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Background: During a simulated coordinated turn in a gondola cen-trifuge, the perceived roll-tilt, quantified as the subjective visual hori-zontal (SVH), may differ tenfold between individuals. One aim of this study was to discern whether this variability reflects real individual char-acteristics or is due to noise or day-to-day variation. We also wanted to establish whether there are any habituation or learning effects of the centrifuge test. Methods: In nine nonpilots (NP) and nine student pilots (SP), with a flight experience of 150 h, the SVH was measured using an adjustable luminous line in darkness. At two test occasions (TI, T2) (interval 5-14 d) subjects underwent two runs (RI, R2; acceleration to 2 ? in 10 s, gondola inclination 60°, 5 min at 2 C, deceleration to 1 gin 10 s, interval between runs 5 min) in a centrifuge (r = 9.1 m). Initial and final SVH was determined for each individual run. Results: Acceleration of the centrifuge induced a tilt of the SVH. AtTI R1, this SVH tilt was, in NP, initially 24 ± 18° and finally 8 ± 10°. The corresponding values for SP were 28 ± 18° and 31 ± 33°. The SVH tilt was slightly larger at R2 than at R1. There was no difference between T1 andT2. Reliability coef-ficients ranged between 0.86 and 0.98 for NP and between 0.78 and 0.99 for SP. Conclusion: The large interindividual variability combined with a very high reproducibility suggests the existence of persistent individual characteristics in the perception of complex vestibular stimuli. Habitua-tion or learning effects of gondola centrifugation appears to be small. [ABSTRACT FROM AUTHOR]

Background: Quantitative data on spatial orientation would be valu-able not only in assessing the fidelity of flight simulators, but also in evaluation of spatial orientation training. In this study a manual indicator was used for recording the subjective horizontal plane during simulated flight. Methods: In a six-degrees-of-freedom hexapod hydraulic motion plat-form simulator, simulating an F-16 aircraft, seven fixed-wing student pilots were passively exposed to two flight sequences. The first consisted in a number of coordinated turns with visual contact with the landscape below. The visually presented roll tilt was up to a maximum 67°. The second was a takeoff with a cabin pitch up of 10°, whereupon external visual references were lost. The subjects continuously indicated, with the left hand on an adjustable plate, what they perceived as horizontal in roll and pitch. There were two test occasions separated by a 3-d course on spatial disorientation. Results: Responses to changes in simu-lated roll were, in general, instantaneous. The indicated roll tilt was ap-proximately 30% of the visually presented roll. There was a considerable interindividual variability. However, for the roll response there was a correlation between the two occasions. The amplitude of the response to the pitch up of the cabin was approximately 75%; the response decayed much more slowly than the stimulus. Discussion: With a manual indica-tor for recording the subjective horizontal plane, individual characteris-tics in the response to visual tilt stimuli may be detected, suggesting a potential for evaluation of simulation algorithms or training programs. [ABSTRACT FROM AUTHOR]

Introduction: +GZ exposure above the tolerance threshold typically induces a sequence of symptoms/signs, with loss of: peripheral vision, central vision (black out), and consciousness (G-LOC). The aims of this study were to investigate: 1) whether G history influences latent time to, or sequence of, symptoms/signs upon G exposures exceeding the toler-ance threshold; and 2) how pilots respond to a sudden loss of pressure in the anti-G garment (AGG) in flight-like scenarios. Methods: There were 14 subjects who were exposed to rapid onset rate +Gz-time pro-files, with plateaus 1 and 2 G above the relaxed tolerance level, without initial pressurization of the AGG (NoAGG) and when losing AGG pres-sure after 10 (AGGJO) and 120 (AGG_120) s at the plateau. Simulated target-chase flights during which AGG pressure was released were per-formed by seven pilots; the pilot was instructed to behave as during real flight. Results: Latent time to symptoms was shorter at +2 G than at +1 G, and shorter in AGG J 0 and AGG_120 than in NoAGG. In AGG_120, 43 and 64% of the subjects experienced serious symptoms (black out, Almost LOC, G-LOC) at +1 and +2 G, respectively, compared to 21 and 54% in AGGJO and 7 and 29% in NoAGG. The incidence of A-LOC/ G-LOC was higher in AGG_10 and especially in AGG_120 than in NoAGG. During the target chase, one pilot did not notice the pressure loss, one experienced G-LOC, and two A-LOC. Discussion:The risk of serious consequences of G exposure exceeding the tolerance level appears to be greater when G-garment failure occurs after a prolonged than after a brief exposure. [ABSTRACT FROM AUTHOR]

Background: During high +G¡, loads, pilots may experience arm pain. It is commonly assumed that such pain is caused by distension of blood vessels and that vascular distensibility adapts to the prevailing transmural pressure. The aim was to investigate whether vascular pressure habituation (PH) is as efficient in alleviating C-induced arm pain as using counterpressure/support garments. Methods: In Series I, 7 subjects underwent a 5-wk PH regimen, consisting of 15 40-min sessions, during which intravascular pressures in one arm were elevated by 65-105 mmHg. Before and after PH, arm pain was determined during incremental +C¡,- exposures in a centrifuge. In Series II, the effect on G-induced arm pain of wearing protective garments around the lower part of the upper arm was investigated in 10 subjects in 4 conditions: 1) counterpressure; 2) rigid support; 3) sham support; and 4) no support (control). Pain was rated using a 10-point graded scale. Results: PH reduced arm pain at 7.5 G from [median (range)] 4 (2-9) to 2 (0-5) in the pressure-habituated arm. The sham support did not affect pain compared to in the control condition (5.2; 3.0-10.0), whereas pain was reduced by both the rigid support (3.7; 1.0-8.0) and the counterpressure (2.5; 0.0-5.5). PH was as efficient in alleviating pain as the counterpressure and more efficient than the rigid support. Discussion: The results support the notion that G-induced arm pain is caused by vascular overdistension. Repeated moderate elevations of local intravascular pressure reduce G-induced arm pain, presumably because such PH reduces vascular distensibility. [ABSTRACT FROM AUTHOR]

Background: It has been shown that flight experience may induce an adaptation of the vestibular system. The aim of the present work was to elucidate whether pilots, in comparison with non-pilots, have an increased responsiveness to angular displacement canal stimuli in the pitch plane during a conflict between the otolith organs and the semicircular canals. Methods: In a large swing-out gondola centrifuge, eight non-pilots, eight fighter pilots, and eight helicopter pilots underwent three runs (2 G, 5 mm) heading forward, centripetally, and centrifugally. The direction of the gravitoinertial force was constant with respect to the subject. The visually perceived eye level (VPEL) was measured in darkness by means of an adjustable luminous dot. Resu/ts: In the forward position the three groups produced similar results. After acceleration there was a sensation of backward tilt and an increasing depression of VPEL. This effect was smaller in the centripetal position and larger in the centrifugal position. The difference in VPEL between the opposite positions constitutes a measure of the ability to sense the pitch angular displacement canal stimulus related to the swing out of the gondola (600). This difference was most pronounced initially at the 2-G plateau (mean ± SD): 13.5 ± 1 2.9° (non-pilots), 41.6 ± 21.10 (fighter pilots), and 19.5 ± 14.0° (helicopter pilots). There was no significant difference between non-pilots and helicopter pilots. Fighter pilots differed significantly from both non-pilots and helicopter pilots. Conclusion: Vestibular learning effects of flying may be revealed in a centrifuge. Fighter pilots had an increased ability, as compared to non-pilots and helicopter pilots, to perceive pitch angular displacements. [ABSTRACT FROM AUTHOR]

Introduction: Bed rest is a terrestrial experimental analogue of unloading experienced during exposure to microgravity. Such unloading causes atrophy predominantly of the postural muscles, especially those of the lower limbs. Methods: We tested the hypothesis that 35 d horizontal bed rest alters thermoregulatory responses of subjects (N = 10) immersed in 15°C water, particularly the heat produced by the shivering tremor of the skeletal muscles. Before and after bed rest we measured the thickness of the gastrocnemius medialis (GM), vastus lateralis (VL), tibialis anterior (TA), and biceps brachii (BB) muscles by ultrasonography. During the immersions, we monitored rectal and skin temperatures, heat flux, heart rate, and oxygen uptake. Results: After bed rest, muscle thickness decreased significantly by 12.2 ± 8.8% and 8.0 ± 9.1% in the GM and VL, respectively. No changes were observed in the TA and BB muscles. The 35-d bed rest caused a significant reduction in aerobic power, as reflected in maximal oxygen uptake. There were no significant differences in any of the observed thermoregulatory responses between the preand post-bed rest immersions. Conclusions: Cardiovascular and muscular deconditioning had no effect on the heat production and heat loss responses. Due to the significant reduction in the mass of the muscles in the lower limbs, concomitant with no change in heat production, we conclude that leg muscles do not play a significant role in shivering thermogenesis. [ABSTRACT FROM AUTHOR]

Background: G protection afforded by the abdominal bladder of a pneumatic anti-G suit is usually attributed to counteraction of G-induced caudad displacement of the heart and pooling of blood in the abdominal veins. The study examined whether the abdominal bladder might provide G protection also via other mechanisms. Methods: Each subject was exposed to +Gz loads while sitting relaxed, wearing a full-coverage anti-G suit modified to permit separate pressurization of the abdominal and leg bladders. In two experimental series (N = 8, N = 14), subjects were breathing at positive airway pressure (PPB); in a third series, five subjects were breathing at atmospheric airway pressure. Intrathoracic pressures were estimated by use of esophageal catheters. Results: During PPB at high G loads, intrathoracic pressure was higher with than without the pressurized abdominal bladder. In 7 of the 14 subjects, basilar intrathoracic pressure exceeded airway pressure during PPB when the abdominal bladder was pressurized. The mean arterial pressure response at high G loads was higher in this subset of subjects (55 ± 23 mmHg) than in the subjects in whom airway pressure exceeded intrathoracic pressure (41 ± 27 mmHg). Without PPB at increased G load, the intrathoracic pressure gradient was higher with than without the pressurized abdominal bladder. Discussion: During PPB, the abdominal bladder acts as an airway counterpressure, thereby facilitating pressure transmission from the airways to the thorax and hence improving G protection. It also appears that in several individuals, pressure may be transmitted from the abdominal bladder to the thorax and heart. [ABSTRACT FROM AUTHOR]

Background: It has previously been reported that nonpilots underestimate the roll tilt angle after acceleration in a gondola centrifuge. The aim of the present work was to elucidate the significance of flight experience for roll tilt perception based on vestibular information. Methods: The subjective visual horizontal (SVH) was measured by means of an adjustable luminous line in darkness. Eight nonpilots (N), nine fighter pilots (F), and eight helicopter pilots (H) underwent two centrifuge runs (2 C, 5 mm) heading forward and backward, respectively. The roll position of the gondola (60° at 2 G) was controlled so that the subject was always upright with respect to the gravitoinertial force. Results: Upon acceleration of the centrifuge there was a tilt of the SVH in a direction compensatory to the inclination of the gondola. This tilt was larger in the forward position [N: 17.2 ± 6.4°, F: 31.2 ± 16.4°, H: 33.6 ± 1 8.2° (means ± SD)l than in the backward position (N: -5.0 ± 6.8°, F: -12.2 ± 17.4°, H: -10.4 ± 15.4°). In N the tilt declined with time, approaching zero by the end of the 2-C plateau. In the pilots it was significantly larger and did not decline. Conclusion: Flight experience results in an increased ability to perceive the roll tilt during movement along a curved path. That this can be revealed in a centrifuge might suggest that acceleration of the centrifuge constitutes a movement pattern which is similar, from a vestibular point of view, to that of an airplane entering a coordinated turn. [ABSTRACT FROM AUTHOR]

Background: Positive pressure breathing (PPB) is commonly used in modern fighter aircraft as part of the anti-G ensemble. PPB is combined with a chest counterpressure bladder which is pressurized to the same magnitude as the breathing mask (balanced PPB). The chest counterpressure is expected to reduce the expiratory work of breathing, reduce the risk for lung rupture, and increase G tolerance. In a previous study we did not find any effect from chest counterpressure on G tolerance or G endurance. The aim of this study was to investigate the effects of chest counterpressure on the work of breathing and the risk for lung rupture. Methods: Eight male test subjects were exposed to 20-s periods of PPB at +1.0, 5.0, 6.0, 7.0, and 8.0 Gz. Each Gz level was accomplished twice, with and without pressurization of the chest bladder. Inspiratory and expiratory flows were measured and esophageal pressures were measured in the lower and upper third of the thorax. Subsequently, work and power of breathing and apical transpulmonary pressure were estimated. Results: The apical transpulmonary pressure was slightly larger without than with chest counterpressure at 1.0 Gz, while chest counter- pressure did not affect apical transpulmonary pressure at increased Gz load. Nor did the chest counterpressure affect work or power of breathing at any Gz load. Conclusion: Inflation of the chest bladder does not seem to have any effects on work or power of breathing or risk for lung rupture during PPB at high Gz loads. [ABSTRACT FROM AUTHOR]

Introduction: The somatogravic illusion (SGI) is easily broken when the pilot looks out the aircraft window during daylight flight, but it has proven difficult to break or even reduce the SGI in non-pilots in simulators using synthetic visual scenes. Could visual-flow scenes that accommodate compensatory head movement reduce the SGI in naïve subjects? Methods: We investigated the effects of visual cues on the SGI induced by a human centrifuge. The subject was equipped with a head-tracked, head-mounted display (MMD) and was seated in a fixed gondola facing the center of rotation. The angular velocity of the centrifuge increased from near zero until a 0.57-C centripetal acceleration was attained, resulting in a tilt of the gravitoinertial force vector, corresponding to a pitch-up of 30°. The subject indicated perceived horizontal continuously by means of a manual adjustable-plate system. We performed two experiments with within-subjects designs. In Experiment 1, the subjects (N = 13) viewed a darkened HMD and a presentation of simple visual flow beneath a horizon. In Experiment 2, the subjects (N = 12) viewed a darkened MMD, a scene including symbology superimposed on simple visual flow and horizon, and this scene without visual flow (static). Results: In Experiment 1, visual flow reduced the SGI from 12.4 ± 1.4° (mean ± SE) to 8.7 ± 1.5°. In Experiment 2, the SGI was smaller in the visual flow condition (9.3 ± 1.8°) than with the static scene (13.3 ± 1.7°) and without HMD presentation (14.5 ± 2.3°), respectively. Conclusion: It is possible to reduce the SGI in non-pilots by means of a synthetic horizon and simple visual flow conveyed by a head-tracked HMD. This may reflect the power of a more intuitive display for reducing the SGI. [ABSTRACT FROM AUTHOR]

Background: In aviation, vestibular-induced spatial disorientation is a significant cause of accidents. Recreating flight-like vestibular stimuli in simulators might be a means for training pilots to respond adequately in disorienting situations. Due to the physical constraints of land-based simulators, the question arises whether a given illusion may be created in different ways. For instance, is it possible to induce sensations of tilt by rotary stimuli? The present study concerns the relationship between sensations of rotation and tilt during complex vestibular stimulation. Methods: The visual sensation of roll rotation was quantified by means of a velocity-matching procedure. In a large gondola centrifuge eight subjects underwent four runs (2 G, 2 mm) with different heading positions (forward, backward, centripetally, and centrifugally). The inclination of the gondola persistently corresponded with the vector sum of the Earth gravity force and the centrifugal force (60° at 2 G). Thus, the semicircular canal stimulus in roll was combined in different ways with stimuli in yaw and pitch, as well as with an increasing or decreasing C vector. Results: The magnitude of the responses was only dependent on the roll component of the stimulus. The gain, defined as the ratio between the response and the roll stimulus, was 7-10%. The responses decayed with a time constant ranging from 4 to 5.5 s. Conclusion: The visual sensation of roll rotation reflects the roll plane canal velocity stimulus independently of other stimulus components. This is in contrast to earlier findings on the sensation of changes in position (roll tilt). [ABSTRACT FROM AUTHOR]

The article presents aviation medicine research into the interaction among positive pressure breathing, antigravity straining maneuvers, pressure suits, and the abdominal bladder in air pilots. Results indicate that the suit and the pressure breathing system provided gravity (G) protection equivalent to or more than 4.6 G.