Seated Work

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Sitting Posture
Anthropometric Dimensions of Seated Workers
Seated Work Place and Layout
Video Display Terminal Users

  1. Chaffin, D.B, & Andersson G.B.J., 1999. Occupational Biomechanics, 3rd ed. New York: John Wiley & Sons. pp.355-392.

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Objectives: After studying this topic, the students will be able to

  1. understand the biomechanics of sitting posture
  2. identify the anthropometric measurements for the seated workers
  3. understand the guideline for seated work place design and layout
  4. understand the common problems and solutions for VDT users

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Sitting Posture

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Definition

  • a body position in which the weight of the body is transferred to a supporting area, mainly by the ischial tuberosities of the pelvis and their surrounding tissues (Schoberth, 1962)
  • body weight transferring through
  • the ischial tuberosity to the seat and then to the floor
  • the foot directly to the floor
  • the forearm to the armrest and then to the floor
  • the back and pelvis to backrest and then to the floor
  • comparisons of sitting posture with standing posture
  • Sitting posture provides stability required on tasks with high visual and motor control.
  • Sitting posture is less energy consuming than standing posture.
  • Sitting posture places less stresses on lower extremities than standing posture.
  • Sitting posture lowers hydrostatic pressure on lower extremity circulation.
  • The pelvis rotates backward and the lumbar spine flattens when standing to sitting.
  • Although seated work provides some advantages for the workers, it is obvious that the work place should be assessed carefully so as not to introduce musculoskeletal problems.
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    Types of Sitting Posture

  • middle sitting
  • COM of the upper body directly above ischial tuberosity
  • floor support ~25%
  • subtypes¡G
  • relaxed middle sitting with the lumbar spine straight or slight kyphosis
  • supported middle sitting¡G with the lumbar spine straight or slight lordosis
  • forward sitting (forward leaning sitting)
  • COM of the upper body in front of ischial tuberosity
  • floor support >25%
  • subtypes¡G
  • forward rotation of the pelvis with the lumbar spine straight or slight kyphosis
  • little rotation of the pelvis but with large kyphosis of the lumbar spine
  • sitting on a chair with a forward sloping seat¡G with the lumbar spine slight lordosis
  • backward sitting (backward leaning sitting)
  • COM of the upper body behind ischial tuberosity
  • floor support <25%
  • subtypes¡G
  • backward sitting without lumbar support¡G backward rotation of the pelvis and kyphosis of the lumbar spine
  • backward sitting with a lumbar roll support¡G backward rotation of the pelvis and lordosis of the lumbar spine
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    Standard Sitting Posture

  • chin in
  • neck flexion 5-10 º
  • keep lumbar lordosis
  • hip¡G 85-100 º
  • tibia¡G perpendicular to the floor
  • foot flat on the floor
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    Sitting on a High Chair

  • should have a foot support
  • without foot support, the weight of leg will form a moment at the hip joint to create anterior tilt of the pelvis, and then increase lumbar lordosis that might result in low back pain
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    Semi-Sitting Posture

  • good for ¡¥active¡¦ worker e.g. grocery check-out person
  • to encourage mobility
  • to allow rapid changes between sitting and standing
  • to preserve lumbar lordosis
  • inclination of the seat starts just in front of the ischial tuberosity to have full support of the trunk and the thigh
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    Anthropometric Dimensions of Seated Workers

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    Vertical Anthropometric Measurements

    All of the anthropometric measurements are based on the position when an individual sits with the popliteal fold 3-5 cm above the seat, with knee flexion of 90º, and with the foot flat on the floor.

  • sitting height¡G the vertical distance from the floor to the posterior aspect of the mid-point of the thigh
  • shoulder height¡G the vertical distance from the sitting height to the superior aspect of the acromion
  • elbow height¡G the vertical distance from the sitting height to the tip of the olecranon with the elbow being flexed to 90º and the upper arm being vertical
  • thigh height¡G the vertical distance from the floor to the highest point of the thigh
  • patellar height¡G the vertical distance from the floor to the superior aspect of the patella
  • orbital height¡G the vertical distance from the floor to the orbit
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    Sagittal Anthropometric Measurements

  • abdominal depth¡G the sagittal distance from the posterior aspect of the buttocks to the anterior aspect of the abdomen
  • external sitting depth¡G the sagittal distance from the posterior aspect of the buttocks to anterior aspect of the patella
  • internal sitting depth¡G the sagittal distance from the posterior aspect of the buttocks to the posterior aspect of the popliteal fold
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    Transverse Anthropometric Measurements

  • shoulder width¡G the transverse distance between the tips of both acromion processes
  • buttocks width¡G the maximum transverse distance at the buttocks
  • external elbow width¡G the transverse distance between the tips of both olecrani when the arms are placed at shoulder abduction of 90º
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    Seated Work Place and Layout

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    Dimensions of the Seat

  • seat height = sitting height
  • 3-5 cm below the knee fold when the low leg is vertical; otherwise it will cause compression of the posterior aspect of the thighs
  • 3-5 cm above popliteal level if the chair is tiltable or the seat slope is forward (Bendix, 1987)
  • seat width
  • seat depth (length)¡G 10 cm less than the internal sitting depth in order to facilitate rising from the chair
  • seat slope
  • backward slope of 5º
  • adjustable seat slope¡G better used in the office
  • forward slope of 20º
  • shape of the seat¡G Front part of seat should be contoured so that the edges of the seat should not be detectable during seated work.
  • friction properties
  • softness¡G pressure should be avoided on the posterior aspect of lower thigh
  • adjustability
  • climatic comfort


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    Dimension of the Backrest

  • Either with backrest or with lumbar support will decrease the pressure under the ischial tuberosity.
  • Backrest should not restrict trunk or arm movements
  • backrest top height = backrest bottom height + backrest height
  • backrest bottom height
  • backrest center height
  • backrest height
  • backrest width
  • backrest horizontal radius¡G concave from side to side to conform the body contour
  • backrest vertical radius¡G convex from the top to the bottom to conform to the lumbar lordosis
  • backrest-seat angle
  • pivoting and recline possibility
  • softness
  • adjustability¡G adjustable in the vertical and/ or horizontal planes
  • climatic comfort
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    Dimension of the Armrest

  • Armrest can reduce the loading on the spine and facilitate the rising from the chair
  • armrest length
  • armrest width
  • armrest height = elbow height
  • shoulders shrug if the armrests are too high
  • trunk slumps or leans to one side if the armrests are too low
  • armrest-to-armrest width
  • distance from armrest front to seat front
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    Dimension of the Chair Base

  • number of feet
  • base diameter
  • use of caster or wheel
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    Dimension of the Workbench

  • Not necessarily the same for all types of work
  • factors affecting workbench dimensions
  • size of the workpiece
  • motions required by the task performer
  • overall work layout
  • workbench top height
  • 3-4cm above the elbow level (Bendix, 1987)
  • Key board height = workbench top height if the computer is used
  • workbench bottom height¡G greater than the thigh height in order to ensure sufficient space for the thigh
  • workbench surface
  • size large enough to accommodate work objects but not too far to reach
  • friction high enough to prevent sliding of work
  • inclination of workbench surface
  • The influence on lumbar posture from inclined table surfaces was actually greater than the influence of the seat slope. (Bendix, 1987)
  • for reading¡G a slope of 45¢X
  • for writing: a flat desk
  • field of vision
  • VDT must be placed to prevent forward head or trunk flexion of the user
  • focal distance: 20-40 cm
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    Video Display Terminal Users

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    Definition

  • maintaining the same posture > 2 hours
  • for one specific computer work
  • repeated using the same key(s) or mouse
  • NOTE¡G In most developed countries, approximately ¾ of labors is sedentary workers (Reinecke et al. 1992)
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    Cumulative Traumatic Syndromes in VDT Users

  • Hultgren & Knave1st, 1974
  • 1streporter about soft tissue problems among VDT users
  • Muscle fatigue, soreness, stiffness, cramps, numbness, and/or pain were frequently found in VDT users
  • associated with the frequency of key strikes
  • More than half of computer users have reported local pain. (1991 US statistics)
  • location of pain
  • neck and shoulder pain: 67%
  • low back pain: 40%
  • wrist pain: 29%
  • resulting in
  • increase in medical expenditure
  • Increase in work compensation
  • decrease in productivity
  • possible causes
  • physiological factors
  • Endurance time decreases significantly when the posture required more than 30% of the strength of back muscles (Jorgensen, 1970)
  • intradiscal pressure changed during various sitting postures
  • If the trunk leans forward, the moment loaded on the lumbar disc increased as the sine of a. For example, if the trunk leans forward at an angle of 30º, then the moment is Wd(sine30º), i.e., 0.5 Wd.
  • flextion of the neck depends on the visual demand and the height of work surface.
  • environmental or task factors
  • malposture or maintaining the same posture for a long period of time
  • improper workplace
  • repetitive motions
  • psychological factor
  • work stress
  • time stress
  • social factors
  • prevention of cumulative traumatic syndromes
  • to decrease the sustained duration
  • muscle cannot sustain contractions over ~15-20% of their maximum strength without fatigue
  • to decrease the frequency
  • to increase muscle strength in the posture where the task requires
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    Biomechanical Considerations in VDT Workplace Design

  • chair
  • chair with armrest
  • seat slope
  • chair base
  • better to have 5-foot support
  • radius = 30-35cm
  • use of casters or wheels
  • computer desk
  • to provide sufficient space for the legs i.e. work bench bottom height ³ thigh height
  • If the desk is too low, an individual tends to lean forward and lower and protract the shoulder joints.
  • If the desk is too high, an individual tends to elevate and shrug the shoulder joint which is susceptible to muscle fatigue.
  • keyboard
  • keyboard height (from middle row to floor): 70-85 cm
  • keyboard distance (from middle row to table edge): 10-26 cm
  • in the position to have minimum wrist extension, flexion, and ulnar deviation
  • screen
  • screen height (from center of screen to floor): 90-115 cm
  • screen inclination: 88-105¢X
  • screen distance (screen to table edge): 50-75 cm
  • body posture
  • visual distance (from eyes to center of screen)
  • viewing angle (from eyes to center of screen)¡G < 20º
  • trunk-seat angle¡G most people uses the backward leaning posture that causes in a decrease in lumbar lordosis and is susceptable to herniation of the intervertebral disc.
  • elbow angle¡G ~ 90º
  • shoulder flexion angle¡G as small as possible
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    Established on 06/01/2003 and Last Updated 06/09/2003 by Huei-Ming Chai, PT Phd
    © 2003 ®ã´f±Ó at School of Physical Therapy, National Taiwan University, Taipei
    All Right Reserved