Motion

Motion

Main Topics

• Vibration

• Acceleration

• Illusions during Motion

• Motion Sickness

Motion

Two General Classes of Motions

Volitional, Low-SpeedIssues Concerned:

Non-Volitional, High-SpeedIssues 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)

Motion

Senses Related to Motion

Sensory ReceptorsExteroceptors

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

Motion

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

Motion

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) vertebrae5 Hz Resonance in shoulder girdle20-30 Hz Resonance between head and shoulders60-90 Hz Resonance in eyeballs

Motion

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)

Motion

Effects of Vibration

3. PerformanceVisual 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

Motion

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

Motion

Limits of Exposure to Whole-Body Vibration

Criterion-basedcomfort, task performance, or

physiological response

ISO 2631most applicable for transportation

and industrial type vibration exposures

Fatigue-Decreased Proficiency (FDP)

Figure 19-7, page 634

Motion

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

Limits of Exposure to Whole-Body Vibration

Motion

Control of VibrationSource 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

Motion

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

Motion

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

Motion

4. Tumbling

5. Spinning

Acceleration

Head over heels

Around main body axis- spiral nose dive- forces alternate +/-

Motion

Acceleration Duration

Sustained

Abrupt

Begins at 2/10 second and continues

Effects are primarily physiological

Shorter acceleration, less than2/10 second

Mainly effects of impact or rapiddeceleration

Effects are primarily physical

Motion

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

Motion

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

Motion

-Gz “Eyeballs Up”

-Gy “Eyeballs Right”

+Gx“Eyeballs In”

-Gx“Eyeballs Out”

+Gz“Eyeballs Down”

+Gy“Eyeballs Left”

Resulting Forces on the Body

Motion

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

Motion

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

Motion

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

Motion

Effects of -Gx

Acceleration spineward

Effects the opposite of +Gx

Tolerance = ~30G

Effects of Directional Forces

Motion

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

Effects of Directional Forces

Motion

Exposures less than 2/10 second

Extremely abrupt

Reverse acceleration

Mainly in forward/backward direction

Deceleration (Impact)

Motion

Rate ofOnset

Duration

t0 t1 t2 t3

G

Deceleration (Impact)

Magnitude

Motion

Factors affecting the impact of an impact

Rate of Onset

Peak G

Stopping Distance

Angle of Impact

Deceleration (Impact)

Motion

Tolerance

Survivablelimit 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

Motion

Protection

1. Restraining Devices seat belt

2. Absorbing Devicesair bag

3. Special Contoured Seatssecondary collision minimized

4. Body Posturedirection-dependent stiffness or

resistance

5. Water Immersiondamping

6. Anti-G Suitscan take up to 9 G

Motion

WeightlessnessTwo 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

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

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

Illusions During Motion

Motion

Motion Sickness

Kinesthetic(body position)

Vestibular(inner eartubes)

Eyes

Cause: incongruities among senses

Motion

Motion Sickness

Head Symptoms– drowsiness

– general apathy

Gut Symptoms– nausea

– vomiting

Sensory Rearrangement Theory– sensory systems provide

contradictory information

Simulator Sickness– exact cause unknown