WORKSTATION DESIGN AND POSTURAL STRESS PART 1: BACKGROUND TO OCCUPATIONAL SYNDROMES Anthony Golding MSc Ergonomics (London), BSc Psychology (CNAA) BACKGROUND Musculoskeletal disorders at the workplace appear to be increasing1, and to judge from the explosion of publications on the subject in the last ten years or so, the problem is beginning to be taken seriously. According to Coleman2 the prevalence and clinical course of disorders have not been widely shown to be affected by conventional prevention ef­ forts. Because of this, more emphasis is now being laid on the assessment of task-related factors in the workplace in the hope that this will contribute to more effective prevention. Surprisingly little seems to be known about the aetiology o f disorders. Troup3 stated that only a small proportion of back disorders can be attributed to trauma; in the majority of problems the cause is obscure, or multifactorial4. The pletho­ ra of syndromes involving the neck and upper limbs appearing more recently, gives cause for concern because these do not conform to any known model. At present it is commonly believed that the pathophysiological mechanism involved in many disorders is acute/chronic muscle or connective tissue strain5. Alternative theories of how muscular strain leads to damage are summarised in the following section. If the mechanism is muscle strain there is a good chance of success with prevention programmes (ie. programmes which prevent strain occurring, as opposed to measures to relieve symptoms). Hagberg and Wegman6 maintain that the proportion of common disorders attributable to occupational exposure is high and that it should be possible to prevent a relatively large number o f these cases. Several intervention projects have used simple models for the development of work-related disorders. These assume an interrelationship between an individual’s physical characteristics and health and the physical demands of the work. These demands can either be imposed by the workstation and machine design or by the w ork organisation eg. the w ork d u ra tio n a n d pauses, work pace, forces applied and repetitiveness of the task. Psychogenic factors also cause tension and contribute to­ wards musculoskeletal stress. In these models an imbalance between the factors eg. a work surface too high for the worker, may cause postural strain and result in localised pain. If this situation is not changed then there is a risk o f developing a neck problem. There are obviously moderating variables and a crucial one is the amount of static muscle effort expended over prolonged periods in relation to the maximum voluntary muscle contrac­ tion the person can exert. If this effort is considerable then the risk increases. SUMMARY The growing incidence of occupational musculoskele­ tal disorders gives cause for concern about the effec­ tiveness of traditional awareness programmes in preventing such problems. Overseas there has been a move towards evaluating work-related factors in an at­ tempt to deal with complaints at their source. This paper outlines the hypothesis that postural constraint, as a result of work design, contributes towards muscu­ lar fatigue which is accompanied by an increased likeli­ hood of developing chronic musculoskeletal disorders. Methods of studying the problem and measures used to improve the situation are discussed. OPSOMMING Die groeiende omvang van beroepsverwante muskulo- skeletale probleme het 'n besorgdheid laat ontstaan oor die doeltreffendheid van tradisionele bewustheid- sprogramme vir die voorkoming van sulke probleme. Oorsee is daar 'n beweging tot die evaluasie van werks- verwante faktore om sodoende te probeer om die probleme by hul oorsprong te hanteer. Hierdie artikel beskryf die hipotese dat beperkte liggaamshoudings wat ontstaan uit werkstasie ontwerp, bydra tot spier uit- putting wat weer gepaard gaan met ’n hoer waarskyn- likheid van die ontwikkeling van kroniese muskuloskeletale probleme. Die metodes wat gebruik is om die probleem te studeer, asook did wat gebruik is om die situasie te verbeter, word bespreek. Intervention studies have focused on improving the workstation7, improving the work organisation , or by in­ fluencing both o f these factors8. Such large-scale studies have shown promising results with quite marked reduction in the incidence of problems. MUSCULOSKELETAL STRESS AND CONSTRAINED POSTURE Repeated isometric contraction of muscles can lead to overuse syndromes in the upper limbs of individuals whose w o rk is rep etitiv e eg. typists, p u n ch -m ach in e o p e ra to rs. S us­ tained isometric contraction of the muscles responsible for maintaining posture in awkward or constrained positions leads to local postural fatigue. The physiological process of muscular strain is similar in both instances and begins with the static nature of the physical work being executed. Aristotle said, “Movement is Life”. This has a message for us today. There needs to be an interplay between the static and dynamic components of activity for the body to function optimally. People who exercise have got part of the message, but the problem is that this interplay must be constant throughout the day. This is the challenge to those who plan how we work and stay healthy. Chief Researcher, Environmental Studies Division, National Institute for Personnel Research, Johannesburg. Address: NIPR, P 0 Box 32410, Braamfontein 2017. Tel: (011) 648-1046. Physiotherapy, February 1990, vol 46 no 1 Page 17 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) Alternate overloading and underloading of the interver- tebral discs has been identified as the principal mechanism of nutritional exchange9. Static positions, such as prolonged sitting, do not assist this process. Similarly with muscles. Cailliet10 found cervical pain ascribed to soft tissues to be due to protracted isometric contraction o f the paravertebral mus­ cles. In some cases pain is generated by overuse of myofascial formations. In most cases prolonged static contraction causes a constant increase in endomuscular pressure, with constric­ tion of the blood vessels and consequent ischaemia: in such conditions there is a reduced supply of nutrients to muscles and energy catabolic products accumulate. The pain experi­ enced is due to oxygen deficiency, action of irritating metabo­ lites, accumulation of lactic acid and the reduced intracellular concentration of potassium. Alternatively, Fassbender and Wegner11 have posited that localised hypoxaemia may cause muscular degeneration through mitochondrial lesions. The resultant biochemical changes lead to mesenchymal transfor­ mation of the tissue. In sum, ischaemia and the resulting biochemical alteration of the tissue initiate an inflammatory process which over time leads to a fibrotic reaction of both the muscle and surrounding tissues. Pain is simultaneously an effect and a cause in maintaining the process since the reac­ tion of the sympathetic nervous system to pain is decreased blood flow to muscles by way of vasoconstriction by which means static contraction of the muscles is even more liable to induce pain and inflammation. It must also be borne in mind that while static contrac­ tions may form up to 20% of maximum voluntary contraction (MVC) and are accompanied by increased blood supply to the muscle, contractions above this level cause a decreased 1 9 blood circulation and relative hypoxaemia begins to occur . It is thus significant that levels of static contraction near or above 20% of MVC are easily reached during keyboard work in the muscles of the upper body and limbs, particularly when the arms are unsupported13. In this way non-maximal pro­ tracted isometric contractions typical of constrained postures may lead not only to pain in the short term, but eventually to disease due to alterations of the soft tissues. MENTAL POSTURAL IMMOBILISATION STRAIN STRESS .*—qp—i MUSCULAR TENSION --------------------- / I \ ISCHAEMIA OEDEMA ACCUMULATION OF CATABOLITES TISSUE TRANSFORMATION PAIN \ / FUNCTIONAL IMPAIRMENT -----------------► Figure 1: Role of irritative stimuli in the pathogenesis of muscular functional impairment and pain. Rest from the fatiguing activity is one way o f avoiding the process attendant with overuse. However, the limited rest which can be provided without taking sick leave is often insufficient to allow for full recovery of muscle function. Grandjean14 states that static loads repeated daily over long periods are associated with a risk of permanent damage to muscles, joints, tendons and other tissues as well as disc troubles. The occupations most affected are those where the load is highest. CONSTRAINED POSTURE AND WORKSTATION DESIGN The most frequent form of static muscular work is con­ strained posture, mainly due to carrying the trunk, head or limbs in unnatural positions. Continuous postural stress oc­ curs when there is a need to maintain the position of unsup­ ported body members, to maintain the body in asymmetrical or twisted positions or where joints are used at the extreme position of their ranges. Sedentary occupations, with their associated hazards of weakened stomach muscles and flexion of the spine, have always posed a risk to the low back. Recently, epidemiological studies have focused on the high incidence of neck and upper limb disorders among office workers. This seems to be in­ fluenced by the automation of office tasks and higher speeds made possible by computerisation. The Visual Display Ter­ minal (VDT) is becoming the central feature of the office worker’s environment. Here the entire body above the waist is typically in a constrained position, the arms with reference to the keyboard, the head with reference to the screen and source documents, the trunk in a supporting role. The speed and repetition of the keystrokes is a major constraining factor. The high speed in data-entry tasks (up to 13000 keystrokes an hour8) necessitates a rigid posture which results in static loading of both the back/neck muscles and upper limb mus­ cles. Terms like “Tension Myalgia”, “Repetitive Strain In­ jury”, and “Occupational Cervicobrachial Syndrome” all refer to similar problems affecting such workers15. METHODS OF INVESTIGATION One of the main aims of research in this area is to examine the epidemiology of musculoskeletal disorders and derive guidelines for preventing them. More recently meth­ ods evolved from different disciplines have been employed together to allow for cross-validation of the results of each technique. Records may provide basic data. Work injuries must be notified by law in many countries and workers’ compensation records and accident reports are often mandatory. Company medical records may exist where there are health personnel and payroll figures may provide data on labour turnover, sickness leave and absenteeism. From this information the incidence rate of disorders over a specified length of time can be calculated and comparisons made between different jobs. A problem with records is that disorders are not always recognised as being work-related and therefore many cases go unreported. Such statistics often fail to identify those jobs where there may be potential problems. Records are thus only a starting point for subsequent investigation. Perhaps the most widely used tool comes from the be­ havioural sciences - the survey questionnaire. Self-ratedc^ues- tionnaires on physical impairments have been used ’ in Bladsy 18 Fisioterapie, Februarie 1990, dee! 46 no 1 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) conjunction with medical examination and subjective comfort rating scales repeated at regular intervals have utilised body part maps to determine the location and level of discomfort1 . A limitation of surveys is that they rely on the worker’s will­ ingness to report her health condition and that the responses will be affected due to differences in individuals’ tolerance to pain. However, this can be counteracted to some extent by medical examination, including anamnesis and physical examination - inspection, range of motion, palpation, neuro­ logical and muscle power assessment etc. In analysing jobs many people have utilised work-meth- ods analysis or ergonomic checklists which assess the bio­ mechanical risk factors involved in jobs such as repetitive, sustained exertions, constrained postures and high mechan­ ical forces. The measurement of forces and muscle strength is more obscure and technically difficult - calculation of bio­ mechanical loads and measurement of forces inferred from the level of muscle activity using Electromyography deter­ mine postural stress in precise, quantitative terms according to the model being used but do not necessarily come to the same conclusions. Conventional ergonomics approaches evaluate worksta­ tions and posture by measuring anthropometric and worksta­ tion dimensions (reach distance, work surface height) and assessing static work, machine-induced postures, and ma­ chine-related repetitive action. The observation of work pos­ tures over time to study postural stress is either performed manually by means of coded sheets18 or by using video film18. This technique has received attention recently because of the ability to use film in conjunction with computers to analyse work postures in detail20,21. The most difficult task is to determine whether the ob­ served biomechanical risk factors pose demands that exceed acceptable ranges of human capacity as limited by the design o f the existing workstation and machine. Fortunately the amount o f information available to assist in making that judge­ ment is increasing with the growth o f the research data base. SOLUTIONS The efficacy of health education is often put in question because there are so many obstacles to putting it into practise. Similarly propaganda will not achieve the desired goal unless it is constantly reinforced. Informing someone about risk factors and the behaviours to avoid may make him aware that something must be done differently, but it may not give him any idea about how the job should be done. Known ways of doing things are generally “easier” because o f old habits, production pressures to take short cuts, the new way may be more time-consuming (“straight-back” lifting versus stoop lifting), the risk of developing a disorder may seem remote, the workstation or job design may not permit the prescribed actions, and other work-related factors are beyond the worker’s control. To be effective, training must be on-going and a major goal o f company policy. Thus it is generally only large organisations which can afford the associated costs. Workstation redesign and job redesign are the main methods which have been used in overseas intervention pro­ grammes. These aim at avoiding unnecessarily fatiguing or strenuous work postures and movements and provide the worker with the opportunity to vary his work posture. If this cannot be achieved by the normal means of job rotation or task variety, the person doing the job must be given suitably disposed breaks to reduce the build-up of fatigue. This alter­ native is work reorganisation, which has been tried with data- entry operators successfully8. CONCLUSION The use o f ergonomic intervention programmes to pre­ vent work-related musculoskeletal disorders is a compara­ tively recent field of endeavour5. Once organisations realise the immense amount of money which is being lost because of sick leave and absenteeism due to poor work design (design of the task, the machine and the workstation) they may show an interest. The calculation based on a Norwegian figure 22 is a loss of R1000 per person per year. The costs of permanent disability run into millions. The prevalence of occupational posturally-related dis­ orders is becoming increasingly more evident and it appears that far too little of the problem is generally appreciated. For example, most workstations are still designed without refer­ ence to ergonomic considerations. Correspondingly there is a new urgency to pursue the development o f a better under­ standing o f posture and preventive techniques. The aetiology of such conditions is complex but through epidemiological studies it is possible to identify risk factors and jobs where there are problems. With this knowledge it should be possible to predict what is safe and acceptable with regard to this important aspect of working life. References 1. PO PE MH. The biomechanical basis for early care programmes. Ergono­ mics 1987:30, (2):351-358. 2. COLEMAN PJ. Descriptive epidemiology in job injuiy surveillance. In Occupational Accident Research. Proceedings of the International Sem­ inars on Occupational Accident Research 1983, edited by U. Kjellen, Sallsjobaden. Sweden: Elsevior. 3. TRO U P JD G. Relation of lumbar spine disorders to heavy manual work and lifting. Lancet 1965;1,7390,857-861. 4. JONSSON B, PERSSON J, & KILBOM A. Disorders of the cervicobra- chial region among female workers in the electronics industiy - a two-year follow up. International Journal o f Industrial Ergonomics 1988, (in press). 5. KILBOM A. Intervention programmes for neck and upper limb disorders. Ergonomics 1988;31,(5):735-747. 6. HADBERG M & WEGMAN DH. Prevalence rates and odds ratios of shoulder-neck diseases in different occupational groups. British Journal o f Occupational Mcdicinc 1987;44,:602-610. 7. AARAS A. Postural load and the development of musculo-skeletal illness. PhD thesis. Institute of Work Physiology, Oslo. 1987. 8. ONG CN. V DT work place design and physical fatigue. A case study in Singapore. In Ergonomics and health in modem offices, ed by E. Grand- jean. London: Taylor and Francis, 1984. 9. KRAEMER J, K OLDITZD , & GOWIN R. W aterand electrolyte content of human intervertebral discs under variable load. Spine 1985;10,:69-71. 10. CAILLIET R. II dolore cervico-brachiale. Rome: Leonardo Ed Scienti- fiche 1973 (quoted in Grieco A. Sitting posture: an old problem and a new one). Ergonomics 1986;29,(3):345-362. 11. FASSBENDER HG & WEGNER K. Morphologie und pathogogenese des weichtrheumatismus. Zeitschrifl fu r Rheumaforschung I973;32. (quoted in Grieco A. Sitting posture: an old problem and a new one. Ergonomics 1986:29,.3). 12. BARNES WS. The relationship between maximum isometric strength and intramuscular circulatoiy occlusion. Ergonomics, 1980;23,:351-357. 13. ONISHI N, SAKAI K, & KOGI K. Arm and shoulder muscle load in various keyboard operating jobs of women .Journal o f Human Ergology 1982;ll,:89-97. ">■ 14. GRANDJEAN E. Ergonomics in computerized offices. London: Taylor and Francis. 1987. 15. WARRIS P. Occupational cervicobrachial syndromes. Scandinavian Physiotherapy, February 1990, vol 46 no 1 Page 19 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. )