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.     

Effects of microgravity on the interaction of vestibular and optokinetic nystagmus in the vertical plane.

The extent to which the slow phase velocity (SPV) of nystagmus elicited by a vertical optokinetic stimulation with constant velocity could be modulated by sinusoidal angular motion in the vertical plane was investigated under normal gravity condition and during the microgravity period of parabolic flight. In normal gravity, when the angular head motion and the optokinetic stimulation were in the same direction, the peak SPV was slower than the velocity of the optokinetic display. When the head motion and the optokinetic stimulation were in opposite directions, the peak SPV was equal to the velocity of the optokinetic display. In microgravity, the peak SPV was approximately equal to the velocity of the optokinetic display when head rotation and optokinetic stimulation were in the same direction, and was faster than the velocity of the optokinetic display when head rotation and optokinetic stimulation were in opposite directions. In addition, the interaction of vestibular and optokinetic nystagmus was found to be non-linear in microgravity, especially when the optokinetic stimulation was directed downward. These results suggest an interaction between the vestibular and the optokinetic systems modulated as a function of the gravitational state, and support the observation that visual input is more effective in reducing sensory conflict experienced in microgravity.     

Dynamics of motor components of optokinetic nystagmus during exposure of the vestibular apparatus to angular accelerations]

The effect of angular accelerations on the optokinetic nystagmus was studied on 25 test subjects using a special vestibulo-optokinetic trainer rotated according to five programs and electrooculography. Angular accelerations were found to induce indirectly changes in the amplitude and time of the optokinetic nystagmus via the vestibular apparatus. Optimization of parameters of the optokinetic nystagmus was reflected in the criterion of positive difference + delta between angular velocities of the slow component of the nystagmus and optokinetic stimuli. It is postulated that the criterion + delta is a necessary factor in the organization of optimal time relations between optokinetic stimuli and central processes that are responsible for the final result--sensory image of movement.     

Relation between the asymmetry of optokinetic nystagmus and optovestibular and vestibulo-autonomic stability

Comparative quantitative analysis of physiological asymmetry of the optokinetic nystagmus produced predominantly by central optokinetic stimulation (field of vision = 30 degrees) or peripheral stimulation (field of vision = 110 degrees) reveals an objective correlation between the asymmetry coefficient of the central optokinetic nystagmus with respect to the angular velocity of its slow phase and susceptibility to motion sickness caused by Coriolis and pseudocoriolis acceleration, the nystagmus velocity being up to 45 deg/sec.     

离心机模拟连续推拉动作训练方法研究

目的建立贴近实战化训练模式的连续推拉动作(PPM)离心机训练方法。方法调研提炼飞行训练中典型的PPM载荷参数,编制离心机模拟连续PPM曲线。16名战斗机飞行员作为志愿者,A组6名,采用-1Gz/3s→+6Gz/10s→2Gz/10s→0Gz/3s→+4.5Gz/10s→2Gz/10s→0.5Gz/3s→+5 Gz/10s连续PPM曲线,B组10名,采用-1Gz/3s→+6Gz/10s→2Gz/10s→0Gz/3s→+4.5Gz/10s→2Gz/10s→-1Gz/3s→+7Gz/5s连续PPM曲线(根据A组建议对曲线进行了改进),进行了HP动作对抗连续PPM离心机训练。志愿者在相对-Gz(小于+1Gz)暴露时采用HP动作的呼吸方式,在向+Gz转换时开始做较用力的HP动作,记录分析+Gz耐力、心率(HR)等指标的变化。结果A组6名志愿者均完成了连续PPM离心机训练,对PPM曲线提出了改进建议。B组9名志愿者采用改进的连续PPM曲线完成离心机训练。两组在相对-Gz时的HR均显著高于安静状态(P<0.05),连续PPM暴露时的HR变化趋势说明心血管调节有持续效应,其适应负荷有滞后。结论建立了高性能战斗机飞行员连续PPM离心机训练方法,模拟连续PPM曲线中不同水平-Gz与+Gz交替作用的模式更能体现实战化训练中长时间反复空战的特点,将应用于后续飞行员离心机训练。     

Release from masking in virtual auditory space during sustained positive acceleration

BACKGROUND: The use of a three-dimensional (3-D) auditory display to significantly lower subject detection level while maintaining comprehension under sustained positive G-stress was explored in this study. METHODS: Auditory threshold levels were measured for detecting a band limited pulsed signal in the presence of a broadband diotic masker at both + 1 Gz (rest position) and under sustained +3 Gz. The pulsed signal was presented diotically and was spatialized at one of four static azimuth positions on the horizontal plane. RESULTS: Results showed that auditory thresholds were not significantly affected by sustained +3 Gz stress. Compared with a diotic presentation, subjects reached an average of 6.8 dB lower auditory threshold at +1 Gz and under sustained +3 Gz when the pulsed signal was spatialized at a static position of 90 degrees azimuth on the horizontal plane. CONCLUSION: The implication of these results and suggestions for further research are discussed.     

前庭刺激对选择注意影响的事件相关电位研究

目的应用事件相关电位(ERPs)技术观察线性改变前庭刺激时晚期选择注意加工的动态变化.方法 33名被试者进行了匀角速度旋转(10°/s)和4种水平匀角加速度旋转刺激,角加速度分别为0.6°/s2、0.8°/s2、1.0°/s2和1.2°/s2.前庭刺激时被试者完成相同的等概率听觉认知任务.听觉刺激为1 000 ms随机间隔的中文数字2到9,任务要求被试者听到奇数(靶信号)快速按键,听到偶数(非靶信号)不按键. 结果与对照和4种水平匀角加速度旋转相比,匀速旋转时非靶信号ERPs的N2幅值在前中央脑区显著降低,靶信号ERPs的N2幅值在F4、Fz和T4电极位置显著降低.与对照相比,匀速旋转时靶信号ERPs的P3潜伏期显著缩短.4种角加速度水平,靶信号ERPs的P3潜伏期在0.8°/s2和1.0°/s2刺激时较长,在0.6°/s2和1.2°/s2时较短.结论匀速旋转对晚期选择注意加工有激活作用,相对于匀速旋转,匀角加速度旋转对选择注意加工有抑制作用,这种抑制作用有几个层次.     

Eye movements to yaw, pitch, and roll about vertical and horizontal axes: adaptation and motion sickness

BACKGROUND: In the search for parameters to predict motion sickness that can be measured in the laboratory, we performed a longitudinal investigation in aviators. Since the vestibular system is involved in the generation of motion sickness as well as eye movements, vestibulo-ocular reflex (VOR) parameters seemed relevant. We investigated three topics: 1) the effect of axis orientation and its orientation to gravity on the VOR; 2) changes in VOR parameters depending on flight experience; and 3) differences in VOR parameters in aircrew with high and low susceptibility to motion sickness. HYPOTHESIS: Nystagmus decay after angular velocity steps would be faster for non-susceptible and trained aviators. METHODS: We recorded eye movements evoked by angular on-axis velocity steps (+/- 90 degrees x S(-2), to and from 90 degrees x S(-1)) in yaw, pitch, and roll, about both the Earth vertical and Earth horizontal axes in 14 subjects with a low susceptibility to motion sickness. These data were compared with those of 10 subjects with a high susceptibility. RESULTS: Horizontal axis rotations are nauseogenic. We found that during (per) and post-condition, left- and rightward rotation responses were equal, and the orientation with respect to gravity did not alter the basic nystagmus decay, apart from a sinusoidal modulation. Moreover, pitch and roll rotations show equal nystagmus decays, significantly faster than for yaw; yaw and pitch peak velocities were equal and were larger than for roll. With regard to changes in VOR parameters depending on flight experience, we found that repeated vestibular stimulation reduced nystagmus decay as well as the otolith modulation. With respect to the changes in VOR parameters and motion sickness susceptibility, we found that subjects highly susceptible to motion sickness showed a slower decay of nystagmus with a larger peak velocity than less susceptible subjects. CONCLUSIONS: Group averages indicate a difference in eye movement parameters, only in yaw, depending on flight experience; and between subjects with low and high susceptibility to motion sickness. The involvement of the velocity storage mechanism as realized by an internal model is given as a plausible explanation.     

Visual-vestibular interactions: the directional component of visual background movement

Legibility of a head-fixed display and visual suppression of the vestibulo-ocular reflex (VOR) were found to be superior when vestibular stimuli and optokinetic stimuli were of like direction (i.e. would produce the same directions of nystagmus) and inferior when they were opposite in direction. Velocities (relating to the head) of peripheral optokinetic stimuli ranging between -18 degrees/s and +180 degrees/s interacted effectively with vestibular stimuli to influence visibility of a head-fixed display. This indicates that peripheral optokinetic stimulation can influence visual suppression of the VOR at velocities that far surpass effective production of optokinetic nystagmus.     

潜艇人员旋转刺激后眼震电图检查及其应用价值

对晕船、不晕船现役潜艇人员各３０名做旋转后眼震电图检查。结果表明：晕船组前庭性眼震的慢相角速度均值及总值都比不晕船组高，两者比较有显著性差异（Ｐ＜０．０５）。晕船组的后相眼震发生率高、发生次数多，与不晕船组相比较有非常显著性差异（Ｐ＜０．０１）。晕船组后相眼震的慢相角速度总值高、持续时间长，与不晕船组比有显著性差异（Ｐ＜０．０５）。作者认为上述结果为将该项检查实际应用于筛选潜艇人员提供了依据。     

Protection to +12 Gz

BACKGROUND: The U.S. Air Force has developed +Gz-protective equipment that will provide most pilots protection to +9 Gz with minimal-to-no straining. This equipment includes a pressure breathing system called COMBAT EDGE (CE), which is currently operational, and the Advanced Technology Anti-G Suit (ATAGS), which is not yet operational. For future high-performance aircraft design it is important to know the upper limit of various protective equipment and techniques. METHODS: Six subjects were randomly exposed to a 12-cell matrix composed of +Gz and the following combinations of protective equipment at three seat-back angles (13 degrees, 30 degrees and 55 degrees from the vertical): 1) the standard CSU-13B/P anti-G suit (STD); 2) the STD suit with CE; 3) the ATAGS; and 4) the ATAGS with CE. Relaxed, followed by straining +Gz tolerance was determined using 15-s rapid onset runs to a maximum of +12 Gz. A comprehensive battery of baseline and post-exposure medical surveillance studies was performed to evaluate the medical consequences of these high +Gz exposures. RESULTS: All 6 subjects were able to achieve +12 Gz with various combinations of +Gz-protective equipment, seat-back angle, and various amounts of straining, from none to maximum. When the data were collapsed over all protective equipment there was a significant (p < 0.05) seat effect. Relaxed tolerance to ROR increased with seat-back angle from 13 degrees to 30 degrees to 55 degrees. There was also a significant protective equipment effect when the data were collapsed over all seat-back angles. CONCLUSIONS: These data confirm that effortless protection to +9 Gz is available using ATAGS/CE with the 13 degree and 30 degree seat-back angle (F-15, F-16 and F-22) and to +10.5 Gz with a 55 degree seat-back angle. Moreover, with ATAGS/CE, and a moderate degree of straining, +12 Gz is definitely achievable at 55 degrees, even with reduced anti-G suit pressure at 55 degrees. With additional straining +12 GC is also achievable at the 13 degree and 30 degree seat-back angles.     

Evaluation of +Gz tolerance following simulation of 8-hr flight]

Tolerance of +Gz (head-pelvis) centrifugation of pilots was evaluated following simulation of a long flight on single-seat fighter. The experiment involved 5 test-subjects who were exposed to +Gz before and after simulated 8-hr flight with a growth gradient of 0.1 u/s without anti-g suits and muscles relaxed; in addition, limiting tolerance of intricate profile +Gz loads of 2.0 to 9.0 units with a growth gradient of 1.0 u/s of test-subjects in anti-g suits (AGS) with a change-over pressure valve in the peak mode using muscle straining and breathing maneuvers. To counteract the negative effects of extended flight, various seat configurations: with a back inclination at 30 degrees to the +Gz vector and changeable geometry with a back inclination at 55 degrees to the vector. The other counter-measures applied were cool air shower, suit ventilation, physical exercises, lower body massage with AGS, electrostimulation of the back and lumber region, profiling of the supporting and soft parts of the seat, and 30-s exposure to +5 Gz. Hemodynamic and respiration parameters as well as body temperature were measured in the course of 8 hrs of flight and during and shortly after centrifugation. According to the results of the investigation, seat inclination at 55 degrees to the +Gz vector and tested system of countermeasures prevent degradation of tolerance of large (9 u.) loads following 8-hr flight simulation with the use of the modern anti-g gear,     

Centrifuge training program with "push-pull" elements

INTRODUCTION: Pilots of fighter aircraft are often exposed to maneuvers that produce negative acceleration (-Gz) immediately followed by positive acceleration (+Gz). This sequence has been found to reduce tolerance to +Gz, a phenomenon known as the "push-pull" effect. We devised a centrifuge training program to demonstrate this phenomenon to pilots. METHODS: The centrifuge of the Military Institute of Aviation Medicine in Warsaw, Poland, was modified in 1996 to allow active positioning of the gondola during rotation. Head-down position of -6 degrees to -40 degrees were used to produce relative -Gz (r-Gz) in a range down to 0.2. As a side effect, this produces Gy acceleration between -1.3 Gy and -1.6 Gy. Pilots completed normal centrifuge training, including a relaxed, gradual-onset run and three rapid-onset runs. They were then exposed to a profile that included a series of push-pull exposures where r-Gz was followed by +Gz with stepwise increases in the latter from +2.5 to +5 Gz. The final profile was a simulated aerial combat maneuver with push-pull elements. RESULTS: The trainees expressed surprise at the push-pull effect, which forced them to begin an anti-G straining maneuver at lower levels than normal. They complained about the presence of the Gy, which rarely occurs in aircraft. DISCUSSION: This type of profile appears useful for training pilots about the push-pull phenomenon. After collection of additional data, the profiles may be refined.     

姜素预防犬运动病的效果观察

目的　观察姜素预防犬运动病的效果和对豚鼠视前庭相互作用的影响。　方法　采用随机双盲和安慰剂对照实验设计 ,观察 7 2mg/kg ,3 6mg/kg和 1 8mg/kg 3种剂量姜素及苯海拉明 (乘晕宁 3 2mg/kg)、盐酸苯环壬酯 (飞塞乐 0 2mg/kg)对犬运动病预防效果和对豚鼠视前庭系统的影响。犬运动病采用三角离心摆动方式 (峰速 2 4 0°/s ,频率 0 0 5Hz)诱发 ,以一定时间内犬的呕吐发生率作为犬运动病的观察指标 ;以正弦摆动 (峰速 6 0°/s,频率 0 0 5Hz)加光条刺激方式诱发豚鼠视前庭相互作用眼震 ,以眼震次数、慢相速度和增益作为视前庭相互作用观察指标。　结果　 3种剂量姜素均可显著降低犬呕吐发生率 ,且在 7 2mg/kg和 3 6mg/kg剂量时其效果较苯海拉明、盐酸苯环壬酯显著 ,各种药物对豚鼠VVOR眼震均无显著影响。　结论　姜素用于预防对抗运动病效果显著且不会影响正常的视前庭功能     

用交替角加速度刺激获得前庭适应的实验研究

<正> 前庭适应的获得方法,各国学者都在积极研究,因为这是航空、航海、航天和运动医学中的一个迫切需要解决的问题。同时,任何人都不可能对各类不同数值的前庭刺激先天就有很高的耐受性,大多数人对某种前庭刺激的耐受性都是后天获得的。因此,根据职业活动的环境条件,模拟职业活动时可能受到的异常前     

Reflex heart rate response to variable onset +Gz

Rapid onset high sustained +Gz is a frequent requirement in air combat maneuvering. The cardiovascular response is inadequate to fully compensate for this rapid +Gz change. The rate of change in heart rate (HR) during gradual (0.1 G.s-1, GOR), rapid (1.0 G.s-1, ROR), and very high (6.0 G.s-1, VHOG) onset acceleration exposures to sustained (15 s) +7Gz, +8Gz, and +9Gz levels was measured in 81 healthy male subjects in a human centrifuge. The time (s) to reach maximum heart rate (T7) was measured as the time for the preacceleration exposure resting heart rate (RHR) to reach maximum heart rate (MHR). The change in heart rate upon reaching maximum +Gz level (delta HRA) from rest was calculated along with the change in HR from rest to the maximum heart rate achieved before maximum +Gz level was attained. During the ROR and VHOG runs, MHR was not achieved until after maximum +Gz level was attained. The change in heart rate from resting HR (immediately prior to acceleration) to the heart rate achieved at the onset of maximum +Gz level (delta HRA), decreased by 50% as the onset rate increased from GOR to ROR and VHOG. The delta HRB for very high onset rates exposures was significantly greater than that for ROR and GOR exposures. Acceleration exposure to levels of +7Gz and above (+7Gz, +8Gz and +9Gz) exhibited similar HR responses. VHOG to sustained +Gz stress levels of +7 to +9Gz for 15 s did not provide a sufficient length of time to allow maximum cardiovascular response.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Effect of muscle tensing on cerebral oxygen status during sustained high +Gz

BACKGROUND: Successful monitoring of oxyhemoglobin during +Gz exposure was recently achieved using near-infrared spectroscopy (NIRS). To assess the effects of muscle tensing on sustained +Gz tolerance, we measured muscle activity and cerebral oxygen status (COS) during anti-G straining maneuvers at sustained high +Gz. METHOD: We exposed 21 male pilots wearing CSU-13/P anti-G suit to two different centrifuge profiles: 1) short-term repeated exposure (5 to 20 s) at 4, 5, 6, 5.5, or 7 Gz; 2) sustained exposure to a + 7Gz plateau for 30 s. During the Gz exposures, surface electromyographic (EMG) measurements were taken from the vastus medialis (VM) and rectus abdominis (RA) muscles. At the same time, the COS was recorded from the left forehead area using a commercial NIRS system. Mean muscular tensing for each muscle was calculated as a percentage of maximal voluntary contraction (% MVC). RESULTS: Oxyhemoglobin (O2Hb) and total hemoglobin (sum of O2Hb and deoxyhemoglobin) were decreased during both short-term and sustained +Gz exposure. RA muscle tensing was positively correlated with changes in the concentration of O2Hb during sustained + 7Gz exposure (r = 0.540, p < 0.05). RA tensing ranged from 6.2 to 36.8%MVC, and O2Hb ranged from -41.3 to -7.28 micromol x L(-1) during the exposures. No significant correlation was observed between VM tensing and O2Hb. CONCLUSION: NIRS measurements confirmed that a muscle straining maneuver increases G tolerance. Higher RA muscle tensing helps preserve brain blood volume during sustained high +Gz.     

+Gz重复暴露对大鼠多脏器脂质过氧化作用的影响

目的：观察重复＋Gz作用下大鼠多脏器脂质过氧化作用的变化。方法：雄性Wistar大鼠20只,随机分为两组（n=10);对照（＋1Gz）组,＋10Gz组,两组分别受＋1Gz,＋10Gz作用。离心机暴露条件：＋G增长率0．5G/s,峰值持续时间30s,间隔1min,5次／d,3d/wk,共3wk。末次离心机暴露后次日,将大鼠断头处死,于冰浴上速取肺、心、肝1肾制备匀浆及心、肾线粒体。检测丙二醛（MDA）、甘谷胱肽（GSH）的含量氧化物歧化酶（SOD）活性。结果：对照组比,重复＋10Gz暴露后,心肌线粒体MDA含量明显增加（P＜0．01）,肝匀浆及肾线粒体SOD含量明显降低（P＜0．01）,而心、肾线粒体GSH含量无明显变化,结论重复＋10Gz应激导致心肌线粒体的脂质过氧化损伤,对肝、肾氧自由基代有一定影响。     

Impairment of cardiovascular and vasomotor responses during tilt table simulation of "push-pull' maneuvers

BACKGROUND: Numerous studies have shown that tolerance to positive acceleration (+Gz) is impaired subsequent to an exposure of less than +1 Gz. HYPOTHESIS: Vasodilation induced by antecedent negative Gz (-Gz) exposure delays sympathetic vasoconstriction during subsequent +Gz, further reducing G-tolerance. METHODS: There were 20 subjects tested on an electronic tilt table, and exposed to the following randomized head-up tilt (HUT) and head-down tilt (HDT) conditions: +75 degrees HUT for 60 s, followed by transition to either 0 degrees (supine) HDT, or -25 degrees HDT, or -45 degrees HDT for 7 or 15 s at tilt rate of 45 degrees x s(-1). This was followed by HUT, divided into three periods: HUT1 (approximately 3-10 s), HUT2 (approximately 15-22 s), and HUT3 (approximately 27-35 s). Systolic blood pressure (SBP) was normalized to heart and head-levels. Stroke volume (SV) was estimated using impedance cardiography; forearm blood flow (FBF) estimated by venous occlusion plethysmography and forearm vascular resistance (FVR) was calculated from FBF and SBP. Total peripheral resistance (TPR) was estimated by MAP/(SV*HR). RESULTS: Heart-level SBP decreased significantly during HDT for both HDT durations (p < 0.01). SBP increased significantly at head-level during HDT (p < 0.001). During HUT1 heart and head-level SBP decreased for all conditions (p < 0.001), recovering to baseline levels by HUT2. TPR decreased significantly for all HDT conditions (p < 0.001), with this decrease related to the degree of HDT angle (p < 0.05). During HUT1, TPR remained depressed below baseline. At HUT2, TPR remained decreased for the -45 degrees/7-s condition only (p < 0.01). FBF decreased significantly during HDT (p < 0.02), with the magnitude related to the HDT angle. FBF remained elevated during HUT1 (p < 0.01). FVR decreased as a function of HDT angle during HDT (p < 0.001), with the decrease persisting into the HUT1 phase (p < 0.01). By the HUT2 and HUT3 periods, FVR were above baseline levels for the -45 degrees HDT condition (p < 0.01). CONCLUSION: These results confirm in humans the delayed recovery of peripheral vascular resistance observed in animal studies when -Gz precedes +Gz. Since SV recovered to baseline levels during the "pull" phase (HUT1-3), with TPR and forearm vascular resistance remaining depressed, baroreflex-mediated peripheral vascular control is delayed. This delay at higher subsequent +Gz levels is dangerous for the military pilot, since symptoms of G-intolerance due to delay in head-level BP recovery will ensue at lower absolute +Gz levels during push-pull type maneuvers.