We are searching data for your request:
Upon completion, a link will appear to access the found materials.
Most animals are horizontal with the ground. When they lie down, they get closer to the ground, but their orientation does not change.
Humans, on the other hand, stand up straight and tall, perpendicular with the ground. And when we lie down, we make a complete 90° turn. So we spent 2/3 of our day at one orientation, and 1/3 of our day at the exactly orthogonal orientation.
It's difficult to think of many other animals that change their orientation so completely on a regular basis, or spend much time in different orientations. (Sure, prairie dogs, bears, lemurs, and apes stand upright now and then, and I've seen pictures of whales sleeping vertically, but it's never for long, sustained periods. Giraffes seem so tall and vertical, but it's all neck and legs - the organs of their body are always horizontal with the ground, and this orientation is maintained when they lie down. Birds, maybe?)
For humans, regular changes in orientation must surely cause biological stresses, and therefore there must be some biological adaptations to those stresses.
Even non-biologists are aware of plenty of orientation-related malfunctions:
- middle-aged people start to get acid reflux and indigestion when they lie horizontally
- elderly people with certain heart conditions and arrhythmias cannot lie horizontally
- some people are affected by changes in blood pressure when they lie down or stand up
- some people have dizziness or balance issues when lying down or standing up
- knees and feet start to have problems from a lifetime of bearing all the weight
What specific adaptations does the human body have to handle these daily changes in orientation?
What adaptations does the human body have to handle frequent and sustained changes in orientation? - Biology
This section contains the following topics:
See the video clips associated with this section.
The user must keep in mind that much is still unknown about the over-all, long term effects of various space environments on performance capabilities. The data included here were derived from past experience with high-performance aircraft, and the relatively limited experience, particularly with respect to long orbital stays, with past space programs. A lot of the information in this chapter has been derived from one-g data to which trend information from the sources cited above was applied. Although less than perfect or complete data were compiled for this chapter, it is the best information in this field known to exist at this time.
This chapter is based on the premise that designers and mission planners will do a better job if they are familiar with the capabilities of the people for whom they are designing. When people go into space their performance capabilities may change in important ways. The purpose of this chapter is to document these changes.
The voluminous data that exists on human performance capabilities under 1-G (Earth) conditions are not included here. This material is covered in other sources (see refs. 4, 19, 143, and especially 336).
This section discusses aspects of visual performance that are, or are likely to be, modified by space travel. For more general information on vision, consult the references provided in Paragraph 4.1.
4.2.2 Vision Design Considerations
Space-related factors that may affect visual perception as listed below.
a. Acceleration - The effects of acceleration on vision depend on the direction of the force vector.
(Refer to Paragraph 5.3, Acceleration, for additional information on the affects of acceleration.)
1. +Gz acceleration (eyeballs down) results in dimming of vision, followed by tunnel vision loss of sight which begins on the periphery and gradually narrows down until only macular (central) vision remains. This is followed by total blackout and then loss of consciousness.
2. +Gx acceleration (eyeballs in) results in loss of peripheral vision. This typically occurs at slightly over 4-G (based on a rate of onset of 1-G per second). Complete loss of vision varies between individuals, and with physical conditioning, training, and experience.
3. -Gz acceleration (eyeballs up) results in diminished vision, red-out (red vision), an increase in the time for the eyes to accommodate, and a blurring or doubling of vision.
4. When exposed to -Gx acceleration (eyeballs out), crewmembers will experience visual symptoms associated with -Gz acceleration (see 3" above).
5. Visual reaction time may be defined as the interval between the onset of a stimulus and the initiation of the crewmember's response. This interval is, in general, lengthened by increased G level.
6. Visual tracking is moderately degraded by increased G level.
b. Vibration - If vibration is sufficiently severe, visual performance will be degraded. The severity depends on the frequency and amplitude of the vibration along with the resonance frequency of the body part involved. Unfortunately, the times when vibration is most likely to be encountered (e.g., liftoff and landing) also tend to be times when vision is important. Displays that must be read during projected periods of high vibration should be designed accordingly. Design techniques to be considered should include display characters which are sufficiently large to be perceived even when blurred and sufficient illumination to avoid scotopic vision which results in a lower Critical Flicker Fusion Frequency.
(Refer to Paragraph 126.96.36.199, Human Response to Vibration, and reference 19 for additional information.)
c. Light in Space - Differences in light transmission and reflectance in space result in some significant differences in available perceptual cues in the extravehicular environment as compared to earth atmosphere.
1. Light Scatter - Atmospheric light scatter does not exist in space due to the lack of particulate and gaseous material. Thus, aerial perspective cues are absent. Figure-ground contrast is increased and shadows appear darker and more clearly defined. Loss of these cues along with other environmental consequences discussed below can degrade perception of object shape, distance, location and relative motion.
2. Luminance Range (Contrast) - The extravehicular environment is marked by a wider range of light intensities than normally encountered on Earth. Shifting gaze from a brighter to a substantially dimmer scene will require time for the eyes to adapt to the lower light level. For example, problems arise on EVA missions when crewmembers go from working in sunlight to working in shadows.
In Figure 4.2.2-1, adaptation time requirements are shown for shifting gaze from a brighter to a dimmer environment. For comparison, Figure 4.2.2-2 indicates luminance values for some typical visual stimuli.
Figure 4.2.2-1 Dark Adaptation Thresholds
Reference: 338, p. 187 NASA-STD-3000
Figure 4.2.2-2 Luminance Values for Typical Visual Stimuli
|Example||Scale of Luminance (millilamberts)||Effect|
|Sun's Surface at noon||10 9||Damaging|
|Tungsten Filament||10 6|
|White paper in sunlight||10 4|
|White paper in moonlight||10 -2|
|White paper in starlight||10 -4|
|Absolute threshold||10 -6|
Reference: 337, p. 26 NASA-STD-3000 194
d. Absence of Other Earth Cues:
1. Absence of a Fixed Vertical Orientation - Recognition of familiar objects, faces, and areas (e.g., workstation) is poor when viewed from an orientation significantly different from the established vertical. The viewer must be oriented within approximately 45 degrees of this vertical to perceive the surroundings in a relatively normal fashion. This fact argues for the establishment of a local vertical for each living and working area within a space module.
2. Absence of Fixed Horizon and its accompanying foreground and background cues can be expected to degrade extravehicular perception of object shape, distance, location and relative motion.
3. Absence of a fixed, overhead sun position and its effects on shadow cues is expected to have similar effects as those in 2 (above).
e. Light Flashes - The perception of light flashes has been reported by many crewmembers during periods of darkness at specific orbital locations. The cause is thought to be cosmic rays and/or heavy-particle radiation traversing the head or eyes and triggering a neural response that results in these perceptions.
f. Potential deficits - While visual perception in space is normal in many respects, there are reports of various changes in vision (some of them contradictory) that point out the complex consequences of the above factors. These include Soviet reports of a shift in perceived colors and a reduction in contrast sensitivity, along with a seemingly contradictory report indicating improved visual acuity for distant objects. Some U.S. astronauts have indicated a reduction in near acuity with no apparent change in far acuity, while some crewmembers who wear reading glasses on Earth found they were more dependent on them while in space. Clearly, more research is needed before we can say more about these effects.
4.3 AUDITORY SYSTEM
There is no evidence that human auditory functioning changes in space. However, there are several factors (e.g., the effects of noise) that should be considered in designing the space habitat.
(Requirements pertaining to acceptable noise levels are described in Paragraph 5.4.3, Acoustic Design Requirements.)
4.3.2 Auditory System Design Considerations
188.8.131.52 Auditory Response
Figure 184.108.40.206-1 shows human auditory responses as a function of frequency.
Figure 220.127.116.11-1 Human Auditory Response as a Function of Frequency
|Human Auditory Response||Decibels (db)|
|Threshold of pain||140|
|Subway - local station with express passing||120|
|Average factory, large store, or noisy office||80|
|Low whisper 1.5 m (5 ft)||20|
Reference: 335 NASA-STD-3000 195
c. Communication - Even low levels of noise can interfere with communication.
d. Task Complexity - Noise can adversely affect performance, with the effects being greater for more complex asks.
e. Intermittent Noise - Intermittent noise has more adverse effects than steady-state noise.
f. Adverse Effects - General effects of a noisy environment include fatigue, distractibility, sleep disturbance, irritation, and aggressive behavior.
g. Psychological Factors - The level of annoyance that noise produces depends on a number of factors. Sensitivity varies greatly among individuals.
1. People are generally less sensitive to noise related to their well-being.
2. People are more sensitive to unpredictable noise.
3. People are more sensitive to noise they feel is unnecessary.
4. People who are most sensitive to noise become increasingly disturbed as the noise persists, whereas the annoyance level of less sensitive individuals remains constant over time.
5. The perceived abrasiveness of certain sounds is subjective and varies considerably among individuals (e.g., consider the potential conflict between opera and rock music lovers).
h. Cabin Pressure - Reduced cabin pressure causes a reduction in sound transmission. This means that crewmembers have to talk louder to be heard which can potentially lead to hoarseness on the part of some crewmembers. The problem becomes more noticeable as the distance between individuals increases.
(Refer to Paragraph 5.4, Acoustics, for additional information.)
18.104.22.168 Noise Design Considerations
Noise can have many adverse effects on humans and must be considered when designing the human habitat. Considerations include:
a. Extreme Noise - Extreme noise can cause pain and temporary or permanent hearing loss. The adverse effects of pure tones occur at a level about 10 dB lower than for broad band noise.
b. Extended Exposure - Exposure to loud noise for extended periods of time can cause permanent hearing loss. The degree of exposure that will result in damage depends on intensity and individual susceptibility.
4.4 OLFACTION AND TASTE
Changes in our senses of smell and taste might occur in space. These changes are described below.
4.4.2 Olfaction and Taste Design Considerations
Aspects of olfaction (smell) that could influence design are presented below.
a. Decreased Sensitivity - There are frequently reported problems with nasal congestion while living in the microgravity environment.
b. Adverse Effects - Unpleasant odors have been associated with a number of medical symptoms including nausea, sinus congestion, headaches, and coughing. Such odors also contribute to general annoyance.
c. Microgravity Odors - Because particulate matter does not settle out in a weightless environment, odor problems in a space habitat may be more severe than under similar Earth conditions. Circulation and filtering will influence the extent of the problem.
d. Visual Cues and Odors - Responses to odors can be accentuated by the presence of visual cues. This increased responsiveness applies to pleasant and unpleasant odors and is something that a designer could potentially put to good use.
Generally there is a decrement in the sense of taste in microgravity. This is probably caused by the upward shift of body fluids and accompanying nasal congestion. Reports indicate that food judged to be adequately seasoned prior to flight tasted bland in space. Given the important role that food is likely to play in maintaining morale on extended space missions, attention should be paid to this problem.
4.5 VESTIBULAR SYSTEM
Microgravity results in two categories of vestibular side effects: spatial disorientation and space adaptation syndrome (space sickness), both of which can impair crewmember performance.
4.5.2 Vestibular System Design Considerations
22.214.171.124 Spatial Disorientation
Spatial disorientation is experienced by some crewmembers and should be considered in the design of hardware and the planning of missions.
a. Spatial Disorientation - Responses include postural and movement illusions and vertigo. For example, stationary crewmembers may feel that they are tumbling or spinning. These illusions occur with the eyes open or closed.
b. Frequency of Occurrence - The percentage of crewmembers who experience spatial disorientation varies from mission to mission, but averages approximately 50%. The conditions that determine the likelihood and intensity of this disorientation are not well understood.
c. Duration - Some crewmembers may experience spatial disorientation for the first 2 to 4 days of a mission.
d. Activity Schedule - While spatial disorientation need not cause any serious problems, it is advisable not to schedule activities that depend heavily on spatial orientation early in a mission.
126.96.36.199 Space Adaptation Syndrome
Aspects of space adaptation syndrome (SAS) relevant to the design of space modules and mission planning are presented below.
a. Symptoms - SAS symptoms range from stomach awareness and nausea to repeated vomiting. Symptoms also include pallor and sweating.
b. Incidence and Duration - It appears that approximately 50% of the crewmembers are affected by SAS. Symptoms last for the first 2 to 4 days of flight.
c. Performance Decrements - A highly motivated crewmember may be able to maintain a high level of performance despite the presence of mild SAS. However, if motion sickness is severe, some crewmembers will be unable to work until the symptoms lessen.
d. Cause - The leading theory as to the cause of SAS is the sensory conflict theory. This theory states that space sickness occurs when patterns of sensory input to the brain from different senses (vestibular, other proprioceptive input, vision) are markedly rearranged, at variance with each other, or differ substantially from expectations.
e. Volume Effects - The severity of SAS tends to increase as the motion which induces sensory conflict and sickness (particularly head movements in the pitch and roll modes) increases. It follows then that as the volume in which a crewmember is working becomes larger, the chances for this sickness inducing motion increases.
f. Space and Motion Sickness - It is assumed that the mechanism of SAS and 1-G motion sickness are similar, but are similar, but it is not possible to predict an individual's susceptibility to space sickness from their susceptibility to Earth motion sickness.
g. Space Sickness Countermeasures.
1. Drugs - Anti-motion sickness pharmaceuticals (usually Scopedex) have reduced the severity of SAS symptoms for some crewmembers, but have appeared to be ineffective for others. It is likely that they would be more universally effective if they were administered prophylactically, either by injection or orally. The drug should be taken before symptoms develop and absorption from the gut is severely hampered due to the cessation of propulsive motions of the stomach., If a swallowed drug becomes trapped in the stomach, little absorption will take place.
2. Head movements - In some cases restricting head movements has been found effective in reducing the incidence of, and ameliorating the symptoms of, space motion sickness.
Kinesthesia is the sense mediated by end organs located in muscles, tendons, and joints, and stimulated by body movements and tensions. Present knowledge of kinesthetic changes occurring when one enters microgravity is limited to estimation of mass and limb position sense.
4.6.2 Kinesthetic Design Considerations
One experiment has indicated that some kinesthetic sensitivity degradation occurs for a few crewmembers. The indications of this experiment are provided below.
a. Mass Versus Weight - In a weightless environment, increments in mass must be at least twice as large as weight increments in a 1-G environment before they can be discriminated (see Figure 4.6.2-1).
b. Barely Noticeable Differences - For two masses to be perceived as different under microgravity conditions, they must differ by at least 10% (see Figure 4.6.2-1).
c. Mass and Acceleration - Differential sensitivity for mass under microgravity conditions can be improved by increasing the acceleration force imposed on the object.
d. Mass Estimation - Absolute judgments of mass tend to be lower under microgravity than under 1-G.
Figure 4.6.2-1 Mean Difference Thresholds (DL) and Associated Standard Deviations (SD) Plotted for Each Standard Under Both Weight and Mass Conditions.
Reference: 78, p. 1-90 NASA-STD-3000 196
4.7 REACTION TIME
There appears to be some slowing of reaction times in space, although little precise data are available.
The subject of this Section is Response Time. This time period consists of two phases: 1) Reaction Time which is the time between the presentation of a stimulus to a subject and the beginning of the response to that stimulus, and 2) the time during which the actual response to the stimulus is accomplished. It is believed that this section is actually referring to Response Time and the titles and references should be changed accordingly. However, Reaction Time should not be slowed in micro-gravity as it has more to do with motivation and the effects of microgravity on the subject's physiological and emotional states. A good definition of the difference between Response Time and Reaction Time would help in the solution of this dilemma.
4.7.2 Reaction Time Design Considerations
Information on reaction time that should be considered by designers is provided below.
a. Object Mass - The time required to move an object in microgravity increases as the mass of the object increases.
b. Control Operation - In microgravity, the speed of operating switches (pushbuttons, toggles, rotary switches) is significantly lower than in the 1-G condition.
(Refer to Reference 171 for more information on visual reaction times and Reference 347, for 1-G muscular-reaction time information.)
4.8 MOTOR SKILLS (Coordination)
There is a minor impairment of motor skills upon first entering microgravity. This decrement is reduced or eliminated after a short period of adaptation.
4.8.2 Motor Skills (Coordination) Design Considerations
Aspects of human motor skills in space that should be considered by individuals designing for space are provided below.
a. Adaptation Period - Motor skills are somewhat affected when crewmembers are first exposed to microgravity, although these effects tend to diminish or disappear with adaptation. During the period that the crew is adapting to microgravity, fine motor movements are more adversely affected than either medium or gross motor movements. Designers should minimize requirements for crewmembers to exercise fine motor control early in the mission. Switches should be easy to manipulate and care should be taken to preclude accidental activation.
During periods that motor coordination is adapted for the micro-g environment, short returns to an altered g-state (as in reentry, maneuvers, landings, etc.) may result in dyskinesia and dysmetria. This can cause undershooting when reaching for switches for buttons or applying force to control sticks, pedals, knobs, handles, etc.
b. Postural Changes - A change in body posture in microgravity results in a change in the relative position of body parts and can cause decrements in coordination until adaptation occurs. Changes in body posture result from the crewmembers assuming the increase in height due primarily to spinal column expansion.
Refer to Section 3, Anthropometric and Mobility, for additional information on microgravity posture.
c. Body Part Weight - When moving in microgravity, the muscular system does not have to compensate for the weight of body parts. This changes the muscular forces required for coordinated movement and requires the system to readapt.
d. Large Mass Handling - When properly planned, no difficulty has been encountered by crewmembers in moving large masses in a microgravity environment.
Physical work can be divided into two parts: power and endurance (anaerobic and aerobic performance).
The next section addresses the first of these (power), and how it can influence the design of facilities and equipment to achieve optimal crewmember performance. (Endurance is addressed in Paragraph 4.10.2a).
4.9.2 Strength Design Considerations
Aspects of human strength that should be understood and considered in designing for the space environment are presented below.
a. Strength - Strength is the ability to generate muscular tension and to apply it to an external object through the skeletal lever system. Sheer muscle mass (thus, body size) is a significant factor, with cross-sectional area of the muscle fibers being a major determinant of the maximum force that can be generated. Maximum muscular force (strength) can be exerted for only a few seconds.
b. Muscular Endurance - Muscular endurance is the duration a submaximal force may be held in a fixed position (Isometric), or the number of times a movement requiring a submaximal force may be repeated (Isotonic). The duration that a fixed percentage of maximum can be held is reasonably constant across individuals.
c. Counterforces - Microgravity does not have certain counterforces that allow people to effectively perform physical work in 1-G. Traction which depends on body weight is absent, as are forces that result from using body weight for counterbalance.
d. Working While Restrained - Crewmembers' work capabilities while restrained can approach the efficiency experienced on Earth-based tasks, but only where workstation design (including fixed and loose equipment) and task procedures are optimized for the microgravity environment.
e. Working Without Restraints - Without proper restraints, a crewmember's work capabilities will generally be reduced and the time to complete tasks increased.
f. Improved Performance - There are situations where a crewmember can achieve improved strength performance in microgravity. These situations occur when the crewmember uses the greater maneuverability of microgravity to achieve a more efficient body position to be able to push off solid surfaces.
g. Deconditioning - Experience in space indicates that both the strength and aerobic power of load bearing muscles in crewmembers decreases during missions exposing them to microgravity. Exercise programs have been used to counter these deficits but to date have been only partially effective.
(Refer to Paragraph 7.2.3, Reduced Gravity Countermeasures, for information on maintaining strength in space.)
h. Kinematics - The linear motion of free-floating crewmembers can be described by relatively simple equations. The time crewmembers can exert force is governed by the distance they can push before losing physical contact. The force exerted during this time will typically vary as in Figure 4.9.2-1.
The important aspects of this curve are the impulse (Fdt, or the area under the curve), which will determine departure velocity and the peak force, which will determine peak acceleration. In the simplest case, for a subject of mass m, an impulse I with a peak force F acting through the subject's center of mass will result in a velocity
v = I /m where v is in ft/x, I is in lbfs, and m is in slugs or v is in m/x, I is in Ns, and m is in kg and a peak acceleration
a=F/m where a is in ft/s2, F is in lbf, and m is in slugs or a is in m/s2, F is in N, and m is in kg.
In reality, of course, an impulse will rarely go exactly through the center of mass to produce pure linear motion. For any offset of the force from the center of mass, a percentage of the impulse will go toward producing angular (tumbling) motion, with a corresponding decrease in linear velocity. This percentage depends on the offset distance and the subject's moment of inertia. (Moment of inertia varies considerably with body position, and so is difficult to analyze parametrically, but there will be some tumbling in practically all cases.)
Figure 4.9.2-1 Representation of Force Generated by Free-Floating Crewmember Pushing Off
Figure 4.9.2-2 shows the time that a particular force can be exerted as a function of the magnitude of the force exerted, the mass of the individual, and the distance pushed. The velocity that the crewmember will have as they lose contact with the surface is also given.
Figure 4.9.2-2 Force Application and Push-Off Velocity
Note: Please be aware that all of the above data was gathered under 1-g conditions.
Reference: 335 NASA-STD-3000 197
4.9.3 Strength Design Requirements
Strength data that shall be used to guide design work are provided below. The weakest crew member in the specified design population shall be accommodated.
(Refer to Reference 16 for additional data on 1-G strength.)
1. Grip strength, as a function of the size of the gripped object, is provided for men in Figure 4.9.3-1.
2. Maximum grip strength for men (5th, 50th, and 95th percentile) is given in Figure 4.9.3-2.
3. Grip strength for females is shown in Figure 4.9.3-3.
b. Arm, Hand, and Thumb/Finger Strength - Figure 4.9.3-4 presents arm, hand and thumb/finger strength for fifth percentile males. These figures must be corrected for females (see Figure 4.9.3-5).
c. Male/Female Muscular Strength - Figure 4.9.3-5 provides a comparison of male and female muscular strength for different muscle groups. These data allow a more accurate extrapolation from male to female strength data than is provided by the old method of assuming females have two thirds the strength of men.
(Refer to Reference 16 for more detailed male/female comparison data.)
d. Static Push Force - Maximal static push forces for adult males are shown in Figure 4.9.3-6. While these data were collected in a 1-G situation, the fact that they do not depend on friction resulting from body weight makes them applicable to microgravity. Corrections will have to be made for females (see Figure 4.9.3-5).
e. Leg Strength - Leg strength for the 5th percentile male as a function of various thigh and knee angles is reported in Figure 4.9.3-7. Estimates of female leg strength can be made from these data using the correction factors provided in Figure 4.9.3-5.
f. Torque Strength - Maximum hand torque data are provided in Figure 4.9.3-8.
Figure 4.9.3-1 Male Grip Strength as a Function of the Separation Between Grip Elements
Note: 44 subjects, all pilots or aviation cadets
Reference: 1, p. 2.5-19 NASA-STD-3000 200
Figure 4.9.3-2 Grip Strength for Males
Reference: 1, p. 2.5-18 NASA-STD-3000 201
Figure 4.9.3-3 Grip Strength for Females
Reference: 1, p. 2.5 - 18 NASA-STD-3000 202
Figure 4.9.3-4 Arm, Hand, and Thumb/Finger Strength (5 th Percentile Male Data)