Earth-Based Research Analogs to Investigate Space-Based Health Risks.

During spaceflight, astronauts are exposed to a variety of unique hazards, including altered gravity fields, long periods of isolation and confinement, living in a closed environment at increasing distances from Earth, and exposure to higher levels of hazardous ionizing radiation. Preserving human health and performance in the face of these relentless hazards becomes progressively more difficult as missions increase in length and extend beyond low Earth orbit. Finding solutions is a significant challenge that is further complicated by logistical issues associated with studying these unique hazards. Although research studies using space-based platforms are the gold standard, these are not without limitations. Factors such as the small sample size of the available astronaut crew, high expense, and time constraints all add to the logistical challenge. To overcome these limitations, a wide variety of Earth-based analogs, from polar research outposts to an undersea laboratory, are available to augment space-based studies. Each analog simulates unique physiological and behavioral effects associated with spaceflight and, therefore, for any given study, the choice of an appropriate platform is closely linked to the phenomena under investigation as well as the characteristics of the analog. There are pros and cons to each type of analog and each actual facility, but overall they provide a reasonable means to overcome the barriers associated with conducting experimental research in space. Analogs, by definition, will never be perfect, but they are a useful component of an integrated effort to understand the human risks of living and working in space. They are a necessary resource for pushing the frontier of human spaceflight, both for astronauts and for commercial space activities. In this review, we describe the use of analogs here on Earth to replicate specific aspects of the spaceflight environment and highlight how analog studies support future human endeavors in space.

Overview of the NASA 70-day Bed Rest Study.

PURPOSE: The purpose of this article was to provide an overview of the National Aeronautics and Space Administration (NASA) 70-day Bed Rest Study. The integrated complement of investigations and the standardized bed rest environment that served as the platform for this study complement are described. Outcomes of the studies will not be presented here but will be reported in separate publications. METHODS: A set of studies running in an integrated fashion along the entire period (pre-, in-, and post-bed rest) and using the same subjects is referred in this article as "the campaign" or "complement." NASA selected eight individual studies to participate in the 70-d bed rest campaign. These studies were integrated to increase efficiency in the utilization of resources and to share common measures among the investigations. In addition to the individual studies addressing specific aims, a battery of standardized measures was included. Standard measures target a wide range of physiologic systems and represent some of the testing routinely done on astronauts. Bed rest subjects underwent rigorous medical and psychological screening. Standardized conditions included 70 d of bed rest in a 6° head-down tilt position. Subjects' vital signs, body weight, and fluid intake and output were measured daily. A standardized diet was provided to ensure consistent nutritional intake across subjects. Exercising subjects were prescribed individualized aerobic and resistance training 6 d·wk performed in a horizontal body position. Subjects in the testosterone supplementation countermeasure group received testosterone enanthate injections at 2-wk intervals during bed rest. CONCLUSION: Long-duration head-down tilt bed rest provided a suitable platform for examining physiologic effects of spaceflight and testing countermeasures in a ground-based model. Integrating studies into a complement is an effective way to support multiple investigations while minimizing the number of subjects to answer many research questions.

Ophthalmological Evaluation of Integrated Resistance and Aerobic Training During 70-Day Bed Rest.

INTRODUCTION: We evaluated ophthalmic changes in healthy individuals who underwent integrated resistance and aerobic training (iRAT) during 70-d 6° head-down tilt (HDT) bed rest (BR). METHODS: Participants were selected using NASA standard screening procedures. Standardized NASA BR conditions were implemented. Subjects were randomly assigned to the iRAT protocol or no exercise during HDTBR. Weekly ophthalmic examinations were performed in the sitting (pre/post-BR only) and HDT (BR only) positions. Mixed-effects linear models compared pre- and post-HDTBR intraocular pressure (IOP), Spectralis OCT circumpapillary retinal nerve fiber layer (cpRNFL) thickness, and peripapillary retinal thickness observations between groups. RESULTS: Six controls and nine exercisers completed the study. There was an overall effect of BR on our outcomes. Except Goldmann IOP (mean pre/post difference in controls and exercisers: -0.47 mmHg vs. +1.14 mmHg), the magnitude of changes from baseline was not significantly different between groups. There was a +1.38 mmHg and a +1.63 mmHg iCare IOP increase during BR in controls and exercisers, respectively. Spectralis OCT detected a +1.33 µm average cpRNFL thickness increase in both groups, and a +9.77 µm and a +6.65 µm peripapillary retinal thickening post-BR in controls and exercisers, respectively. Modified Amsler grid, red dot test, confrontational visual field, color vision, and stereoscopic fundus photography were unremarkable. CONCLUSIONS: HDTBR for 70 d induced peripapillary retinal thickening and cpRNFL thickening without visible signs of optic disc edema. The magnitude of such changes was not different between controls and exercisers. A slight IOP increase during BR subsided post-BR. Further study should evaluate whether different physical exercise paradigms may prevent/mitigate the risk of space-related visual impairment.Taibbi G, Cromwell RL, Zanello SB, Yarbough PO, Ploutz-Snyder RJ, Godley BF, Vizzeri G. Ophthalmological evaluation of integrated resistance and aerobic training during 70-day bed rest. Aerosp Med Hum Perform. 2017; 88(7):633-640.

Ocular Outcomes Comparison Between 14- and 70-Day Head-Down-Tilt Bed Rest.

PURPOSE: To compare ocular outcomes in healthy subjects undergoing 14- and/or 70-day head-down-tilt (HDT) bed rest (BR). METHODS: Participants were selected by using NASA standard screening procedures. Standardized NASA BR conditions were implemented. Subjects maintained a 6° HDT position for 14 and/or 70 consecutive days. Weekly ophthalmologic examinations were performed in the sitting (pre/post-BR only) and HDT positions. Mixed-effects linear models compared pre- and post-HDT BR observations between 14- and 70-day HDT BR in best-corrected visual acuity, spherical equivalent, intraocular pressure (IOP), Spectralis OCT retinal nerve fiber layer thickness, peripapillary and macular retinal thicknesses. RESULTS: Sixteen and six subjects completed the 14- and 70-day HDT BR studies, respectively. The magnitude of HDT BR-induced changes was not significantly different between the two studies for all outcomes, except the superior (mean pre/post difference of 14- vs. 70-day HDT BR: +4.69 µm versus +11.50 µm), nasal (+4.63 µm versus +11.46 µm), and inferior (+4.34 µm versus +10.08 µm) peripapillary retinal thickness. A +1.42 mm Hg and a +1.79 mm Hg iCare IOP increase from baseline occurred during 14- and 70-day HDT BR, respectively. Modified Amsler grid, red dot test, confrontational visual field, color vision, and stereoscopic fundus photography were unremarkable. CONCLUSIONS: Seventy-day HDT BR induced greater peripapillary retinal thickening than 14-day HDT BR, suggesting that time may affect the amount of optic disc swelling. Spectralis OCT detected retinal nerve fiber layer thickening post BR, without clinical signs of optic disc edema. A small IOP increase during BR subsided post HDT BR. Such changes may have resulted from BR-induced cephalad fluids shift. The HDT BR duration may be critical for replicating microgravity-related ophthalmologic changes observed in astronauts on ≥6-month spaceflights.

Ocular outcomes evaluation in a 14-day head-down bed rest study.

INTRODUCTION: We evaluated ocular outcomes in a 14-d head-down tilt (HDT) bed rest (BR) study designed to simulate the effects of microgravity on the human body. METHODS: Healthy subjects were selected using NASA standard screening procedures. Standardized NASA BR conditions were implemented (e.g., strict sleep-wake cycle, standardized diet, 24-hour-a-day BR, continuous video monitoring). Subjects maintained a 6° HDT position for 14 consecutive days. Weekly ophthalmological examinations were performed in the sitting (pre/post-BR) and HDT (in-bed phase) positions. Equivalency tests with optimal-alpha techniques evaluated pre/post-BR differences in best-corrected visual acuity (BCVA), spherical equivalent, intraocular pressure (IOP), Spectral-domain OCT retinal nerve fiber layer thickness (RNFLT), optic disc and macular parameters. RESULTS: 16 subjects (12 men and 4 women) were enrolled. Nearly all ocular outcomes were within our predefined clinically relevant thresholds following HDTBR, except near BCVA (pre/post-BR mean difference: -0.06 logMAR), spherical equivalent (-0.30 D), Tonopen XL IOP (+3.03 mmHg) and Spectralis OCT average (+1.14 µm), temporal-inferior (+1.58 µm) and nasal-inferior RNFLT (+3.48 µm). Modified Amsler grid, red dot test, confrontational visual field, and color vision were within normal limits throughout. No changes were detected on stereoscopic color fundus photography. DISCUSSION: A few functional and structural changes were detected after 14-d HDTBR, notably an improved BCVA possibly due to learning effect and RNFL thickening without signs of optic disc edema. In general, 6° HDTBR determined a small nonprogressive IOP elevation, which returned to baseline levels post-BR. Further studies with different BR duration and/or tilt angle are warranted to investigate microgravity-induced ophthalmological changes.

Effects of 30-day head-down bed rest on ocular structures and visual function in a healthy subject.

INTRODUCTION: We report ocular changes occurring in a healthy human subject enrolled in a bed rest (BR) study designed to replicate the effects of a low-gravity environment. CASE REPORT: A 25-yr-old Caucasian man spent 30 consecutive days in a 6 degrees head-down tilt (HDT) position at the NASA Flight Analogs Research Unit. Comprehensive ophthalmologic exams, optic disc stereo-photography, standard automated perimetry (SAP), and optic disc Spectralis OCT scans were performed at baseline, immediately post-BR (BR+0), and 6 mo post-BR. MAIN OUTCOME MEASURES: changes in best-corrected visual acuity, intraocular pressure (IOP), cycloplegic refraction, SAP, and Spectralis OCT measures. At BR+0 KIOP was 11 and 10 mmHg in the right (OD) and left eye (OS), respectively (a bilateral 4-mmHg decrease compared to baseline); SAP documented a possible bilateral symmetrical inferior scotoma; Spectralis OCT showed an average 19.4 microm (+5.2%) increase in peripapillary retinal thickness, and an average 0.03 mm3 (+5.0%) increase in peripapillary retinal volume bilaterally. However, there were no clinically detectable signs of optic disc edema. At 6 mo post-BR, IOP was 13 and 14 mmHg in OD and OS, respectively, and the scotoma had resolved. Spectralis OCT measurements matched the ones recorded at baseline. DISCUSSION: In this subject, a reduction in IOP associated with subtle structural and functional changes compared to baseline were documented after prolonged head-down BR. These changes may be related to cephalad fluid shifts in response to tilt. Further studies should clarify whether decreased translaminar pressure (i.e., the difference between IOP and intracranial pressure) may be responsible for these findings.

The effect of microgravity on ocular structures and visual function: a review.

Ocular structural and functional changes, including optic disk edema and reduction of near visual acuity, have been recently described in some astronauts returning from long-duration space travels. It is hypothesized that ocular changes related to spaceflight may occur, in predisposed individuals, as a result of cephalad shift of body fluids, possibly leading to elevated intracranial pressure (ICP). Results from head-down bed-rest studies (used to simulate the effects of microgravity) and from parabolic flight experiments (used to produce transient periods of microgravity) indicate that ocular blood flow and intraocular pressure (IOP) may undergo changes in a low-gravity environment. Recent studies suggest that changes in translaminar pressure (i.e., IOP minus ICP) may be implicated in the pathophysiology of optic disk neuropathies. Because postural changes exert an effect on both IOP and ICP, the head-down bed-rest analog may also be used as a platform to characterize the relationship between IOP and ICP, and their reciprocal influence in the pathophysiology of conditions such as optic disk edema or glaucoma.

Sensorimotor adaptation training's effect on head stabilization in response to a lateral perturbation in older adults.

The goal of this study was to determine if exposure to sensorimotor adaptation training improved head stabilization in older adults. Sixteen participants, age 66-81 yr, were assigned at random to the control group (n = 8) or the experimental group (n = 8). Both groups first completed 6 trials of walking a foam pathway consisting of a moveable platform that induced a lateral perturbation during walking. Head-in-space and trunk-in-space angular velocities were collected. Participants from both groups then trained twice per week for 4 wk. Both groups walked on a treadmill for 20 min. The control group viewed a static scene. The experimental group viewed a rotating visual scene that provided a perceptual-motor mismatch. After training, both groups were retested on the perturbation pathway test. The experimental group used a movement strategy that preserved head stabilization compared with the controls (p < .05). This training effect was not retained after 4 wk.

Outcome measure to assess head-down bed rest subject performance.

BACKGROUND: The purpose of the present study was to develop an objective outcome measure to assess the performance of head-down bed rest subjects. The rationale behind the development was that the current outcome measure is subjective and dependent upon how much the clinical psychologist knows about subject behaviors during the study to rate them accurately. METHODS: The behaviorally anchored rating scales (BARS) were developed through the use of the critical incident technique, along with traditional BARS development procedures, and the use of focus groups. RESULTS: Use of these methodologies yielded 86 usable critical incidents, which were used as anchors for the scales. DISCUSSION: Further research will be necessary to determine whether or not associations exist between data from the BARS and psychological screening data.

Effects of sensorimotor adaptation training on functional mobility in older adults.

The goal of this study was to determine if prolonged exposure to perceptual-motor mismatch increased adaptability and retention of balance in older adults. Sixteen adults, aged 66 to 81 years, were randomized to one of two groups: either the control group (n=8) or the experimental group (n=8). Both groups first completed six trials of walking an obstacle course. Participants then trained twice a week for 4 weeks. In the training, the control group walked on a treadmill for 20 minutes while viewing a static visual scene and the experimental group walked on a treadmill for 20 minutes while viewing a rotating visual scene that provided a perceptual-motor mismatch. Following training, both groups were post-tested on the obstacle course. The experimental group moved faster through the obstacle course with fewer penalties. This training effect was retained for 4 weeks. Exposure to perceptual-motor mismatch induced an adaptive training effect that improved balance and locomotor control in older adults.

The effect of increasing inertia upon vertical ground reaction forces and temporal kinematics during locomotion.

The addition of inertia to exercising astronauts could increase ground reaction forces and potentially provide a greater health benefit. However, conflicting results have been reported regarding the adaptations to additional mass (inertia) without additional net weight (gravitational force) during locomotion. We examined the effect of increasing inertia while maintaining net gravitational force on vertical ground reaction forces and temporal kinematics during walking and running. Vertical ground reaction force was measured for 10 healthy adults (five male/five female) during walking (1.34 m s(-1)) and running (3.13 m s(-1)) using a force-measuring treadmill. Subjects completed locomotion at normal weight and mass and at 10, 20, 30 and 40% of added inertial force. The added gravitational force was relieved with overhead suspension, so that the net force between the subject and treadmill at rest remained equal to 100% body weight. Ground reaction forces were affected by the added inertial force, but not to the magnitude predicted by the increase in mass, suggesting that adaptations in motion occurred. Vertical ground reaction force production and adaptations in gait temporal kinematics were different between walking and running. Peak vertical impact forces and loading rates increased with increased inertia during walking, and decreased during running. As inertia increased, peak vertical propulsive forces decreased during walking and did not change during running. Stride time increased during walking and running, and contact time increased during running. The increased inertial forces were utilized independently from gravitational forces by the motor control system when determining coordination strategies.

Adaptive changes in gait of older and younger adults as responses to challenges to dynamic balance.

The study proposed to identify balance strategies used by younger and older adults during gait under proprioceptive, visual, and simultaneous proprioceptive-visual challenges. Participants ambulated under 4 conditions: consistent, noncompliant surface; inconsistent, compliant surface (C); consistent, noncompliant surface with vision obscured (NCVO); and inconsistent, compliant surface with vision obscured (CVO). Balance adaptations were measured as changes in gait velocity, cadence, and gait-stability ratio (GSR). Participants were 5 younger (mean age = 27.2) and 5 older (mean age = 68) healthy adults. Significant age differences were found for GSR (p = .03) on all surfaces. Older adults adopted a more stable gait pattern than younger adults regardless of the challenge presented by surface. Significant condition differences were found for velocity (p < .001) and cadence (p = .001). All participants exhibited significantly decreased velocity and increased cadence on surfaces C and CVO. Gait speed and cadence did not significantly change in NCVO. Younger and older adults exhibited similar adaptive balance strategies, slowing and increasing steps/s, under proprioceptive and proprioceptive-visual challenges to dynamic balance.

Tae Kwon Do: an effective exercise for improving balance and walking ability in older adults.

BACKGROUND: Age-related declines in balance and walking ability are major risk factors for falls. Older adults reduce the dynamic components of walking in an effort to achieve a more stable walking pattern. Tae Kwon Do is an exercise that trains dynamic components of balance and walking that diminish with age. METHODS: Twenty participants from a Tae Kwon Do exercise class (average age 72.7 years) and 20 nonexercising controls (average age 73.8 years) participated. Balance and walking ability for all participants were pretested and posttested using the following measures: single-leg stance (SLS), Multidirectional Reach Test (MDRT), Timed Up-and-Go (TUG), walking velocity, cadence, gait stability ratio (GSR), and sit-and-reach (S&R). Analysis of variance for a mixed design was used to assess differences at the 0.05 level of significance. RESULTS: For nonexercising controls, no differences were found between pretest and posttest measures. Tae Kwon Do participants showed significant improvements on the MDRT when reaching backward, right, and left. TUG, walking velocity, GSR, and S&R also showed significant improvement in this exercising group. CONCLUSIONS: Tae Kwon Do exercise was effective for improving balance and walking ability in community-dwelling older adults. These improvements were attributed to Tae Kwon Do movements that emphasize dynamic movement components typically deficient in the older adult walking pattern. Improving balance and walking ability through Tae Kwon Do exercise may serve to restore function that has declined with age and preserve mobility for older adults.

Association of balance measures and perception of fall risk on gait speed: a multiple regression analysis.

Gait speed, commonly modified to adapt to the balance and stability challenges of aging, is related to measures of balance and mobility. This study investigated associations between age, Berg Balance Scale, Activities-Specific Balance Confidence (ABC) Scale, One Question Fear of Falling (1QFOF), and gait speed in adults using regression analysis. Results suggested an interaction between 1QFOF and ABC scores. An expanded five-variable model explained 49% of gait speed variance. Age, ABC, and 1QFOF-ABC interactions were significantly associated with gait speed. Regression analysis is useful in investigating associations between performance variables and function. Continued research needs to identify optimal variable combinations and improve prediction of function.

Adaptations in horizontal head stabilization in response to altered vision and gaze during natural walking.

The purpose of this study was to determine adaptations in head stability resulting from altered gaze control and vision during over-ground walking. Using over-ground walking permitted adaptations in walking velocity and cadence that are otherwise not possible during treadmill walking or walking-in-place. Gaze control and vision were manipulated by having 20 young adult subjects 1) walk naturally, 2) view a distant, earth-fixed target to enhance the vestibulo-ocular reflex (VOR), 3) view a head-fixed target to suppress the VOR, and 4) walk in darkness. Horizontal head and trunk angular velocities in space, walking velocity and cadence were measured. Root-mean-square head and trunk angular velocities were calculated and frequency analyses determined head-trunk movement patterns. Results demonstrated that when given the opportunity, subjects slowed down and decreased cadence in response to challenging tasks. Despite strongly reduced walking velocity and cadence, walking in darkness proved most challenging for head stabilization, indicating the importance of vision during this process. Viewing the earth-fixed target demonstrated the greatest head stability thereby, facilitating gaze stabilization. However, comparisons between the earth-fixed and head-fixed target conditions suggest a reciprocal relationship where gaze stability also facilitates head stability. This contribution of gaze stability to head stability is more important than vision alone as the head stabilization response was diminished during the VOR suppressed condition.

Relationship between balance and gait stability in healthy older adults.

Age-related adaptations during walking create a more stable walking pattern, which is less effective for forward progression and might be related to balance deficiencies. This study determined the relationship between walking stability and measures of balance in older adults. Seventeen older and 20 young adults performed the Berg Balance Test (BBT) and walked 10 m. Walking velocity (WV) and cadence were measured, and a gait-stability ratio (GSR) was calculated. Higher GSR indicated that a greater portion of the gait cycle was spent in double-limb support. Age-group comparisons established declines in BBT scores and WV and increases in GSR with age. Significant relationships were identified for BBT Item 12 (alternate stepping on a stool) with WV (r =.58, r(2) =.34) and GSR (r = -.74, r(2) =.54). The correlation of BBT Item 12 with GSR was stronger than with WV (p <.05). Results indicated a strong relationship between increased gait stability and decreased balance for a dynamic weight-shifting task. Therefore, GSR is a better indicator of balance deficits during walking than is WV alone.

Movement strategies for head stabilization during incline walking.

Changes in body orientation with respect to space during incline walking can alter vestibular information requiring a different solution to the problem of head stabilization. Eleven young adults walked along a level walkway, and ascended and descended an inclined surface. Head, neck and trunk angular positions in space were collected. Changes in the gravitoinertial vector imposed by the inclined surface, produced concomitant changes in body segment orientation that decreased head stability during the inclined walking tasks. Head, neck and trunk segments were least stable while ascending the incline creating the greatest challenge to head stability during this task. Movement strategies reflected adjustments of head-neck and neck-trunk patterns to accommodate changes in the gravitoinertial vector and insure balance of the head over the trunk.

Influence of vision on head stabilization strategies in older adults during walking.

BACKGROUND: Maintaining balance during dynamic activities is essential for preventing falls in older adults. Head stabilization contributes to dynamic balance, especially during the functional task of walking. Head stability and the role of vision in this process have not been studied during walking in older adults. METHODS: Seventeen older adults (76.2 +/- 6.9 years) and 20 young adults (26.0 +/- 3.4 years) walked with their eyes open (EO), with their eyes closed (EC), and with fixed gaze (FG). Participants performed three trials of each condition. Sagittal plane head and trunk angular velocities in space were obtained using an infrared camera system with passive reflective markers. Frequency analyses of head-on-trunk with respect to trunk gains and phases were examined for head-trunk movement strategies used for head stability. Average walking velocity, cadence, and peak head velocity were calculated for each condition. RESULTS: Differences between age groups demonstrated that older adults decreased walking velocity in EO (p =.022). FG (p = .021), and EC (p = .022). and decreased cadence during EC (p = .007). Peak head velocity also decreased across conditions (p < .0001) for older adults. Movement patterns demonstrated increased head stability during EO. diminished head stability with EC, and improved head stability with FG as older adult patterns resembled those of young adults. CONCLUSIONS: Increased stability of the lower extremity outcome measures for older adults was indicated by reductions in walking velocity and cadence. Concomitant increases in head stability were related to visual tasks. Increased stability may serve as a protective mechanism to prevent falls. Further, vision facilitates the head stabilization process for older adults to compensate for age-related decrements in other sensory systems subserving dynamic balance.