Chapter%203:Physical%20Ergonomics

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Chapter 3Physical Ergonomics
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Human Activity Continuum

• As a result of activity, the body undergoes
  various changes
• Physiological
• Psychological
• Cognitive
• Etc.
• Stress and strain
• Stress some undesirable condition,
  circumstance, task, or other factor that impinges
  upon the individual (outside effector)
• Examples heavy work, immobilization, heat and
  cold, sleep loss, etc.
• Strain refers to the effects of stress on the
  individual
• Examples blood chemistry, oxygen consumption,
  electrical activity of the muscles or the brain,
  heart rate, work rate, errors, attitudes
• People vary on what creates their stress and the
  level of stain associated with stress

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Human Activity Continuum
Dynamic activity
Sensory functions
Static loading
Thinking calculating
Strain measures
HR O2 consumption HR recovery
EMG
EOG
HRV EEG Dual-task Subjective ratings ex NASA
TLX
Abbreviations EMG - electromyogram EOG
electroculogram HRV heart rate variability EEG
- electroencephalogram
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Stress and Performance

• Stress does not always lead to ill effects
• Sources of job stress
• Workplace arrangement (dimensions, relative
  location, etc.)
• Work content (speed and accuracy requirements,
  etc.)
• Work organization (work-rest cycles, start-stop
  times, work control, etc.)
• Work environment (co-workers, supervisor, heat,
  cold, fear of job loss, etc.)

Peak Performance
Performance
Overload
Boredom
Stress
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Muscular-Skeletal System

• Movement Terminology
• Flexion decreasing joint angle
• Extension increasing joint angle
• Abduction away from the body
• Adduction towards the body
• Pronation inward rotation
• Supination outward rotation
• Planes of Reference
• Sagittal vertical plane dividing left and right
  half of body
• Lateral (frontal) vertical plane dividing front
  and back of body
• Horizontal (traverse) horizontal plane dividing
  top and bottom of body

Sagittal
Lateral (frontal)
Horizontal (traverse)
YZ
XZ
XY
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Muscular-Skeletal System

• Skeletal System
• 206 bones establish framework
• Give body structure
• Provide protection to vital organs (e.g., skull,
  ribs)
• Basis for activity such as bones in arms, legs,
  etc. Levers that produce movement
• Bones are connected to each other via ligaments
• Bones connected to muscles via tendons
• Produce elements of blood within bone marrow
• Store calcium and phosphorus

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Muscular-Skeletal System

• Bones - specialized form of connective tissue
• Composition
• Calcium carbonate/calcium phosphate (60-70)
• Stiffness (stress/strain)
• Compressive strength
• Collagen (protein)
• Flexibility, tensile strength
• Decrease with age
• Water (25-30)
• Contributes to strength
• Connective Tissue in Skeletal System
• Cartilage firm yet elastic and flexible, rapid
  growth and handles moderate stress
• End of ribs (expansion)
• Spinal disks (cushion)
• Accounts for bone growth at ends
• Hardens when fully developed

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Muscular-Skeletal System

• Bones connected to each other at joints
• Hinge joints (wrist)
• Pivot joints (elbow)
• Ball and socket joints (shoulder, hip)
• Joints
• Fibrous no movement (e.g., skull)
• Cartilaginous limited range of movement (e.g.,
  spinal column)
• Synovial large range of movement
• Fluid cavity to lubricate (e.g., knee)

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Muscular-Skeletal System

• Type of Movement (joints)
• Gliding or angular (elbow, knee)
• One dimensional
• flexion/extension, abduction/adduction
• Circumdunction (shoulder, hip, wrist)
• Two dimensional, no rotation
• Rotation (shoulder, hip, forearm)
• Two dimensional about longitudinal axis
• Limiting Factors for Range of Motion
• Tension of ligaments elbow or knee extension
• Tension of antagonistic muscles flex/stretch
  with extended knee
• Contact of soft tissue flex hip with knee
  relaxed, elbow or knee flex

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Muscular-Skeletal System

• Mobility study by Staff (1983)
• 32 major body joints
• Electric bubble goniometer used to measure range
  of motion
• Females exhibited significantly greater mobility
  except in ankle flexion and wrist abduction
• Bone development
• Change shape, size, and structure depending on
  mechanical demands
• Wolffs law bone deposited where needed and
  resorbed where not needed

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Muscular-Skeletal System

• Muscles major prerequisite for human activity
• 600 muscles in the body 80 account for most
  vigorous activity
• 3 types of muscles
• Striated (skeletal)
• Cardiac (heart)
• Smooth (walls of blood vessels and internal
  organs)
• Skeletal is the most concern to ergonomists
• Composed of bundled muscle fibers (length range
  from 0.2 to 5.5 inches, but some are as large as
  12 inches
• Mechanical leverage muscle applies forces on
  the bone(s) to which it is attached
• Maximum force in extended state little force in
  contracted state
• Muscle generates mechanical work by converting
  chemical energy into mechanical energy

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Muscular-Skeletal System

• Muscles (cont)
• Three thin filaments
• Actin (primary filament)
• Troponin
• Tropomyosin B proteins
• Myosin (thick filament) slides over to bring
  about contraction
• Figure 3.3 Pulat
• Antogonist Pairs
• Can only contract and pull, cannot push
• For each motion, there is a muscle to perform
  opposite motion
• Act as break to slow motion and prevent damage
• Precise motor movement
• Example Bicep
• flexion (bicep contract tricep inhibit)
• extension (tricep contract bicep inhibit)

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Muscular-Skeletal System

• Physiological Characteristics
• Irritability (excitability) react to stimuli
  (electrical stimulation)
• Chemical reaction creates muscle contraction
• Contractility increase tension
• Shorter and thicker
• Extensibility stretched beyond resting length
• Requires antagonist or gravity force
• Elasticity return to resting length

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Muscular Skeletal System

• Classification of muscle contraction
• Isometric no change in muscle length
• No physical work performed
• Tension usually constant
• Concentric decreasing muscle length
• Positive work
• Acceleration of limb during movement
• Tension decreases
• Eccentric increasing muscle length
• Negative work
• Deceleration of limb
• Tension increases
• Isotonic applied force is constant
• Rare in practice

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Muscular Tension

• Length of muscle
• Maximum tension occurs at resting length (or
  slightly longer)
• All active myosin sites lined up with actin
  attachment sites
• Joint angle changes length

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Muscles

• Velocity of Contraction
• Maximum velocity at zero tension
• Maximum force at zero velocity
• Cross sectional area
• Max force (0.3-0.4 N/mm2)
• Only gender difference is cross-sectional area
• Women narrower muscle
• Women 2/3 force of men
• Electrical Process of muscle
• Resting potential of muscle fiber
• 90 mV with inside negatively charge relative to
  exterior
• Due to imbalance of ions
• Action Potential is reversal of resting potential
• Positive charge applied (depolarization)
• Lasts 2-4 msec, speed 5 m/s
• Refractory period is where muscle has decreased
  ion permeability
• 1-3 msec after action potential

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Physiological Strain-Basic Physics Concepts

• Force a unit of force is a newton (N) 1
  kg-m/s2
• 1 N 0.225 lbf (pounds force)
• Work or Energy work is done or energy is
  consumed when a force is applied over a distance
• Measures
• 1 N x 1 m 1 J (joule)
• Kilocalorie (kcal) amount of heat required to
  raise the temperature of 1 kg of water from 15
  degrees Celsius to 16 degrees Celsius
• The Calorie which is used for energy content of
  food is actually a kilocalorie
• 1 kcal 1000 cal 1 Cal (food)
• 1 kJ 1000 J
• 1 kcal 4.1868 kJ
• 1 kcal 3087.4 ft lbs
• Power work per unit time
• Measures
• Watt (W) 1 J/s
• Horsepower (hp) 736 W

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Muscular Activity

• Metabolism
• Supplies the energy needed to slide the actin
  filaments over the myosin filaments. It is a
  chemical process of converting food into
  mechanical work and heat.
• Some mechanical work is consumed by the body
  while other is consumed by physical activity
• Basic source of energy for contraction of the
  muscle is glycogen or glucose which is abundant
  in the blood
• Sources of energy (next time)