evaluation of work chairs

Provided by :Hamed Nadri

MS.c of Industrial hygiene (Tehran, Iran)

Evaluation of Work ChairsEuropean Community Standards Authorities (CEN).

German DIN 4551 and Italian UNI 7498.

The purpose of this chapter is to illustrate an organized approach for objectively evaluating the ergonomic features of a work chair

BASIC ERGONOMIC REQUIREMENTS FOR WORK CHAIRS(the principal requirements that a work chair should meet to be defined as ergonomic:)

Evaluation of Work Chairs

Safety: the chair should never be the cause or the means of accidents.

Adaptability: the chair and its components should have dimensions or be adjustable to meet the anthropometric needs of a wide range of users, and of at least 90% of the population.

Comfort: the chair and its components (i.e. especially the seat and the backrest) should be upholstered, body contoured and reciprocally adjustable so as to meet the physiological needs and characteristics of many different “body shapes, curves and sizes.”

Practicality : Adjusting the various chair components should be made as easy as possible for the user.

Solidity:The sturdiness and resistance to wear of the seat and its components

Suitability for the intended use:every workplace has its peculiarities and special needs from an

ergonomic standpoint.

Safety: Stability of the chair: the chair-to-ground projection of vertical

force application points, placed 5 cm apart Chair components should be made of non-flammable material The chair should come in a choice of castors for different types of

floors; Absence of sharp edges. Armrests should never pinch


Adaptability:

Height of the seat, “popliteal height.” adjustable(5th percentile in women 95th percentile in men).

width of the seat plane“hip breadth” (95th percentile in women)

Depth of the seat “buttock–popliteal length.” 5th percentile subjects.


A : SEAT HEIGHT

B : SEAT DEPTH

C : BACKREST SEAT PLANE HEIGHT

D : BACKREST HEIGHT

Backrest height: support the trunk,

correspondent values of the 95th percentile in men from low back curvature(L5–S1) up to maximum thoracic kyphosis.

Backrest width:(corresponding to the 95th percentile in men)

maximum lumbar lordosis and maximum thoracic kyphosis.

B : BACKREST WIDTH (LUMBAR LEVEL)C : BACKREST WIDTH (THORACIC LEVEL)

Frontal view

Height, depth and width of the armrests:

Height: supporting upper limbs,keeping balance“elbow–seat plane height”, 5th to 50th percentile

Depth: recommended depth of the armrest: “elbow-to-wrist length” in the 95th percentile of men.

Width: width of 4 cm or more arerecommended.

COMFORT: Inclination of the backrest: Backrests equipped with an

inclination ranging between 90 and 115º (with respect to the horizontal plane) are preferable (120º for high backrests).


Contours of the seat plane:To avoid compressing the neurovascular structures at the popliteal

level, the front edge of the seat should be rounded and possibly made of non-rigid material,

A slope ranging between 3 and 10ºThe center concavity, 10 cm of the most protruding point of the

lumbar support

A : LUMBAR SUPPORT HEIGHT (FROM SEAT PLANE)B : DEPTH OF LUMBAR SUPPORTC : EXTENTION OF LUMBAR SUPPORT ALONG BACKRESTD : SPACE FOR BUTTOCKS (FROM BACKREST)E : SEAT PLANE AREA FOR MEASURING INCLINATIONVS. HORIZONTALF : ROUNDED FRONT EDGE

Lumbar support:Height of maximum protrusion:defined “lumbar lordosis–seat plane

height.” maximum protrusion is fixed between 20 and 24 cm(not adjustable backrest)

Depth: according to anthropometry data of low back curvature.If it is not adjustable, set between 2 and 4 cm.

Length: determined by using anthropometric parameters such as: “distance

between 1º sacral vertebra and 1º lumbar vertebra” and “distance between 1º sacral and maximum thoracic kyphosis ”

Padding:providing an appropriate distribution of body pressure,Chairs should be manufactured using semi-rigid upholstery (with a

deformation due to the weight of a person 2.0–2.5 cm),

Covering materials: surface that comes in contact with the user should not create any

thermal undesirable sensation. seats and backrests should be made of porous material

PRACTICALITY:The covering material of the chair should also be washable in order to

guarantee the minimum hygienic require ments. This aspect is particularly important in industrial or in very dirty environments, or in cases where the chair is intended for multiple users.

SOLIDITY:The sturdiness and resistance of the seat and its components can be

evaluated using the specific tests and interpretation criteria developed by National Standards Authorities,


SUITABILITY FOR THE WORKPLACE AND INTENDED USE:

Covering materials and padding crowded areas, or areas with flames and sparks, both materials should be flameproof.

type of floor used (smooth, carpeting, etc.). Chairs with castors for carpeting should not be used on smooth surfaces.


Figure 4.1 Posterior tilting of the pelvis and flattenting of the lumbar curve during the transition from standing to sitting.Much of the postural adaptation to sitting takes place in the back, rather than the legs.

Figure 4.2 Anterior wedging of the intervertebral disc occurs in the slumped sitting position (a = posterior ligaments). Soft tissues between the anterior and posterior elements of the spine may be pressurised, resulting in pain. (Adapted from Keegan, 1953, with permission of the The Journal of Bone and Joint Surgery, Inc.).

Disc pressures are lower in standing than in sitting and lower still when lying down. In standing, the load is shared between the facet joint. In sitting, the discs bear more of the load (Adams and Dolan, 1995)

Rohlmann et al. (2001) found that disc pressure was lower in relaxed sitting than in standing, but higher when subjects attempted to extend the spine to sit erect.

Both Nachemson (1966) and Rohlmann et al. report lower disc pressures when subjects recline against a backrest. This implies that seated workers should be able to adopt relaxed postures.

evaluation of work chairs

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