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