Chapter 9 Guidelines for Seated Work. Sitting: weight of the body transferred to supporting area...
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Transcript of Chapter 9 Guidelines for Seated Work. Sitting: weight of the body transferred to supporting area...
Chapter 9
Guidelines for SeatedWork
Sitting: weight of the body transferred to supporting
area• Main Contact points==> seatMain Contact points==> seat
• Ischial tuberositiesIschial tuberosities• Soft tissuesSoft tissues
• Secondary contact Secondary contact points==>otherpoints==>other• feet to floor/chairfeet to floor/chair• back to backrest (trunk)back to backrest (trunk)• arms to armrestarms to armrest
Advantages of seated posture
1. Stability for tasks with fine motor 1. Stability for tasks with fine motor movement or concentrated vision.movement or concentrated vision.
2. Less energy than standing.2. Less energy than standing.
3. Reduces stress on lower 3. Reduces stress on lower extremities.extremities.
4. Reduce hydrostatic pressure on 4. Reduce hydrostatic pressure on legslegs
Back Anatomy ReviewFigure 9.2 from text
http://www.scoi.com/spinanat.htm• Cervical & Lumbar are mobileCervical & Lumbar are mobile• Thoracic relatively immobileThoracic relatively immobile• Three curves: C(l), T(k), L(l)Three curves: C(l), T(k), L(l)• Vertebral bodiesVertebral bodies support support
weightweight• Vertebral spinesVertebral spines for muscle for muscle
attachmentattachment• IntervertebralIntervertebral disks disks: nucleus & : nucleus &
annulusannulus• Flexibility & SupportFlexibility & Support
Vertebra Structures
Vertebral Column Anatomy
Three Seated Postures
• Reflects the task and chair (& individual)Reflects the task and chair (& individual)• Based on Center of Mass Based on Center of Mass (CofM)(CofM) location location
relative to the ischial tuberosities relative to the ischial tuberosities (IT)(IT)
• Affects % TBW transferred to different Affects % TBW transferred to different support surfaces.support surfaces.
• Affects/reflects lumbar spine positionAffects/reflects lumbar spine position• Figure 9.3Figure 9.3
• anterior, middle, posterioranterior, middle, posterior
Three Seated Postures Classified
• Reflects the task and chair (& individual)Reflects the task and chair (& individual)• Based on Center of Mass Based on Center of Mass (CofM)(CofM) location location
relative to the ischial tuberosities relative to the ischial tuberosities (IT)(IT)
• Affects % TBW transferred to different Affects % TBW transferred to different support surfaces.support surfaces.
• Affects/reflects lumbar spine positionAffects/reflects lumbar spine position• Figure 9.3Figure 9.3
• anterior, middle, posterioranterior, middle, posterior
Middle Position
• CofM directly above ITCofM directly above IT• Floor supports about 25% of TBWFloor supports about 25% of TBW
• Spine in neutral position or, if relaxed, Spine in neutral position or, if relaxed, in slight kyphosis (round back)in slight kyphosis (round back)
Anterior Position (forward leaning)
• CofM in front of IT CofM in front of IT • Floor supports > 25% of TBWFloor supports > 25% of TBW
• Forward rotation of the pelvis Forward rotation of the pelvis with the spine in neutral positionwith the spine in neutral position
• By assuming a slight kyphosisBy assuming a slight kyphosis• Little or no rotation of the pelvis Little or no rotation of the pelvis
but with large induced kyphosis but with large induced kyphosis of the spineof the spine
Posterior Position(backward leaning)
• CofM behind ITCofM behind IT• Floor supports < 25% of Floor supports < 25% of
TBWTBW• backward rotation of the backward rotation of the
pelvis and simultaneous pelvis and simultaneous kyphosis of the spinekyphosis of the spine
Posture selected/exhibited reflects
• Individual seating habitsIndividual seating habits• Task demandsTask demands• Height & inclination of chair seatHeight & inclination of chair seat• Position, shape, inclination of Position, shape, inclination of
backrest backrest • Presence of other types of supportPresence of other types of support
• arm rests, footrestarm rests, footrest
A good chair
Functionally adapts Functionally adapts to the occupantto the occupant
and and the taskthe task
Anthropometric Measures for Seated Work
The
Occ
upan
t
Anthropometric Measures: Seated Work
Comfort•Remains an unexplored Remains an unexplored
concept concept • frequently defined as the frequently defined as the
absence of discomfortabsence of discomfort•5 minutes as good as 1/2 hour 5 minutes as good as 1/2 hour
as good as 4 hours to as good as 4 hours to subjectivelysubjectively rate comfort rate comfort
Determining Comfort
• Based onBased on• observe body posture & movementobserve body posture & movement• observe task performanceobserve task performance• direct subjective ratings of direct subjective ratings of general general
comfortcomfort using using •chair feature checklistschair feature checklists•body area comfort rankingsbody area comfort rankings•general comfort rankingsgeneral comfort rankings
The Spine and Sitting
Clinical aspects of seated posture
• Does extended sitting cause LBP?Does extended sitting cause LBP? • Research says “Yes”Research says “Yes”• Research says “No”Research says “No”
• Seated in a vehicle: Seated in a vehicle: “Yes”“Yes”• interaction interaction
•lumbar posture lumbar posture •vibrationvibration
Disc Pressure Measures
Disc loading reflectsDisc loading reflects•PosturePosture•Supported loadSupported load
Disc Pressure Measures
Disc loading reflectsDisc loading reflects•PosturePosture•Supported loadSupported load•TaskTask•EnvironmentEnvironment
•Basis for the Basis for the “backless chairs” “backless chairs” where the user rests on where the user rests on the knees and elbowsthe 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 Use of a lumbar support more support more important than important than backrest inclinationbackrest inclination
Lumbar Support
Adjustable Lumbar Support
Maintaining Lumbar lordosis
Pantagraph, 11/12/99
Functional backrest
To reduce disc pressure when seated
• Use lumbar supportsUse lumbar supports •support part of TBW support part of TBW •maintains lumbar lordosis maintains lumbar lordosis
• Increase backrest inclinationIncrease backrest inclination•increases load transferred to the increases load transferred to the
backrestbackrest•reduces muscle activity reduces muscle activity •Use arm rests Use arm rests
• supports the weight of the arms/supports the weight of the arms/trunktrunk
Task effects on disk pressure
Disc loading reflectsDisc loading reflects•PosturePosture•Supported loadSupported load•TaskTask•EnvironmentEnvironment
Task effects on disk pressure
• Writing at a deskWriting at a desk• decrease noted compared with other decrease noted compared with other
tasks tasks • arms ==> trunk supported by the deskarms ==> trunk supported by the desk
• Typing and lifting a phone Typing and lifting a phone • increased pressureincreased pressure
• larger external load moments larger external load moments Leaning forward unsupportedincreases disc pressure
The Legs and
Sitting
• Foot support affects circulationFoot support affects circulation• Pressure applied close on thighs close to Pressure applied close on thighs close to
the knees the knees • leg swelling ==> pressure on sciatic nerveleg swelling ==> pressure on sciatic nerve• should not feel edges of the chairshould not feel edges of the chair• Semi-seated positionSemi-seated position ==> eases transition ==> eases transition
to standing but increased lower extremity to standing but increased lower extremity stressstress
The Legs and Sitting
• Seat height affects buttocks pressureSeat height affects buttocks pressure• Low seat height ==> too small contact areaLow seat height ==> too small contact area
Leg posture & pressure
• Crossing or not crossing legs. Crossing or not crossing legs.
Oborne, 1982
The Table (work surface)
• Desired field of vision Desired field of vision • affects neck & trunk flexaffects neck & trunk flexion ion
• maintain < 15 degreesmaintain < 15 degrees• slanted work surface???slanted work surface???
• Work surface heightWork surface height• TopTop
• 3-4 cm above elbow level3-4 cm above elbow level
• BottomBottom• provide clearance for thighs (no standardprovide clearance for thighs (no standard
Interaction of task &
environment
Clinical aspects of seated posture
• Avoid Avoid Postural RigidityPostural Rigidity• maintaining same position for long maintaining same position for long
time time • Workers MUST be alerted to Workers MUST be alerted to
problems occurring from long-problems occurring from long-lasting low-level muscular effortslasting low-level muscular efforts• tension in same muscle groupstension in same muscle groups• loading of same soft tissuesloading of same soft tissues• loss of “muscle pump”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 Maintain normal lordosis with minimal
muscle activity to reduce back stressmuscle activity to reduce back stress• Proper backrest most important factor to Proper backrest most important factor to
reduce back stressreduce back stress• Disk pressure reduced with lumbar supportDisk pressure reduced with lumbar support
• Feet should be supportedFeet should be supported• Full contact with chair seatFull contact with chair seat• Alter seated posture every 15 minutesAlter seated posture every 15 minutes