Motion

1
Motion

• Main Topics
• Vibration
• Acceleration
• Illusions during Motion
• Motion Sickness

2
Two General Classes of Motions

• Volitional, Low-Speed
• Issues Concerned
• Non-Volitional, High-Speed
• Issues Concerned
• Tolerance
• Safety and protection
• Impact and acute effects
• Illusion
• Vibration a special entity

• Permanence (S-A trade-off)
• Strategy to minimize stress
• Accumulative effects (low force)
• Acute effect (forceful exertion)

3
Senses Related to Motion

• Sensory Receptors
• Exteroceptors
• Proprioceptors
• Semicircular Canals
• Vestibular Sacs
• postural/balance sensors
• (Figure 19-1)
• Conflict between visual perceptions and actual
  gravity

deal with stimuli external to the body
stimulated by bodys own actions
acceleration/deceleration sensors
4
Ways to Describe Vibration

• Type (Wave Form)
• Sinusoidal vs. Random
• Direction
• Forward-Backward
• Left-Right
• Up-Down
• Frequency
• Cycles per second
• Intensity

• Amplitude (Displacement)
• Velocity
• Acceleration
• Rate of acceleration change

5
Effects of Vibration on the Body

• 1. Transmission
• Attenuation
• Amplification
• Resonance

3-4 Hz Resonance in cervical (neck
vertebrae) 4 Hz Peak resonance in lumbar
(upper torso) vertebrae 5 Hz Resonance in
shoulder girdle 20-30 Hz Resonance between
head and shoulders 60-90 Hz Resonance in
eyeballs
6
Effects of Vibration

• 2. Physiological Effects
• Short-term exposure
• increased HR
• increased muscle tension
• urge to urinate
• chest pain
• Long-term exposure
• increased risk of disc herniation
• increase risk of low-back pain
• increased risk of Reynauds Syndrome or Traumatic
  Vasopastic Disease (TVD)

7
Effects of Vibration

• 3. Performance
• Visual Performance
• impaired by vibration of 10-25 Hz
• minor effect in low frequency range due to
  head/eye compensatory motion
• Motor Performance
• vertical sinusoidal vibration of 4-20 Hz most
  detrimental
• dependent on display and control
• Neural Process

• central neural processes (e.g., RT, pattern
  recognition) highly resistant to vibration effect
• tension in muscle increases vigilance

8
Subjective Responses Whole-Body Vibration

• Comfort scale
• mildly uncomfortable
• annoying
• very uncomfortable
• alarming
• Attempt to link frequency acceleration to
  comfort scales
• Equivalent-comfort contours
• Large inter-person variability makes design
  considerations challenging

9
Limits of Exposure to Whole-Body Vibration

• Criterion-based
• comfort, task performance, or physiological
  response
• ISO 2631
• most applicable for transportation and industrial
  type vibration exposures
• Fatigue-Decreased Proficiency (FDP)
• Figure 19-7, page 634

10
Limits of Exposure to Whole-Body Vibration

• Criticisms of FDP
• 1. Comfort and FDP limits for short exposures
  maybe too high
• 2. Appear to be based on mean results
• 3. Imply the effects of multiple single-axis
  vibrations are additive
• 4. Similar shaped contours are an
  oversimplification
• 5. Comfort contours may be inaccurate at extreme
  frequencies
• 6. Assumes time/intensity trade-off with little
  support

11
Control of Vibration

• Source Control
• Reduce intensity
• Avoid resonance
• Provide tool balancing
• Use non- or less vibratory tools
• Path Control
• Provide rest period
• Reduce transmission (attenuate)
• Use isolator
• Receiver Control

• Use isolating or damping apparatus
• Adopt more resistant postures
• Reduce grip force
• Reduce contact area

12
Acceleration

• Terminology
• 1. Acceleration
• Rate of change of motion
• 2. Linear acceleration
• Rate of change of velocity
• 3. Rotational acceleration
• Rate of change of direction
• Radial (centrifugal) acceleration
• Angular (tangential) acceleration
• Nystagmus

involuntary oscillatory movement of the eyeball
13
Acceleration

• Direction (Figure 19-8)
• 1. X-Axis Forward/Backward
• 2. Y-Axis Left/Right
• 3. Z-Axis Headward/Footward
• Follows right-hand rule (RHL)

Look at motion of the eyeballs to determine the
direction of acceleration Eyeballs go opposite
of acceleration, and same direction as
deceleration
14
Acceleration

• 4. Tumbling
• 5. Spinning

Head over heels
Around main body axis - spiral nose dive - forces
alternate /-
15
Acceleration Duration

• Sustained
• Abrupt

Begins at 2/10 second and continues Effects
are primarily physiological
Shorter acceleration, less than 2/10
second Mainly effects of impact or
rapid deceleration Effects are primarily physical
16
Acceleration Duration

• Three Categories
• Short
• Intermediate
• Long

- less than 1 second - impact or acute effect
- 1/2 to 2 second duration - very abrupt
- greater than 2 seconds through several minutes
17
Methods of Study

• Tracks
• Centrifuges
• Suicides/Accidents

• Usually acceleration/deceleration studies
  performed on tracks
• Slide/ejection tests in impact laboratories

• Help to study the effect of non-linear
  acceleration
• Rotary chairs or vehicles

• Haven - Golden Gate Bridge and Brooklyn Bridge
• Real field studies if caught on tape
• Reconstruction or simulation

18
Resulting Forces on the Body
-Gz Eyeballs Up
-Gy Eyeballs Right
Gx Eyeballs In
-Gx Eyeballs Out
Gy Eyeballs Left
Gz Eyeballs Down
19
Effects of Directional Forces

• Effects of Gz (Figure 19-9)
• Acceleration headward
• Increase in weight drooping of face and soft
  tissues
• Difficult or impossible to raise oneself
• Blackout loss of consciousness
• Cardiac output and stroke volume decrease while
  HR, aortic pressure, and vascular resistance
  increase
• Maximum Tolerance 16 G

20
Effects of Directional Forces

• Effects of -Gz
• Acceleration footward
• facial congestion
• headache
• blurring, graying, reddening of vision
• Limit at -5 G is about 5 s
• Maximum Tolerance 10 G

21
Effects of Directional Forces

• Effects of Gx (Fig 19-10, -11)
• Acceleration sternumward
• Speech difficult
• Progressive tightness and pain in chest
• Difficulty in lifting body parts
• Blurring of vision
• Dyspnea
• Maximum Tolerance 30 G

22
Effects of Directional Forces

• Effects of -Gx
• Acceleration spineward
• Effects the opposite of Gx
• Tolerance 30G

23
Effects of Directional Forces

• Effects of /- Gy
• little information on these effects
• mainly encountered in an aircraft
• magnitude is relatively small compared to other
  directions
• less common in occupational settings

24
Deceleration (Impact)

• Exposures less than 2/10 second
• Extremely abrupt
• Reverse acceleration
• Mainly in forward/backward direction

25
Deceleration (Impact)
Magnitude
26
Deceleration (Impact)

• Factors affecting the impact of an impact
• Rate of Onset
• Peak G
• Stopping Distance
• Angle of Impact

27
Tolerance

• Survivable
• limit around 30-40 Gs
• can only endure for 0.25 seconds
• Injury
• Death

60 G with rate of onset 5000 G/sec
200 G with rate of onset 5000 G/sec
28
Protection

• 1. Restraining Devices
• seat belt
• 2. Absorbing Devices
• air bag
• 3. Special Contoured Seats
• secondary collision minimized
• 4. Body Posture
• direction-dependent stiffness or resistance
• 5. Water Immersion
• damping
• 6. Anti-G Suits
• can take up to 9 G

29
Weightlessness

• Two Aspects
• absence of weight itself
• tractionless condition
• Both remain to be fully investigated
• Physiological Effects
• Performance Effects

• space sickness (space adaptation syndrome)
• anthropometric change height growth 3
• relaxed posture assumed
• (Figure 19-13)

• exhaustion due to the added third dimension in
  locomotion

30
Illusions During Motion

• Human senses are not designed for extremely
  dynamic motions and unusual, prolonged forces
  encountered in special settings
• Disorientation from False Sensations (due to
  inaccurate sensory information)
• disrupted vestibular-visual coordination
  illusion of spinning in opposite direction
• Coriolis illusion illusion of roll during
  turning or circling motion

• oculogravic illusion impression of tilt during a
  sudden increase of forward speed

31
Illusions During Motion

• Disorientation from Misperception
• (due to brains misinterpretation or
  misclassification of accurate sensory information
• Autokinesis
• fixed light appears to be moving against a dark
  background

32
Motion Sickness

• Cause incongruities among senses

Vestibular (inner ear tubes)
33
Motion Sickness

• Head Symptoms
• drowsiness
• general apathy
• Gut Symptoms
• nausea
• vomiting
• Sensory Rearrangement Theory
• sensory systems provide contradictory information
• Simulator Sickness
• exact cause unknown