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Motion

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

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  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 body’s 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 Reynaud’s 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. Duration Rate of Onset G t0 t1 t2 t3 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 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

  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 brain’s misinterpretation or misclassification of accurate sensory information • Autokinesis: • fixed light appears to be moving against a dark background

  32. Eyes Kinesthetic (body position) 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

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