Change of voice characteristics during +3 Gz acceleration

BACKGROUND: This study was performed to test the feasibility of an experimental approach for assessing voice changes during exposure to increased +Gz acceleration. Such changes are probably due to mechanical alterations of the structures involved in voice production. This may be relevant to automatic speech analysis for flight control. Because voice control by means of auditory feedback may compensate for acceleration effects, the investigations included conditions with masked auditory feedback. METHOD: Four male subjects read standard speech test material while seated in a human centrifuge both during sustained +3 Gz acceleration and in a reference condition. Both test runs were repeated with masking of the auditory feedback by a white noise presented via headphones. Microphone and acceleration signals were recorded on a PC-based dynamic signal acquisition board. Long-time average spectra (LTAS), fundamental frequency (F0), and the frequency of the first, second, and third formant (F1, F2, F3) of the vowels /a/, /o/, and /i/ were extracted from the microphone signal for the different conditions. RESULTS: LTAS clearly differed between the masking conditions, but not between reference and +3 Gz conditions. F0 clearly rose with auditory masking and showed a small increase under +3 Gz acceleration. Several effects of +3 Gz acceleration on formant frequencies were found, all of rather small magnitude. Increased acceleration lowered F2 for the vowel /i/. A decrease of F3 was observed for vowels /i/ and /o/. CONCLUSION: This pilot study has shown the feasibility of an experimental approach to assessing voice changes during exposure to increased +Gz acceleration. Exposure to +3 Gz showed small effects on FO and several formant frequencies. A definitive forthcoming study should assess the significance of these effects by investigating a greater number of subjects during exposure to acceleration higher than +3 Gz.     

Human tolerance to +Gz acceleration during heating]

Eighty-six measurements of individual tolerance of 25 healthy test subjects to +Gz accelerations were carried out at two body temperatures. At moderate heating (body temperature 37.5+/-0.2degreesC) acceleration tolerance decreased by 1.4 g (maximally by 3 g) without an antigravity suit and by 0.5 g with an antigravit suit. In case of simulated poor fitting of the suit the tolerance decrease was 1.1 g. At high heating (body temperature 38.0+/-0.2degreesC) the decrease was great (2.1 g). In some cases centrifugation was terminated because of poor tolerance to hyperthermia. The regression analysis suggests that at body temperature of 37.3degreesC the acceleration tolerance decreases in some individuals only and at 37.6degreesC and over it declines in most subjects. The correlation between the level of water losses (0.8--2.4% of body weight) and decline in +Gz tolerance was low (r=0.33) and insignificant. The resulting data allow practical recommendations in relation to acrobatic flights in hot climate.     

Rigid gas-permeable contact lens wear during + Gz acceleration

This study was designed to determine how well rigid gas-permeable contact lenses maintain position on the cornea under high G forces, and the effect on visual acuity. One hyperopic and five myopic subjects were fitted with lenses made from Pasifocon C material (specific gravity = 1.07). Two lens diameters (8.8-9.4 mm and 9.6-10.0 mm) were compared upon the myopic subjects for centering on the cornea. Visual acuity was measured at + 1 Gz (baseline), +3 Gz, +4 Gz, +6 Gz, and +8 Gz in the straight-ahead, lateral, and up-gaze positions from three acuity charts mounted in the gondola. All lenses, as estimated from the videotape, decentered down the z axis 2-3 mm at high + Gz. Visual acuity was reduced at the higher +Gz levels to similar measurements in most subjects for both the contact lens and spectacle control rides. No lens displaced from the cornea or dislodged from the eye during any of the 25 centrifuge runs.     

A cardiovascular model for studying impairment of cerebral function during +Gz stress

A digital computer model of the human cardiovascular system has been developed which can be used for studying impairment of cerebral function during +Gz stress. The model includes simulation of the arterial and venous systems, the heart, baroreceptor control of heart rate, control of venous tone, and the effect of gravity. Model predictions suggest that, for unprotected subjects, carotid pressure at eye level decreases to 50 mm Hg (beginning of peripheral light loss) at approximately 2.7 Gz. The pressure decreases to 20 mm Hg (beginning of central light loss) at approximately 3.6 Gz. An anti-G suit provides an extra 1.1 to 1.5 Gz protection. Even though blood pressure supplying retinal vessels drops significantly at the above G levels, cerebral blood flow is maintained due to protective and compensatory mechanisms. These observations compare favorably with results reported in the literature. The results suggest that this model can be used to improve our understanding of the cardiovascular system's response to +Gz stress.     

Degradation of visual pursuit during sustained +3 Gz acceleration.

BACKGROUND: During positive acceleration, there is a diminished flow of blood to all regions above the heart. This is manifested by the commonly described loss of peripheral vision, greyout and blackout, which have been investigated extensively. The ability to select appropriate scanning patterns and to efficiently process visual information is one of the important determinants of scan effectiveness. This study investigates the performance of the smooth pursuit system under sustained +3 Gz before any signs of loss of vision. METHODS: Eleven subjects with no known oculomotor and vestibular anomalies participated in the study. Horizontal and vertical pursuit at amplitudes of 10 and 20 degrees were investigated in each of the subjects over 4 separate days. During each test session, pursuit targets of a predictable sine wave, oscillating at 0.2, 0.4, 0.8, 1.2 and 1.6 Hz were presented to the subjects in a random order. Horizontal and vertical eye movements were recorded using the El-Mar eye tracking system. The subjects were tested in 4 trials: 1) at 1 G before exposure to increased acceleration; 2) during sustained +3 Gz; 3) immediately after the +3 Gz exposure; and 4) 5 min after the +3 Gz exposure. RESULTS: Breakdown in smooth pursuit in response to horizontal and vertical sinusoidal stimuli during +3 Gz is indicated by a statistically significant decrease in gain and an increase in phase lag (p < 0.01). This is most obvious when the stimulus frequency is greater than 0.4 Hz. Qualitatively, the pursuit response during acceleration was ataxic and disorganized in appearance. CONCLUSION: It is postulated that degradation of pursuit gain and phase could be due to central hypoxia, and that the increase of G loading on the vestibular system could affect the neural integration of the pursuit signal in the vestibular nuclei with its direct output to the oculomotor system.     

Errors in measurement of +Gz acceleration tolerance

Most acceleration studies estimate a subject's G-level tolerance by taking only one determination (test) for a given condition. The purpose of this study was to examine the error structure and reliability of an individual's acceleration tolerance and to provide design considerations for future experimentation. A hierarchical (nested) design was used to estimate the sources of variation in measuring G-level tolerance. Six males rode relaxed in the USAF School of Aerospace Medicine human-use centrifuge and were exposed to a 0.1 G/s onset rate profile until greyout. Each subject was tested on three randomly selected days with three repeated determinations within a day. This design allowed for an estimate of both day-to-day and measurement error within a testing session. A single +Gz tolerance determination was found to be moderately unreliable (reliability coefficient = 0.74). Under the best of circumstances a subject's G-level tolerance cannot be estimated with any more accuracy than about +/- 0.3 G with 95% confidence. This degree of accuracy can only be obtained with multiple measurements.     

Isometric force production in experienced fighter pilots during +3 Gz centrifuge acceleration

INTRODUCTION: Previously, we have shown that na?ve subjects produce exaggerated isometric forces when exposed to increased acceleration (+Gz) for the first time. The present study investigates whether +G,-experienced PA-200 Tornado pilots show similar deficits. METHODS: Experiments were conducted in the stationary (+1 Gz) or rotating (+3 Gz) gondola of a human-rated centrifuge. With their dominant hand, seven pilots produced visually prescribed forces of specific direction and magnitude using an isometric joystick. In practice trials, subjects received continuous visual feedback about their performance, while in test trials they did not. RESULTS: Peak forces during test trials were significantly higher in +3 Gz than in +1 Gz, although this increase of about 25% referring to the +1 Gz value was somewhat smaller in pilots than in nonpilot controls (increase of about 36%). DISCUSSION: Since pilots' responses were exaggerated in +3 Gz, it seems that frequent exposure to varying +Gz levels is not sufficient for a profound adaptation of force-producing mechanisms to +3 Gz. In consequence, pilots' performance on isometric tasks could be compromised during flight maneuvers in +Gz.     

Vestibulo-ocular responses in man to +Gz hypergravity

The influence of high +Gz gravito-inertial force on the vestibular system in man was investigated in a 4-m centrifuge with a freely swinging gondola. The Gz profile was: acceleration +0.2 Gz/s, +3 Gz sustained for 3 min, deceleration -0.2 Gz/s. The subject was exposed to this profile under two conditions in randomized order: facing forward and facing backward. Under these conditions, the effective angular velocity in the plane of the vertical semicircular canals is opposed. Adding the slow phase velocity responses from these conditions yields the Gz effect only; subtracting yields the angular velocity effect only. Vertical vestibular nystagmus was analysed in five subjects. Results indicate that +3 Gz induced a subject-dependent vertical nystagmus with slow phase downwards. The average amplitude of this nystagmus reached a maximum of 27 degrees/s at 16 s from G onset, and was 11 degrees/s after 3 min of sustained +3 Gz. The vestibular stimulation by +Gz could result in false subjective perception of attitude, and play a major role in spatial disorientation in flight.     

Tolerance of +Gz acceleration in pilots with health problems

Fighter pilots with health abnormalities were examined for their tolerance to +Gz acceleration. It was found that pilots with psychoneurological disorders and cardiac arrhythmias showed a lower acceleration tolerance. Pilots with arterial hypertension displayed a markedly higher tolerance as compared to the average pilot population. Other health disturbances produced no effect on +Gz acceleration tolerance.     

Regional cerebral blood flow in conscious miniature swine during high sustained +Gz acceleration stress

The two major factors limiting performance during high +Gz acceleration stress are loss of vision, and loss of consciousness. These symptoms are believed to occur as a result of insufficient blood flow to the retina and brain. This study was conducted to determine the effects of +Gz stress on regional cerebral blood flow. Cerebral blood flow (CBF) was measured in 22 conscious, female, miniature swine with the radio-labeled microsphere technique. Acceleration exposures consisted of 60-S plateaus at +3Gz, +5Gz or +7Gz. Microsphere infusions were made before, during, 1.0-6.0 min after, and 10 min after +Gz. Blood flow to the retina was significantly decreased during exposure to +5Gz and ceased during exposure to +7Gz stress. Mean, resting control CBF was 34 +/- 4 ml/min/100 g. Exposure to +3Gz and +5Gz had no significant effect on CBF. Exposure to +7Gz appeared to cause a redistribution of CBF, with blood flow to the brain stem being preserved and flow to the cerebrum being diminished.     

The application of positive pressure breathing for improving +Gz acceleration tolerance

Investigations on +Gz acceleration tolerance were carried out in pilots using various values of positive pressure breathing (PPB) during centrifugation. The greatest improvement of +Gz tolerance--2.2 +Gz--was achieved while applying PPB=45 mm Hg and using a counterpressure suit. PPB prolonged the time at +5Gz from 2 min 35 s under control conditions to 6 min 53 s at PPB=45 mm Hg. The author discusses the mechanism of the increase in acceleration tolerance at PPB, stressing its protective effect on the circulatory system.     

Heart biochemical responses in miniature swine subjected to +Gz acceleration.

Myocardial biochemical systems which are sensitive to hypoxic and ischemic insult were studied to determine the possible etiology of ventricular endocardial hemorrhage in miniature swine following +GZ stress. Unanesthetized animals were subjected to a single, 120-s +9 GZ acceleration. Approximately 1-2 h following +GZ exposure, the animals were anesthetized and the hearts removed for analyses. Acceleration exposure resulted in the loss of acid phosphatase enzyme activity from the membrane-bound lysosomal fraction with concomitant increased activity in the soluble fraction. This suggests that lysosomal membrane integrity had been disrupted. Mitochondrial preparations from +GZ-stressed hearts exhibited marked increases in active respiratory rate and rate of calcium transport while oxidative phosphorylation efficiency was unchanged. The results clearly indicate that +GZ acceleration is capable of altering myocardial biochemical systems. However, the results tend to suggest that these alterations in cellular processes may be mediated by influences other than hypoxia or ischemia.     

Short-arm (1.9 m) +2.2 Gz acceleration: isotonic exercise load-O2 uptake relationship

BACKGROUND: The deconditioning syndrome from prolonged bed rest (BR) or spaceflight includes decreases in maximal oxygen uptake (VO2max), muscular strength and endurance, and orthostatic tolerance. In addition to exercise training as a countermeasure, +Gz (head-to-foot) acceleration training on 1.8-2.0 m centrifuges can ameliorate the orthostatic and acceleration intolerances induced by BR and immersion deconditioning. PURPOSE: Study A was designed to determine the magnitude and linearity of the heart rate (HR) response to human-powered centrifuge (HPC) acceleration with supine exercise vs. passive (no exercise) acceleration. Study B was designed to test the hypothesis that moderate +Gz acceleration during exercise will not affect the respective normal linear relationships between exercise load and VO2max, HR, and pulmonary ventilation (VEBTPS). Study C: To determine if these physiological responses from the HPC runs (exercise + on-platform acceleration) will be similar to those from the exercise + off-platform acceleration responses. METHODS: In Study A, four men and two women (31-62 yr) were tested supine during exercise + acceleration and only passive acceleration at 100% [maximal acceleration (rpm) = Amax] and at 25%, 50%, and 75% of Amax. In Studies B and C, seven men (33+/-SD 7 yr) exercised supine on the HPC that has two opposing on-platform exercise stations. A VO2max test and submaximal exercise runs occurred under three conditions: (EX) exercise (on-platform cycle at 42%, 61%, 89% and 100% VO2max) with no acceleration; (HPC) exercise + acceleration via the chain drive at 25%,50%, and 100% Gzmax (35%, 72% and 100% VO2max); and (EXA) exercise (on-platform cycle at 42%, 61%, 89%, and 100% VO2max) with acceleration performed via the off-platform cycle operator at +2.2+/-0.2 Gz [50% of max (rpm) G]. RESULTS: Study A: Mean (+/-SE) Amax was 43.7+/-1.3 rpm (mean = +3.9+/-0.2, range = 3.3 to 4.9 Gz). Amax run time for exercise +acceleration was 50-70 s, and 40-70 s for passive acceleration. Regression of X HR on Gz levels indicated explained variances (r2) of 0.88 (exercise) and 0.96 (passive). The mean exercise HR of 107+/-4 (25%), to 189+/-13 (100%) bpm were 43-50 bpm higher (p < 0.05) than comparable passive HR of 64+/-2 to 142+/-22 bpm, respectively. Study B: There were no significant differences in VO2, HR or VEBTPS at the submaximal or maximal levels between the EX and EXA runs. Mean (+/-SE) VO2max for EX was 2.86+/-0.12 L x min(-1)(35+/-2 ml x min(-1) x kg(-1)) and for EXA was 3.09+/-0.14 L x min(-1) (37+/-2 ml-min(-1) x kg(-1)). Study C: There were no significant differences in the essentially linear relationships between the HPC and EXA data for VO2 (p = 0.45), HR (p < 0.08), VEBTPS (p = 0.28), or the RE (p = 0.15) when the exercise load was % VO2max. CONCLUSION: Addition of + 2.2 Gz acceleration does not significantly influence levels of oxygen uptake, heart rate, or pulmonary ventilation during submaximal or maximal cycle ergometer leg exercise on a short-arm centrifuge.     

Transfer functions for arterial oxygen saturation during +Gz stress.

Discrete, finite, Fourier transforms of input +Gz stress and output arterial oxygen saturation (Sao2) were used to obtain transfer functions between G and Sao2 in 12 subjects. Ensemble averaging of transfer functions based on responses to variable G stress provided reasonable capability for predicting Sao2 responses to sustained 6-G stress. Prediction of responses to variable G stress from transfer functions based on responses to sustained G stress was unsuccessful, however. A synthetic transfer function with physiologic explicability and substantial predictive ability was obtained from an impulse response fabricated from simple mathematical functions.     

＋Gz作用下兔颅内压的改变

采用侧脑室插管法记录颅内压的技术，观察不同Ｇ值水平及增长率的＋Ｇｚ作用下兔颅内压变化。结果表明：（１）在增长率和峰值作用时间恒定量，麻醉兔在＋３、＋５及＋７Ｇｚ作用下，颅内压降低。随Ｇ值增加，颅内压负值亦随之增大，呈显著负相关。     

Effect of physical training in cool and hot environments on +Gz acceleration tolerance in women

Rectal temperature (Tre), sweat rate, plasma volume (PV), peak oxygen uptake (peak VO2), and relaxed +Gz acceleration tolerance (0.5 G X min-1 linear to grayout) were measured in 15 healthy women 21-41 years old before and after submaximal isotonic exercise training for 2 h X d-1 on a cycle ergometer. The women had 2 weeks of acceleration runs and Vo2 testing, followed by 8 d of exercise training, post-training acceleration runs on day 9, and peak Vo2 tests on day 10. They were divided into three groups: an exercise (heat) group, ambient temperature (Ta) 40.6 degrees C, relative humidity (rh) 42%, and a peak Vo2 of 52%; an exercise (cool) group, Ta = 18.7 degrees, rh = 48%, and Vo2 peak = 55%; and a sedentary control (cool) group. There was no change in peak ventilation, peak heart rate (HR), peak Vo2, or in resting PV in any group after training. Heart rate and Tre were significantly lower after training in both cool and hot environments; HR by 17 b X min-1 (p less than 0.05) and 27 b X min-1 (p less than 0.05), respectively, and Tre by 0.4 degrees C (p less than 0.05) and 0.4 degrees C (p less than 0.05), respectively. Sweat rates were not different in any group. In all groups, acceleration tolerances were not different after training; they ranged from 3.5 to 3.8 G (373 - 410 s). The loss (shift) in PV during acceleration ranged from -5.8% to -10.3% (nonsignificant).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of exercise-heat acclimation on fluid, electrolyte, and endocrine responses during tilt and +Gz acceleration in women and men

Plasma fluid, electrolyte, protein, renin, and vasoactive hormone (epinephrine, norepinephrine, vasopressin) responses were measured in six women (21-23 yr) and four men (21-38 yr) before and immediately following an orthostatic tolerance test (70 degrees head-up tilt) and a +Gz (head-to-foot) acceleration tolerance test (0.5 G X min-1 linear ramp to grayout). These tests were conducted before and after 12 consecutive days of exercise-heat acclimation when the subjects exercised on a cycle ergometer at a relative oxygen uptake of 44% to 49% peak oxygen uptake in a hot environment (Ta = 40 degrees C, 42% rh). During acclimation plasma volume increased by 10.6% (p less than 0.05) in the women and by 11.9% (p less than 0.05) in the men; in both groups exercise heart rate decreased significantly. After acclimation, acceleration tolerance was unchanged in both groups (range 3.1 to 3.4 G); the women's tilt tolerance was unchanged (range 33.6 to 39.5 min), but the men's tilt tolerance increased from 30.4 min before to 58.3 min (delta = 91%, p less than 0.05) after acclimation. Since the pattern of fluid, electrolyte, and protein shifts and acceleration tolerances in the women and men were virtually the same, the hormone responses were highly variable, and the men's tilt tolerance increased significantly after acclimation, it is clear that responses to tilting cannot be used to predict responses to acceleration. Analysis of data from the present study and the literature suggests that current exercise training regimes should be unrestricted for astronauts who have not previously been highly endurance trained.(ABSTRACT TRUNCATED AT 250 WORDS)     

＋Cz作用下兔心脏功能的改变

Changes of cardiac function was studied during +Gz exposure in rabbits. Seven New Zealand rabbits were anesthetized and exposed to +2, +4, and +6 Gz (each for 30s, onset rate 1G/s, with 15 min interval in between). The left ventricular systolic pressure (LVSP), peak rate of left ventricular pressure rising (+dp/dtmax) and ECG were recorded. The results showed that during +2, +4 and +6 Gz exposures, LVSP decreased by 62.96% (P<0.01), 63.34% (P<0.01) and 82.01% (P<0.01) respectively, and +dp/dtmax decreased by 58.46% (P<0.01), 53.59% (P<0.01) and 63.06% (P<0.01) respectively, and the left ventricular end diastolic pressure decreased by 67.78 % (P>0.05), 332.74 % (P<0.01) and 500.54% (P<0.01) respectively, as compared with those of control. It is suggested that cardiac function is depressed with +Gz exposure.