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

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Motion
Motion

  • Main Topics

    • Vibration

    • Acceleration

    • Illusions during Motion

    • Motion Sickness


Two general classes of motions
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)


Senses related to motion
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 body’s own actions

—acceleration/deceleration sensors


Ways to describe vibration
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


Effects of vibration on the body
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


  • Effects of vibration
    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 Reynaud’s Syndrome or Traumatic Vasopastic Disease (TVD)


    Effects of vibration1
    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


    Subjective responses whole body vibration
    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


    Limits of exposure to whole body vibration
    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


    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


    Control of vibration
    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


    Acceleration
    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


    Acceleration1
    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


    Acceleration

    • 4. Tumbling

    • 5. Spinning

    Head over heels

    • Around main body axis

      • - spiral nose dive

      • - forces alternate +/-


    Acceleration duration
    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


    Acceleration duration1
    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


    Methods of study
    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


    Resulting forces on the body
    Resulting Forces on the Body

    -Gz “Eyeballs Up”

    -Gy

    “Eyeballs Right”

    +Gx

    “Eyeballs In”

    -Gx

    “Eyeballs Out”

    +Gy

    “Eyeballs Left”

    +Gz

    “Eyeballs Down”


    Effects of directional forces
    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


    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


    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


    Effects of Directional Forces

    • Effects of -Gx

      • Acceleration spineward

      • Effects the opposite of +Gx

      • Tolerance = ~30G


    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


    Deceleration impact
    Deceleration (Impact)

    • Exposures less than 2/10 second

    • Extremely abrupt

    • Reverse acceleration

    • Mainly in forward/backward direction


    Deceleration impact1

    Duration

    Rate of

    Onset

    G

    t0

    t1

    t2

    t3

    Deceleration (Impact)

    Magnitude


    Deceleration (Impact)

    • Factors affecting the impact of an impact

    • Rate of Onset

    • Peak G

    • Stopping Distance

    • Angle of Impact


    Tolerance
    Tolerance

    • Survivable

      • limit around 30-40 G’s

      • 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


    Protection
    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


    Weightlessness
    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


    Illusions during motion
    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


    Illusions during motion1
    Illusions During Motion

    • Disorientation from Misperception

    • (due to brain’s misinterpretation or misclassification of accurate sensory information

      • Autokinesis:

        • fixed light appears to be moving against a dark background


    Motion sickness

    Eyes

    Kinesthetic

    (body position)

    Motion Sickness

    • Cause: incongruities among senses

    Vestibular

    (inner ear

    tubes)


    Motion sickness1
    Motion Sickness

    • Head Symptoms

      • drowsiness

      • general apathy

    • Gut Symptoms

      • nausea

      • vomiting

    • Sensory Rearrangement Theory

      • sensory systems provide contradictory information

    • Simulator Sickness

      • exact cause unknown


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