Chapter 9
Guidelines
for
Seated
Work
Sitting: weight of the body
transferred to supporting area
• Main Contact points==> seat
•Ischial tuberosities
•Soft tissues
• Secondary contact points==>other
•feet to floor/chair
•back to backrest (trunk)
•arms to armrest
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
Back Anatomy Review
Figure 9.2 from text
http://www.scoi.com/spinanat.htm
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•
•
•
•
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
Vertebra Structures
Vertebral Column Anatomy
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
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
Middle Position
• CofM directly above IT
• Floor supports about 25% of TBW
• Spine in neutral position or, if relaxed, in
slight kyphosis (round back)
Anterior Position
(forward leaning)
• CofM in front of IT
• Floor supports > 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
Posterior Position
(backward leaning)
• CofM behind IT
• Floor supports < 25% of TBW
• backward rotation of the pelvis
and simultaneous kyphosis of
the spine
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
A good chair
Functionally adapts to
the occupant
and
the task
The Occupant
Anthropometric Measures for
Seated Work
Anthropometric
Measures:
Seated Work
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
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
The Spine and Sitting
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
Disc Pressure Measures
Disc loading reflects
•Posture
•Supported load
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
Testing effect of lumbar
support & backrest
Two factor
repeated
measures
ANOVA
Backrest
inclination
effects on
muscle
activity
Interaction of backrest
inclination and lumbar support
Use of a
lumbar
support more
important
than backrest
inclination
Lumbar Support
Adjustable Lumbar Support
Maintaining
Lumbar
lordosis
Pantagraph, 11/12/99
Functional backrest
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
Task effects on disk pressure
Disc loading reflects
•Posture
•Supported load
•Task
•Environment
Task effects on disk pressure
• Writing at a desk
• decrease noted compared with other tasks
• arms ==> trunk supported by the desk
• Typing and lifting a phone
• increased pressure
• larger external load moments
Leaning forward unsupported
increases disc pressure
The Legs
and Sitting
• Foot support affects circulation
• Pressure applied close on thighs close to
the knees
• leg swelling ==> pressure on sciatic nerve
• should not feel edges of the chair
• Semi-seated position ==> eases transition to
standing but increased lower extremity stress
The Legs and Sitting
• Seat height affects buttocks pressure
• Low seat height ==> too small contact area
Leg posture & pressure
• Crossing or not crossing legs.
Oborne, 1982
The Table (work surface)
• Desired field of vision
• affects neck & trunk flexion
• maintain < 15 degrees
• slanted work surface???
• Work surface height
• Top
• 3-4 cm above elbow level
• Bottom
• provide clearance for thighs (no standard
Interaction of
task &
environment
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”
Special Situations
Conflict of
mobility and
stability
Interaction of task &
environment
State
Farm
Workstation
Evaluation
SF cont’d
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