 420 ng/l.

Serum prolactin/ cortisol and plasma ACTH concentrations were detected by immunoassays (prolactin and ACTH, sandwich principle; cortisol, competition principle). The electrochemiluminescence immunoassays (ECLIA^®) for prolactin, cortisol and ACTH were provided by Roche (Mannheim, Germany) and processed on an immunoassay analyser (Elecsys 2010; Roche). Catecholamines were analysed by high-pressure liquid chromatography using electrochemical detection (HPLC—reagents, Chromsystems, Muenchen Germany; hardware, Shimadzu, Langenfeld, Germany).

Statistics

Due to analytical problems, the number of blood samples for the control group was reduced to 10 participants for cortisol and prolactin and to 6 participants for ACTH. A mixed analysis of variances (ANOVA) with the intra-individual factor MEASUREMENT and the inter-individual factor GROUP was applied to the blood data to identify time-dependent changes in hormone concentrations. As blood hormone concentrations show high interpersonal variation even from day to day, group main effects (centrifuge, control) are reported but not discussed. All data were tested for normal distribution by a one sample Kolmogorov–Smirnov test. Mixed-measures ANOVAs were adjusted for multiple comparisons by Bonferroni correction. Two-tailed level of significance was set at *p < 0.05, **p < 0.01, ***p < 0.001. Data are presented as means ± confidence intervals.

Adjective list for the assessment of physical and psychological states: MoodMeter^®

The MoodMeter^® consists of two modules (Bodyfinder and Feelfinder). A detailed description of the development and operating mode of the MoodMeter is given by Kleinert (2006). The Module Bodyfinder has been developed to determine the current perceived physical state (PEPS) in a traditionally more biomedically orientated research (e.g. exercise physiology, internal medicine) and is sensitive for short-term alterations. It was validated during 2001–2005 on a total of 645 participants (Kleinert 2006). The module Feelfinder includes a short form of the “Eigenzustandsskala” (EZ-scale (Nitsch 1976). In contrast with other psychological adjective scales (e.g. the POMS), the EZ-scale allows the measurement of not only the emotional or psychological strain, but also the motivational state. In the present study we used a short 16-item-form of the EZ-scale developed and validated by Nitsch (1976) which forms eight subdimensions (Table II).

The version of MoodMeter^® used here consisted of three catalogues, each consisting of 32 adjectives (16 PEPS, 16 EZ-scale) presented in a random order. A different catalogue of adjectives was presented at each of the measurement time points, i.e. PRE, POST and POST 1HR. It took approximately 2 min for each catalogue to be completed. MoodMeter^® adjectives were presented on a standard touch screen handheld PC in German. Adjectives presented in this text are translated into English. Preliminary instructions given to the participants were: “Please name, without any hesitation, to what extent the following adjective applies to your physical state at this moment”. The endpoints of a 6-step ranking scale were anchored (0 = not at all; 5 = totally). There was a time limit of 5 s to complete the answers.

Out of the 32 adjectives in each catalogue one dimension for the PEPS and two EZ-scale dimensions (Psychological Strain, Motivational State), each containing four subdimensions, were formed (Table II). Each dimension was checked for significant changes using Friedmann's repeated measures ANOVA with the intra-individual factor MEASUREMENT (#1–#3). Wilcoxon paired samples test was used as a post-hoc test if the Friedmann's ANOVA revealed significance. If so, the catalogues were checked against each other. Two-tailed level of significance was set at *p < 0.05, **p < 0.01, ***p < 0.001. Data in the text are presented as means ± SD.

All statistical analyses were performed by Statistica program 7.1 (StatSoft, Tulsa, USA).

Results

Cortisol

Plasma cortisol concentrations were significantly increased after the 15 min of exposure to 3G_z (vs. PRE, p < 0.001) and decreased significantly within one hour back to the baseline values that were measured prior to the start of the 3G phase (p < 0.001, Table I). Although no significant changes were found for the control group, the slight decrease in the cortisol concentrations reflects the typical circadian rhythm, with higher cortisol concentrations in the morning followed by a decrease over the course of the day (Figure 1a).

Table I.  ANOVA results for centrifuge vs. control.

PLASMA HORMONE	N (ce|co)	GROUP	MEASUREMENT	GROUP^*MEASUREMENT
Cortisol	11*10	F(1,19) = 5.71, p < 0.05*	F(2,38) = 10.64, p < 0.001***	F(2,38) = 14.22, p < 0.001***
Prolactin	11*10	F(1,19) = 7.55, p < 0.05*	F(2,38) = 3.12, p = 0.06	F(2.38) = 4.06, p < 0.05*
ACTH	10*6	F(1,14) = 3.95, p = 0.07	F(2,28) = 4.30, p < 0.05*	F(2,28) = 4.64, p < 0.05*
Epinephrine	11*11	F(1,20) = 2.45, p = 0.13	F(2,40) = 4.33, p < 0.05*	F(2,40) = 3.90, p < 0.05*
Norepinephrine	11*11	F(1,20) = .097, p = 0.76	F(2,40) = 2.36, p = 0.11	F(2,40) = 10.06, p < 0.001***

Note: ce- centrifuge group; co- control group.

Figure 1 Changes in plasma hormone concentrations for the centrifuge group who were exposed to 3G_z for 15 min and for the control group. Measurements were made before the centrifuge started (PRE) after it stopped (POST) and one hour later (POST 1HR). Measurements for the control group were chronologically comparable. Data show means ± 95% confidence intervals. Asterisks indicate significant changes compared with the previous measurement (PRE vs. POST vs. POST 1HR): *p < 0.05; **p < 0.01; ***p < 0.001.

Prolactin

Plasma concentrations of prolactin significantly increased (twofold) after the 3G_z phase (p < 0.05). The POST 1HR measurement missed the significance level marginally (p = 0.08). No significant changes were observed for the control group prolactin concentrations (Figure 1b).

Adrenocorticotropic hormone

Similar to cortisol concentrations, the plasma ACTH concentration increased from PRE to POST measurement (p < 0.05) and decreased again to baseline values at the POST 1HR measurement (p < 0.01). There were no significant changes for the control group (Figure 1c).

Epinephrine and norepinephrine

Although the increase in plasma epinephrine concentration POST missed significance vs. the PRE measurement marginally (p = 0.08), epinephrine and norepinephrine values showed similar patterns with an increase after the 3G_z phase (norepinephrine p < 0.01) and a decrease within the following hour (epinephrine p < 0.01, norepinephrine p < 0.01; Figure 1d,e). Again, neither norepinephrine nor epinephrine values showed any significant changes within the control groups.

Perceived physical state (MoodMeter^®, module Bodyfinder)

Within the PEPS dimension (Figure 2a) a 15-min acceleration to 3G_z showed only effects on the perceived physical well-being which was decreased in the POST measurement (p < 0.05; Figure 3a). Although all subdimensions showed similar progressions, e.g. a decrease POST followed by an increase back to baseline POST 1HR (Table II), neither of them reached significance level. No changes in any dimension could be observed for the control group (Table II).

Figure 2 Changes in perceived physical state (PEPS) (left) and the two EZ-scale dimensions motivational state (middle) and psychological strain (right) for measurements at time points PRE, POST and POST 1HR. Data show means, standard error (boxes) and standard deviation. Crosses indicate changes compared to POST: ^†p < 0.05.

Figure 3 Detailed view on the significant PEPS subdimension perceived physical health (left) and on the significant EZ-scale subdimensions calmness (second), readiness to strain (third) and self-confidence (right) during measurements PRE, POST and POST 1HR. Data show means, standard error (boxes) and standard deviation. Asterisks indicate significant differences from PRE measurements: *p < 0.05. Crosses indicate significant differences from POST measurement: ^††p < 0.01. Note that although self-confidence showed a significant effect in the ANOVA (Table II), post-hoc comparison showed no significant differences between the measurements.

Table II.  Friedmann's results and values for each of the three measurements for perceived physical state (PEPS) and the two EZ-scales psychological strain and motivational state and each of their subdimensions.

Group	PRE	POST	POST 1HR	ANOVA results (N = 11, FG = 2)
Perceived Physical State
Centrifuge	3.50 ∀ 1.55	3.03 ∀ 1.43	3.66 ∀ 0.85	χ^2 = 2.28, p = 0.32
Control	3.48 ∀ 0.74	3.32 ∀ 0.92	3.32 ∀ 0.69	χ^2 = 1.81, p = 0.40
Perceived physical energy
Centrifuge	3.69 ∀ 1.36	2.98 ∀ 1.25	3.39 ∀ 1.25	χ^2 = 3.35, p = 0.19
Control	3.73 ∀ 0.99	3.45 ∀ 1.19	3.39 ∀ 1.07	χ^2 = 3.15, p = 0.21
Perceived physical fitness
Centrifuge	3.14 ∀ 1.07	3.00 ∀ 1.52	3.57 ∀ 0.85	χ^2 = 3.32, p = 0.19
Control	3.27 ∀ 0.81	3.17 ∀ 0.91	3.16 ∀ 0.75	χ^2 = 0.40, p = 0.82
Perceived physical flexibility
Centrifuge	3.33 ∀ 1.18	2.97 ∀ 1.73	3.70 ∀ 0.77	χ^2 = 2.25, p = 0.32
Control	2.98 ∀ 0.96	2.86 ∀ 0.91	2.81 ∀ 0.69	χ^2 = 0.16, p = 0.92
Perceived physical health
Centrifuge*	3.86 ∀ 0.91	3.12 ∀ 1.69*	3.97 ∀ 0.80	χ^2 = 6.68, p < 0.05*
Control	3.93 ∀ 0.81	3.77 ∀ 1.02	3.93 ∀ 0.89	χ^2 = 1.87, p = 0.39
Perceived Psychological Strain
Centrifuge	3.38 ∀ 1.24	3.02 ∀ 1.49	3.67 ∀ 0.91	χ^2 = 4.90, p = 0.09
Control	3.69 ∀ 0.74	3.59 ∀ 0.65	3.62 ∀ 0.60	χ^2 = 0.15, p = 0.93
Calmness
Centrifuge*	3.36 ∀ 1.36	3.41 ∀ 1.76	4.22 ∀ 0.72*	χ^2 = 7.31, p < 0.05*
Control	3.91 ∀ 0.66	3.91 ∀ 0.74	3.95 ∀ 0.42	χ^2 = 0.31, p = 0.86
Positive mood
Centrifuge	3.77 ∀ 1.40	3.32 ∀ 1.62	3.50 ∀ 1.18	χ^2 = 2.55, p = 0.28
Control	4.00 ∀ 0.89	4.00 ∀ 0.89	3.95 ∀ 0.96	χ^2 = 0.00, p = 1.00
Recovery
Centrifuge	3.09 ∀ 1.56	2.32 ∀ 1.69	3.68 ∀ 1.21	χ^2 = 2.89, p = 0.24
Control	3.59 ∀ 1.14	3.32 ∀ 1.10	3.14 ∀ 1.31	χ^2 = 1.63, p = 0.44
Relaxation
Centrifuge	3.27 ∀ 1.57	3.05 ∀ 1.62	3.27 ∀ 1.10	χ^2 = 2.88, p = 0.24
Control	3.27 ∀ 0.96	3.14 ∀ 0.74	3.45 ∀ 0.69	χ^2 = 1.40, p = 0.50
Perceived Motivational State
Centrifuge*	3.10 ∀ 1.10	2.80 ∀ 1.27	3.42 ∀ 0.70^††	χ^2 = 7.32, p < 0.05*
Control	3.44 ∀ 0.58	3.40 ∀ 0.73	3.48 ∀ 0.63	χ^2 = 1.11, p = 0.57
Social acceptance
Centrifuge	3.00 ∀ 0.87	2.95 ∀ 1.01	3.27 ∀ 0.68	χ^2 = 3.89, p = 0.14
Control	3.73 ∀ 0.68	3.45 ∀ 0.76	3.68 ∀ 0.81	χ^2 = 4.45, p = 0.11
Willingness to seek contact
Centrifuge	3.32 ∀ 1.29	2.77 ∀ 1.51	3.27 ∀ 1.00	χ^2 = 4.42, p = 0.11
Control	3.73 ∀ 0.75	3.73 ∀ 0.90	3.77 ∀ 0.88	χ^2 = 0.33, p = 0.85
Self-confidence
Centrifuge*	3.00 ∀ 1.30	2.81 ∀ 1.35	3.50 ∀ 0.74	χ^2 = 6.06, p < 0.05*
Control	3.09 ∀ 0.38	3.27 ∀ 0.61	3.36 ∀ 0.64	χ^2 = 4.32, p = 0.12
Readiness to strain
Centrifuge**	3.09 ∀ 1.24	2.64 ∀ 1.47	3.64 ∀ 0.90^††	χ^2 = 10.76, p < 0.01**
Control	3.23 ∀ 0.90	3.14 ∀ 0.92	3.09 ∀ 0.86	χ^2 = 0.00, p = 1.00

Displayed are dimensions, PRE, POST, POST 1HR and Friedmann's ANOVA results below for each group (centrifuge and control). Values are means ± ( ∀ ) standard deviations (SD). Asterisks indicate significant difference when compared with PRE: *p < 0.05. Crosses indicate changes to measurement POST: ^††p < 0.01.

EZ-Scale (MoodMeter^®, module Feelfinder)

Whereas significant changes were observed within the Perceived Motivational State in the POST 1HR measurement (p < 0.01; Figure 2b) the Perceived Psychological Strain missed significance (p = 0.09; Table II). Within the subdimensions, an increase of calmness (p < 0.05; Figure 3b) and the readiness to strain (p < 0.01; Figure 3c) was seen POST 1HR. Although self-confidence (Figure 3d) showed significant changes in the ANOVA (p < 0.05), Wilcoxon post-hoc test revealed no significant changes (Table II). Except for calmness, all subdimensions again were decreased in the POST measurement and returned to baseline after approximately 1 h post-acceleration. Neither of the dimensions showed any changes within the control group.

Heart rate

Heart rate was recorded for 3 min immediately before the centrifuge was started (PRE), in the middle of the 15 min of 3G_z (IN) and 4 min after deceleration (POST). Bonferroni post-hoc test (ANOVA results: F_(2,14) = 27.186, p < 0.001) revealed significant increases (PRE: 53.36 ± 8.51 beats per min; IN: 83.95 ± 21.92 beats per min; p < 0.001) for the IN measurement followed by a significant decrease (POST: 58.00 ± 14.19 beats per min; p < 0.001) in the POST measurement.

Discussion

Previous observations have led to the hypothesis that the initial contact with changed gravity levels influences motor performance, but with a prolonged exposure to hypergravity, participants are able to adapt to this altered environment assuming general principles of plasticity within the central nervous system (Gobel et al. 2006). Similar results have been reported for microgravity (Manzey et al. 1998; Hermsdörfer et al. 1999). Although it is known that changed gravity conditions might lead to increase of stress hormone secretion (Schneider et al. 2007), to date it has remained unclear whether CPNP under increased gravity conditions are evoked by changes of gravity level itself or by an increase of arousal.

We therefore attempted to record changes in stress hormone concentration before and after 15 min of acceleration to 3G_z as well as changes in PEPS, motivational state and psychological strain.

Hormones

Increased plasma concentrations of cortisol and ACTH in the POST measurement indicate that the experience of the 3G_z acceleration was stressful. It is likely that the experience of 3G_z leads to an activation of the corticotropin-releasing hormone neurons in the hypothalamus, and a consequent release of ACTH, and hence the release of cortisol from the adrenal glands. Whereas the declining drift of the plasma cortisol values in the control group may reflect a circadian trend, the values of the group experiencing centrifugation nearly doubled after 3G_z for cortisol and more then tripled for ACTH.

Similar to cortisol, epinephrine and norepinephrine are well known to respond rapidly to acute stress. Within this study, both values were found to be substantially increased after 15 min of acceleration to 3G_z.

In the centrifugal group, prolactin concentrations were elevated after experiencing 3G_z, consistent with prolactin secretion in response to stress as found by others (Chrousos and Gold 1992; Udelsman and Holbrook 1994; Farrace et al. 1996). The increase in heart rate during 3G_z indicates increased physical cardiovascular load, though increased aerobic workload per se is unlikely to have stimulated the increased secretion of cortisol, prolactin or epinephrine, but norepinephrine release has been reported to be associated with aerobic workload (Odink et al. 1986; Rojas Vega et al. 2006; Acevedo et al. 2007). However, the increase in heart rate, which has been shown before during phases of hypergravity (Bjurstedt et al. 1974), may not due to the increased workload of the cardiovascular system under 3G_z but instead may be a consequence of the release of epinephrine and norepinephrine. Both catecholamines are well known to be correlated to physiological and psychological reactions expressed by changes in heart rate, blood pressure, sweat production, voice pitch and skin conductivity (Wolf et al. 2001; Schommer et al. 2003).

Perceived physical state (MoodMeter^®, module Bodyfinder)

Although there seemed to be an overall decrease of PEPS immediately after exposure to 3G_z (Figure 2), only perceived physical well-being reached significance, with a decrease in the POST measurement followed by an increase back to baseline in the POST 1HR measurement. This probably reflects the physical burden of the 15 min of acceleration and goes along with the experience of tiredness and debility of participants after the deceleration. POST 1HR values of perceived physical well-being show a recovery, which suggests that the physical burden of the 3G_z phase is temporary. Neither of the other subdimensions in the PEPS showed significant changes. This might indicate that the chosen adjectives for describing theses dimensions were not adequate, e.g. perceived flexibility and fitness might be more correlated with physical strain evolving out of exercise related burden. Nevertheless, it seems worth mentioning that all of the PEPS subdimensions showed similar characteristics.

EZ-Scale (MoodMeter^®, module Feelfinder)

Perceived Motivational State seems to be impaired POST but to be improved POST 1HR to acceleration. This is important for earlier findings in so far that previously reported deficits in CPNP during phases of hypergravity are not dependent on lack of motivation. That both self-confidence and readiness to strain are elevated POST 1HR might reflect satisfaction in successfully managing the whole procedure and is consistent with the increase in perceived physical well-being. This also suggests that the physical burden of the 3G_z acceleration is temporary and only briefly affects motivation of participants. That supports earlier findings by Manzey et al. (1998) showing that an increased emotional load could be linked to a decrease in CPNP. This has been reported especially within the post-transition phases of long-term missions (i.e. after entry to weightlessness and re-entry >to 1G), which are associated with drastic physiological and psychological changes. Within the Psychological Strain, which missed significance only marginal, the increase in calmness POST 1HR might result from a decrease of worries before and after the 3G_z acceleration. In the EZ-scale, too, a rising number of adjectives presenting one subdimension might lead to an increase of reliability for each subdimension.

Overall discussion

There appears to be a close connection between concentrations of stress hormones and perceived physical and mental states, as both sets of parameters show changes after 15 min of acceleration to 3G_z. This shows that physiological changes are displayed by participants, which is important for our general hypothesis that psycho-physiological reactions to 3G_z might play a role in previously observed impairments in CPNP, although this link was not directly addressed in the present study. Increased arousal is known to affect central processes (Jones and Hardy 1988; Simpson et al. 2001a,b) as well as peripheral processes, e.g. an epinephrine-induced tremor, which might affect fine motor activities in particular (Brunton et al. 2006).

In addition, facilitating effects of increased hormone concentrations also have to be taken into account. Previous studies have pointed to an influence of stress on memory consolidation, which is also achieved by the intravenous administration of norepinephrine (Cahill and Alkire 2003) or cortisol (Buchanan and Lovallo 2001). In addition, the impact of increased blood cortisol concentration on cognitive task performance has previously been demonstrated by Wolf et al. (2001). These authors concluded that catecholamines and cortisol (exogenously applied) modulate memory performance such that increased concentrations of these hormones result in increased memory consolidation. Kleinert (2003) demonstrated that induction of slight fear of injury increases motor performance due to an increase in the attentional processes. Especially, when comparing CPNP of hyperG-trained participants, and participants who are exposed to hypergravity for the first time, this may play a major role (Guardiera et al. 2007).

In the present study, a decrease in Perceived Physical Well-being was observed in the after experiencing hypergravity. Although there were no signs of motion sickness, in previous work this has been reported to be related to an increase of stress hormone secretion (Schneider et al. 2007). It could be that the increased circulating concentrations of cortisol, prolactin, ACTH, epinephrine and norepinephrine are a precursor to ongoing sickness, provoked by stimulating of the vestibular system. This would support earlier findings by Takeda et al. (1990) showing an increased release of epinephrine and norepinephrine after stimulating the vestibular system by axial rotations.

With respect to the present study, further research is necessary to clearly distinguish between the primary effects of hypergravity and the secondary, stress related effects, particularly as they seemed to be related to psychomotor functions. Furthermore, it is essential for future studies to differentiate between hyper G-experienced and non-experienced participants.

Acknowledgements

We would like to thank the analytical team of the Institute of Cardiology and Sports Medicine, Anke Manderfeld and Astrid Hofrichter for their outstanding experience and help. Also, thanks to all our participants. A special thanks goes to Prof. Dr Gerzer, Dr Samel, N. Luks and H. Friedrich from the Institute of Aerospace Medicine at the DLR in Köln, for their support in centrifuge operations and Dr G. Kluge from the same institute for his medical supervision. Thanks to V. Brümmer and A. Noppe for their help during data acquisition. This study was made possible by a grant of the German Space Agency (DLR) 50WB0519 and a young investigator grant dedicated to S. Schneider and S. Guardiera by the German Sport University. A last thanks goes to two unknown referees for significantly improving the quality of this manuscript by valuable advice.