Postural adaptations to workbench modifications in ...

I forces ~

ERGONOMICS, 1995, YOL.38, NO. 12,2485-2503

lumbar spine, Spine, 6,

33, Use of lumbar trunk tanding tasks, Journal of

d NACHEMSON, A. 1987,

Postural adaptations to workbench modifications in standing workers

;. Journal of Orthopaedic

d CORLETI, E. N. 1989, j

by magnetic resonance

. 1992, Working postures :rvation and recording on

R. S. WHISTANCE, L. P. ADAMS;! B. A. VAN GEEMS,t and R. S. BRIDGER Department of Biomedical Engineering, University of Cape Town Medical School, Observatory 7925, Cape Town, South Africa t Medical Biophysics, South African Medical Research Council, c/o Department of Biomedical Engineering, University of Cape Town Medical School, Observatory 7925, Cape Town, South Africa

:tivity of individual trunk

Keywords: Posture; Standing; Workspace design. Surveys have shown that many workers operate under conditions that require constrained standing. The aim of this study was to investigate postural adaptations in constrained standing to facilitate the development of design guidClines for standing workspaces. Standing postures were observed in six different workspaces that were designed using combinations of task distance (which was either constrained or unconstrained) and foot position (which was constrained, unconstrained or employed a footrest). Subjects at work were recorded stereophotogrammetrically and postural variables were obtained in three dimensions. Postural adaptation to increased task distance was found to be characterized by increased trunk flexion and increased hip flexion while adaptation to close work was found to be characterized by increased neck flexion and increased thoracic kyphosis. Constrained foot position resulted in increased hip flexion accompanied by increased plantar flexion. Although usc of the footrest resulted in some reduced lumbar lordosis, it increased trunk flexion and was not associated with significantly less discomfort than any of the other workspaccs. ·

1. Introduction In many work situations, operators must adopt a relatively fixed standing posture throughout the working day over a period of years or even a lifetime. It has been found by van Wely ( 1970) that there is a high correlation between postures repeated daily at work and specific pain sites. More specifically, Grandjean eta!. ( 1968) concluded that prolonged standing in one place is a common cause of ailments affecting the legs and feet of saleswomen. In the long term, postural misalignments at work increase the risk of disorders. Adaptation during prolonged standing is no exception to this (Floyd and Ward 1967, Fox and Jones 1967). Prolonged standing has been linked with a variety of symptoms, ranging from discomfort and fatigue that merely distract the worker and can be cured by rest; to serious long-term disorders such as deterioration of the intervertebral discs, varicose veins and thromboses of the leg (Grandjean 1980). Basmajian ( 1978) points out that, once in the upright posture, man has the most economical anti-gravity mechanism of all animals. Fatigue is associated with direct tensions on inert structures and circulatory inadequacies (Basmajian 1978). However, in order to maintain an upright posture with a minimum of muscular effort, the line of the centre of gravity must fall through the major weight-bearingjoints and be equidistant from each foot (Roaf 1977). Typical working postures rarely meet this requirement. 0014-0LN/95 $10·00 © 1995 Taylor & Francis Ltd.

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•·'*' ·;. ,-~------·--~- ... -...,.,~··-~-~·~-~--·-·----.c~·--.. ith 50S{- more intradiscal pressure than 5.tanding erect. f\1o~hcr (1913. 1914. 1919. in Zacharkow llJ88) \lrc~scJ that ~tanding po~turc i~ largely dependent on the p~..1sition oft he feet. Kirby cr a!. ( J9g7) found that foPl po..,itinn was an important determinant of standing balance. \\'hen indiYiduals need to stand relatively still for prolonged periods they adopt'") mmetrical standing attitudes. It has been found that asymmetrical standing attitudes arc adopted four times as often a~ ~ymmetrical attitudes (Hcllcbrandt cr a/. 1943, Smith. 1953 ). Shifting the \>..eight from foot to foot provides an important relief mechanism (Carlsilo 1961 ). In the commonly obser\'cd asymmetrical position. mo~t of the body weight is carried on one leg with the other leg placed forward ;_rnd to one -..ide (Zacharkow 19SS). This stabilizes the indi\ idual by pro\·iding a bwadcr ba~~ I

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Degree-=-'---~==----93 r--.

Degrees 1

____..____Conal rained Task

(d)

(a)

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2

1

3

;coot Position -

Unconatr11l~ Taak

(c)

--+--Constrained Taak

Foot Position -

Unconatralned Tuk

-+- Conatralned Taak

(f)

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Po\Wral adoptmion' in swnding 11·orkcrs

R . .)·. \\'hi.,runce ct al.

~496

24Y7

Ho\\c\·er. the pel\· is \\a.;; not held completely ~tati(_lnary. \\'hen ta~k Ji...,tancc \\'as ainL'd. pch ic inclination and hip Jkxion increa'->cd . ...,)10\\ ing that !kxJon from the !umharrcgion wa ... acnHnpanicd hy ~omc pchic atbptation. A~ the lo\\L'r limh pn'-.turc did not change \ignitlcantl:- for task di.-.tance, hip fto.ion was incrca.'led Jue to forward inclinJtion of the pc!Yic line. Ulihll

Plantar Flexion Degrees

/:

:~ f 79:-

771 76

f

75

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for by

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l

78

Thu\. ru~tural ;tdaptation to incre;!',l'd ta~k Jiqance'i:-. l'haractcri;eJ by incJT;hed lkxinn at the lumbar ret; inn (trunk flexion) anJ incrca-.ed flexion at the hip. accounted

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74-

2

Foot Position -

Unconelramed Task

----'"-Constrained Task

I g)

Figure 5. Graphic repre\entation of rc\ulung averJge po~tural angle~ fnr all \Hlrk~pacc combinations: (a) trunk tle\inn. 1h) hip flexion. (c) peh"ic inclination. (d) trunk inclinatirm. (t') neck flexion, (f) knee flexion. tg) plantar flnion. Foot pn-.iti(ln l 1) i~ uncon...,trt~incd. C.!) is conqraincJ, and 0) U\1..'~ the foot rail.

Table 3.

for the

Mean comfort

Uncon~trained

~ix \~

2·52

2·43

2·60

2·60

The _c..tereo-triplcts in figures 6 (a)-(/) depict in three dimensions the 3\"Cragc po_c..tural adaptation of the hack and pch·is to each of the ..,jx work~tations a~ ~ccn from the front. Figure 7 il]u..,trate . . on the subject exactly \\hich points are u~ed in the ositirm The results of this study confirm the claim made 80 years ago (Mosher 1913. 1914. 1919 in Zacharkow 1988) that \tanding po~ture is largely dependent on foot po~ition. The con_c..trained foot po_c..ition resulted in more hip flexion than the uncon\traincd

Table 4.

Frequency of discomfort for experimental constraints A-F.

Con~traintt

rrcqucncy ( S:{:)

A

39 45 55 36 44 42

B

c D

E F

Zone of di\comlort

foot position. With the constrained foot position. the front of the subject's foot could not go under the bench. forcing the subject to stand further back and to compensate by increased flexion at the hip~. The constrained foot po:-,ition also resulted in incrca\ed plantar flexion. This could

Neck Neck Thigh and leg on supporting side · Both legs Both legs Thigh and leg on supporting \ide

t See table 1 for definitions of constraints.

be accounted for by subjects placing their feet as far forward as they could go while increased forward flexion at the hips resulted in compensatory backward displacement of the hips. Usc of the footrail re_c..ultcd in more trunk flexion. more posterior tilting of the pelvis. less knee flexion in the supporting leg and more plantar flexion in the supporting foot. Posterior pelvic tilt is recommended by clinicians as a means of decreasing lumbar lordosis voluntarily as a person stands and investigators have confirmed that tilting the

pelvis posteriorly does decrease the depth of the lumbar curve (Bridger et a/. 1992. Day et a/. 1984 ).