Guidelines

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Chapter 9

• Guidelines
• for
• Seated Work

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Sitting weight of the body transferred to
supporting area

• Main Contact pointsgt seat
• Ischial tuberosities
• Soft tissues
• Secondary contact pointsgtother
• feet to floor/chair
• back to backrest (trunk)
• arms to armrest

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Advantages of seated posture

• 1. Stability for tasks with fine motor movement
  or concentrated vision.
• 2. Less energy than standing.
• 3. Reduces stress on lower extremities.
• 4. Reduce hydrostatic pressure on legs

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Back Anatomy ReviewFigure 9.2 from
texthttp//www.scoi.com/spinanat.htm

• Cervical Lumbar are mobile
• Thoracic relatively immobile
• Three curves C(l), T(k), L(l)
• Vertebral bodies support weight
• Vertebral spines for muscle attachment
• Intervertebral disks nucleus annulus
• Flexibility Support

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Vertebra Structures
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Vertebral Column Anatomy
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Three Seated Postures

• Reflects the task and chair ( individual)
• Based on Center of Mass (CofM) location relative
  to the ischial tuberosities (IT)
• Affects TBW transferred to different support
  surfaces.
• Affects/reflects lumbar spine position
• Figure 9.3
• anterior, middle, posterior

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Three Seated Postures Classified

• Reflects the task and chair ( individual)
• Based on Center of Mass (CofM) location relative
  to the ischial tuberosities (IT)
• Affects TBW transferred to different support
  surfaces.
• Affects/reflects lumbar spine position
• Figure 9.3
• anterior, middle, posterior

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Middle Position

• CofM directly above IT
• Floor supports about 25 of TBW
• Spine in neutral position or, if relaxed, in
  slight kyphosis (round back)

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Anterior Position (forward leaning)

• CofM in front of IT
• Floor supports gt 25 of TBW
• Forward rotation of the pelvis with the spine in
  neutral position
• By assuming a slight kyphosis
• Little or no rotation of the pelvis but with
  large induced kyphosis of the spine

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Posterior Position(backward leaning)

• CofM behind IT
• Floor supports lt 25 of TBW
• backward rotation of the pelvis and simultaneous
  kyphosis of the spine

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Posture selected/exhibited reflects

• Individual seating habits
• Task demands
• Height inclination of chair seat
• Position, shape, inclination of backrest
• Presence of other types of support
• arm rests, footrest

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A good chair

• Functionally adapts to the occupant
• and
• the task

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Anthropometric Measures for Seated Work
The Occupant
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Anthropometric Measures Seated Work
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Comfort

• Remains an unexplored concept
• frequently defined as the absence of discomfort
• 5 minutes as good as 1/2 hour as good as 4 hours
  to subjectively rate comfort

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Determining Comfort

• Based on
• observe body posture movement
• observe task performance
• direct subjective ratings of general comfort
  using
• chair feature checklists
• body area comfort rankings
• general comfort rankings

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The Spine and Sitting
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Clinical aspects of seated posture

• Does extended sitting cause LBP?
• Research says Yes
• Research says No
• Seated in a vehicle Yes
• interaction
• lumbar posture
• vibration

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Disc Pressure Measures

• Disc loading reflects
• Posture
• Supported load

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Disc Pressure Measures

• Disc loading reflects
• Posture
• Supported load
• Task
• Environment
• Basis for the backless chairs where the user
  rests on the knees and elbows

Note low stress with reclining position
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Testing effect of lumbar support backrest
Two factor repeated measures ANOVA
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Backrest inclination effects on muscle activity
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Interaction of backrest inclination and lumbar
support
Use of a lumbar support more important than
backrest inclination
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Lumbar Support
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Adjustable Lumbar Support
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Maintaining Lumbar lordosis
Pantagraph, 11/12/99
Functional backrest
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To reduce disc pressure when seated

• Use lumbar supports
• support part of TBW
• maintains lumbar lordosis
• Increase backrest inclination
• increases load transferred to the backrest
• reduces muscle activity Use arm rests
• supports the weight of the arms/trunk

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Task effects on disk pressure

• Disc loading reflects
• Posture
• Supported load
• Task
• Environment

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Task effects on disk pressure

• Writing at a desk
• decrease noted compared with other tasks
• arms gt trunk supported by the desk
• Typing and lifting a phone
• increased pressure
• larger external load moments

Leaning forward unsupported increases disc
pressure
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The Legs and Sitting

• Foot support affects circulation
• Pressure applied close on thighs close to the
  knees
• leg swelling gt pressure on sciatic nerve
• should not feel edges of the chair
• Semi-seated position gt eases transition to
  standing but increased lower extremity stress

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The Legs and Sitting

• Seat height affects buttocks pressure
• Low seat height gt too small contact area

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The Table (work surface)

• Desired field of vision
• affects neck trunk flexion
• maintain lt 15 degrees
• slanted work surface???
• Work surface height
• Top
• 3-4 cm above elbow level
• Bottom
• provide clearance for thighs (no standard

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Interaction of task environment
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Clinical aspects of seated posture

• Avoid Postural Rigidity
• maintaining same position for long time
• Workers MUST be alerted to problems occurring
  from long-lasting low-level muscular efforts
• tension in same muscle groups
• loading of same soft tissues
• loss of muscle pump

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Special Situations
Conflict of mobility and stability
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Interaction of task environment
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Summary

• Maintain normal lordosis with minimal muscle
  activity to reduce back stress
• Proper backrest most important factor to reduce
  back stress
• Disk pressure reduced with lumbar support
• Feet should be supported
• Full contact with chair seat
• Alter seated posture every 15 minutes