Document Sample
                                       D. Wirth(1), N. Rohleder(2), H. Welsch(3)
        Dresden University of Technology, Department of Psychology, Biopsychology, Zellescher Weg 17, D-01069
                              Dresden, Germany, Email:
     Dresden University of Technology, Department Psychology, Biopsychology, Zellescher Weg 17, D-01069 Dresden,
                              Germany, Email:
       German Air Force Institute of Aviation Medicine, Division Aviation Physiology, SteinbornerStraße 43, D-01936
                                Königsbrück, Germany,

                                                              individuals who are able to mount a large stress response
1.          ABSTRACT
                                                              are better protected against loss of blood pressure during
Prediction of student pilots’ +Gz tolerance by stress         acceleration and thus could be better capable of tolerating
hormone levels would be a useful tool in aviation             a high +Gz load.
medicine. The aim of the present study was to analyze the     Based on this assumption, the aim of the present study
relationship between neuroendocrine parameters with           was to search for associations between HPA axis and
performance during acceleration training on the human         SAM system parameters at rest and in response to
centrifuge (HC).                                              acceleration with performance parameters of student
We investigated 21 student pilots during self-controlled      pilots on the human centrifuge.
acceleration training on the HC. Adrenocorticotropic
                                                              3.     METHODS
hormone       (ACTH),      cortisol,   epinephrine,     and
norepinephrine were measured after individual training        3.1. Subjects and Experimental Protocol
sessions and at rest. Performance was defined by several      Subjects were 21 student pilots (22.6 yrs ± 1.8 SEM;
characteristics including maximum tolerated acceleration.     BMI 23.1 kg/m2 ± 0.4 SEM) enrolled in training on high-
ACTH and cortisol, were significantly higher 20 minutes       performance jet aircraft. The study protocol was
after acceleration training compared to the resting           approved by the local ethics committee and written
condition. Subjects tolerated a maximal acceleration of       informed consent was obtained from all subjects.
+6.69 Gz. HPA hormone levels and responses were               Acceleration training took place on the human centrifuge
associated with maximum tolerated acceleration +Gz.           of the German Air Force Institute of Aviation Medicine,
These findings support the expectation that acceleration-     Königsbrück, Germany. After a “warm-up” session, the
induced increases in stress hormones may enable the           main acceleration training started. Student pilots were
organism to tolerate a higher acceleration and could          instructed to reach their personal maximum +Gz tolerance
therefore be used as predictors for acceleration tolerance.   by self-controlled acceleration. The goal was to achieve
2.          INTRODUCTION                                      +7 Gz and to endure it for at least 15 seconds. Blood
                                                              samples were drawn 20 minutes after cessation of the
The prediction of individual +Gz tolerance would be of        training into EDTA tubes. Resting samples were taken
great use in physiological assessment of student pilots.      three days after the training between 07:00 and 07:30h.
However, the predictive value of single anthropometric
                                                              3.2. Endocrine and Performance Parameters
and physiological variables has been shown to be low [1],
                                                              Plasma concentrations of stress hormones were
while the combination of physiological, respiratory and
                                                              determined using commercial immunoassays (ACTH:
muscular parameters proved useful in one study [2].
                                                              DPC Biermann, Bad Nauheim; cortisol: Bayer Vital,
Since +Gz tolerance largely depends on the maintenance
                                                              Fernwald; epinephrine and norepinephrine: IBL-
of blood flow to the central nervous system [3], it could
                                                              Hamburg, all Germany). Several performance parameters
be assumed that activation of physiological stress
                                                              were recorded during each training session. (1) Mean
systems, such as the sympatho-adrenal-medullary (SAM)
                                                              segment duration (of training segments and breaks), (2)
system and the hypothalamus-pituitary-adrenal (HPA)
                                                              integrated +Gz during all acceleration periods and mean
axis should be positively associated with +Gz tolerance
                                                              +Gz, (3) maximum acceleration +Gz and time on
and their assessment could prove useful in the prediction
                                                              maximum acceleration.
of +Gz tolerance.
In fact, acceleration in a human centrifuge leads to          3.3. Statistical analyses
activation of the SAM system with release of epinephrine      Analyses of variance for repeated measures were
and norepinephrine, and of the HPA axis with release of       calculated to test for differences between resting and post
cortisol (e.g. [5]). Since the activation of these stress     training levels of stress hormones. Pearson correlations
systems enables the organism to better endure adverse         were calculated to test for associations between hormone
environmental conditions, it could be assumed that those      levels and performance parameters.
4.     RESULTS                                                      the time spent on maximum +Gz (r=-.40*) and the
                                                                    maximum +Gz itself (r=-.44*). The difference between
4.1. Endocrine parameters                                           resting and post training cortisol was positively
Twenty minutes post acceleration, ACTH levels in                    associated with the time spent on maximum acceleration
plasma were significantly higher compared to those                  (r=.39*; see table 1 for all correlations). No associations
measured at rest (F1,206.67; p=0.02). Cortisol differences          of SAM hormones and performance parameters were
failed significance in ANOVA (F1,20=2.63; p=0.12), but              found (data not shown).
were significant in Wilcoxon Test: Z=-2.1; p<.05; Fig. 1)
                                                                    5.     DISCUSSION
                                                                    We set out to test whether stress hormone levels are
                                                                    useful predictors of student pilots’ performance during
                                                                    acceleration training on the human centrifuge. We found
                                                                    increased levels of the HPA axis hormones ACTH and
                                                                    cortisol twenty minutes after acceleration training
                                                                    compared to a resting condition. The SAM hormones
                                                                    epinephrine and norepinephrine, however, were not
                                                                    increased after acceleration training. This was most likely
                                                                    caused by the fact that the blood samples were taken too
                                                                    late after training to capture the peak response.
                                                                    Performance characteristics and HPA axis hormones
                                                                    were correlated in a way that higher resting levels and
Fig. 1: The HPA axis hormones ACTH and cortisol at rest and         higher increases of ACTH and cortisol were associated
20 minutes after +Gz acceleration.                                  with higher performance during training, i.e. higher
                                                                    maximum acceleration tolerated, and longer toleration of
Epinephrine and norepinephrine levels did not differ                maximum acceleration. SAM hormones, i.e. epinephrine
significantly between resting and post-acceleration                 and norepinephrine, were not associated with
conditions (all F<1; data not shown).                               performance parameters.
4.2. Performance characteristics                                    The results obtained in this study support the hypothesis
Mean duration of the complete training session was 9                that the ability to activate the stress systems in a
min. 4:51 minutes were spent on accelerations >1.4Gz,               sufficient way enables the organism to better tolerate the
total duration of breaks was 3:27 min. Individual                   high +Gz acceleration forces. HPA axis hormones could
segments lasted 56 s (range: 21 to 95 s). A mean of 5.67            therefore be seen as predictors of acceleration tolerance
(±2.18SD) acceleration periods were completed, during               given that these preliminary results are replicated in
which a mean acceleration of +3.91 Gz (±0.46SD) was                 future studies.
reached. The maximum acceleration ranged from +4Gz to
+7Gz (mean: +6.69 Gz; ±0.68SD) and was tolerated for an             6.     REFERENCES
average of 15.7 s (range 2 to 25s).                                 1. Webb J.T., et al. Unpredictability of fighter pilot G
4.3. Associations of hormones and performance                       tolerance using anthropometric and physiologic variables,
Resting ACTH levels were positively associated with the             Aviat Space Environ Med, Vol. 62, 128-35, 1991.
maximum acceleration +Gz (r=.49*) and inversely related             2. Burton R.R. Mathematical models for predicting G-
with mean segment duration (r=-.55**). Post training                level tolerances, Aviat Space Environ Med, Vol. 71, 506-
ACTH levels and the difference between resting and post             13, 2000.
training levels were positively associated with integrated          3. Eiken O., et al. The effect of straining maneuvers on
acceleration +Gz (r=.45*, and r=.47*, resp.).                       G-protection during assisted pressure breathing, Aviat
                                                                    Space Environ Med, Vol. 74, 822-6, 2003.
Table 1: Pearson correlation coefficients between HPA axis          4. Leverett S.D., Jr., Burton R.R. Physiological effect of
hormones and performance parameters.                                high, sustained +Gz forces on man, Life Sci Space Res,
                           mean      time on                        Vol. 17, 171-85, 1979.
                          segment      max.     max.     Integral
  Endocrine parameters    duration     +Gz      +Gz         Gz
                                                                    5. Obminski Z., et al. Effect of acceleration stress on
                  at rest r=-.55**   r=.30‡    r=.49*     r=-.05    salivary cortisol and plasma cortisol and testosterone
ACTH                post                                            levels in cadet pilots, J Physiol Pharmacol, Vol. 48, 193-
                            r=-.03   r=.31‡     r=.07    r=.45*
(pmol/l)    acceleration                                            200, 1997.
              difference     r=.18    r=.20    r=-.12    r=.47*
                  at rest   r=-.14   r=-.40*    r=.14    r=-.19     7.     ACKNOWLEDGEMENTS
Cortisol            post
(nmol/l)                   r=-.36‡    r=.22    r=-.45*   r=-.23     We would like to thank Prof. (em.) Dr. med. habil. P.
              difference     r=.04   r=.39*    r=-.05    r=.32‡     Dettmar, S. Hommel, Dr. med. R. Kowoll, Dr. rer. med..
** p>.01; * p<.05; ‡ p<.10                                          M. Menschikowski, Dipl. med. R. Mörlin, Dr. med. M.
                                                                    Nehring, and K. Wähnert for their valuable technical and
Resting levels of cortisol were negatively associated with          organizational help.