Source
OHTA STUDENT MANUAL
Ergonomics Essentials
April 2009
This manual was originally developed by BP and University of Wollongong. The Occupational Hygiene Training Association Ltd would like to acknowledge the contribution of these organisations in funding and developing the material and is grateful for their permission to use and modify it.
Supported by OHTA, IOHA
This work is licensed under a Creative Commons Attribution-No Derivative Works Licence
WORK-RELATED UPPER LIMB DISORDERS (WRULD)
4.2.1 The Nature and Causes of WRULD/Repetitive Strain Injuries/Cumulative Trauma Disorders
The phrase “work-related upper limb musculoskeletal disorders” includes a variety of upper limb degenerative and inflammatory diseases and disorders, which result in pain and functional impairment. Affected areas typically include the neck, shoulders, elbows, forearms, wrists and hands (Buckle & Devereux, 2002). Names for these types of disorders include: cumulative trauma disorder (CTD), repetitive strain disorder (RSI), occupational cervicobrachial disease (OCD), occupational overuse syndrome (OOS) and more commonly work related upper limb disorder (WRULD).
These types of occupational injuries have been well known over history, examples of occupations include tailors, shoe makers, milk maids; and more recently VDU workers involved with repetitive data entry and vibration induced hand and wrist disorders, such as jack hammer operators.
Colombini et al. (2002) state that the incidence of occupational illnesses of the upper limbs has been constantly increasing and that in 1990, ‘cumulative trauma disorder’ accounted for over 60% of all occupational illnesses in the USA.
From the evidence it is clear that posture is a definite risk factor for the neck and shoulder, while a combination of factors are a risk for the elbow and wrist/hand.
Use of our upper limbs in functional activity, whether at work or at home usually involves static loading of the postural muscles and active use of the dynamic muscles for task completion. For example, when typing on a keyboard, the shoulder, neck and scapular musculature is stabilising the arms for dynamic work of the hands and forearms. However, using the evidence such as the findings of Bernard, allow us to determine whether or not manual tasks involving the upper limbs pose a risk to musculoskeletal health for workers.
To add to the complexity of WRULD, other research into the causes of WRULD indicates that age, gender (female), psychosocial factors and work organisational factors are also significant.
As outlined in Section 4.1, the causes of manual handling injury relates to demands exceeding tissue tolerances. In WRULD, the link between cause and effect is more difficult to tease out. The nexus between the physical risk factors, psychosocial factors, and individual risk factors is complex.
a) The Upper Limb
Let us now examine the structure of the upper limb; it comprises of the hand, wrist, forearm, upper arm and shoulder. The hand is comprised of 19 bones with a further eight in the wrist. It is highly flexible but also delicate and has evolved to manipulate and feel small items with a great degree of sensitivity and skill. It does not have intrinsic strength or mechanical power as the muscles of the hand are very small and are adapted to fine movements and precision.
Some mechanical power, however, can be achieved through the larger forearm muscles acting on the fingers and through body leverage. Therefore the hand can perform two different types of grasp - the pinch or precision grip and the palmar or power grasps.
Both require the forearm and particularly the hand to be stabilised. This is achieved by forearm, upper arm, shoulder and trunk muscles which for the most part are working statically.
The manipulative ability of the hand is improved by the full range mobility of the shoulder joint, the hinge action of the elbow and by the rotation of the forearm at the elbow and wrist.
b) Injury Mechanisms
The hand is a delicate and highly complex machine at the end of a highly flexible lever on a mobile body but its ability to perform depends to some extent on the rest of the body. Where the shoulder or the trunk cannot be positioned for optimum movement of the hand all three areas may suffer strain. This possibility must be considered in relation to the physical layout of work or the demands of the task.
Similarly where joints are in their outer or inner positions (away from neutral position) repeatedly or long periods, all structures – capsules (connective tissue around a joint), ligaments (strengthening tissue in a capsule), tendons (joining muscles to bones) and muscles – may be stressed. For most people such positions are held for short periods and are desirable intermittently. They are not difficult or damaging unless maintained for long periods or repeated many times. Sitting, reading or sewing with the neck bent is an extreme posture and may lead to considerable discomfort if continued over any length of time.
Work that requires the human body to adopt fixed postures and to repeat movements has become common over the last two to three hundred years. In such jobs the body must work in a way that it was never evolved to do. Repeated movements, be they light or forceful, and fixed work postures, such as at a computer terminal, a work bench or conveyor line put mechanical stress on the body. This cumulative loading on muscles, their capsules and ligaments sooner or later results in fatigue and perhaps strain, and eventually WRULD.
When considering work activities and potential risk for WRULD, it is useful to consider the postural clues that can be observed. Ranney (1997, p. 54,55) provides a list of risks to look for in work tasks for the shoulder, and then the hand and forearm. He notes that the potential damage these risk factors may cause is dependent upon exposure. Possible risk signals outlined by Colombini et al (2002) have also been included in Tables 4.5 and 4.6 outlined below:
Table 4.6 - Risk Factors for the Hand and Forearm
Risk Factors for the Shoulder High moments at the shoulder • Hands far from body, especially if with load • Arms out from body or above mid-torso (weight of arms alone ~ 4 kg.) Static load on the shoulder (NB continuous low loads can be just as fatiguing and lead to injury as infrequent high loads • Arms held out from body continuously without support • Shoulder girdle elevated • Tools held continuously Awkward shoulder posture • Working above shoulder height (eg: painting) • Working with arm behind the trunk (eg: reversing machinery) No time for tissue recovery • Continuous repetition of same activity • More than 1/3 strength required for 1/3 of task time. This may lead to incomplete recovery
(Source: UOW - adapted from Ranney 1997 & Colombini et al. 2002)
Risk Factors for the Hand and Forearm
High forces and very repetitive work • Cycle time of up to 15 seconds for at least 4 hours within a shift • Little ‘rest’ time between cycles (arms in constant movement) High forces required by task • Use of fine grips rather than gross grips • Fine grip required to lift object > 900g • Individual fingers used for task (eg: pressing action) • Fingers in hyperextension • Gloves used for task (increases grip requirements due to fit issues) • Lifting activity with palm down (pronation) • Handling of objects > 2.5 kg
Risk Factors for the Hand and Forearm
Non-optimal postures (extreme, end of range)
• Sustained flexion/extension > 30º • Rapid, continual wrist movements • Sustained ulnar or radial deviation • Jerky, flicking or tossing movements of the wrist • Sustained full pronation Static loads • Use of gloves for task • Activity requires wrist to remain in extension • Continuously holding an object Power tools used • High vibration • High torques/poor torques • ‘Kickback’ observed from power tool • Vibration – leads to mechanical injury of tissue and may lead to nerve damage Sharp edges and hard surface • Contact with sharp/hard surfaces by fingers or palm • Hitting/hammering trim or parts with palm of hand High precision placement requirements • Increases time • Increases static loading • Increases force requirements • Holding parts to fit together • Sustained awkward posture to assemble parts
(Source: UOW - adapted from Ranney 1997 & Colombini et al. 2002)
The following illustrations and photographs demonstrate potentially ‘unsafe’ postures of limbs and joints.
4.2.2 Risk Assessment
From the evidence discussed in the previous section, several indicators for risk of WRULMD in the workplace emerge: weight and load (force), frequency of activity (repetition), distances moved (force, posture), workplace layout (posture, vibration), duration of tasks, psychosocial factors, work organisational factors and personal variables.
As in any risk management process, the first step is to identify, then assess, control and recheck to ensure there is no residual risk, or the new controls do not in fact create another, unforseen, risk.
a) Risk Assessment Techniques
i) Simple Techniques
The simple risk assessment described in the standard has four components:
1. Gather preliminary information of the job task
2. Conduct hazard identification and risk estimation procedure and checklist
3. Overall evaluation of the risk
4. Remedial action required
The standard rates risk with a three colour zone approach: green (acceptable risk), yellow (conditionally acceptable) and red (not acceptable). Should the initial risk assessment produce yellow or red zones, the higher order and more complex risk assessment is to be undertaken, and this is to be done with a prescribed tool, the Occupational Repetitive Action Index (OCRA Index), Colombini, et al 2002.
An alternate and extremely useful tool for assessing the risk of WRULD in the workplace is that of the HSE in the UK. The HSE Risk Assessment Worksheet focuses on the key risk factors of repetition; working posture for each of the upper limb body segments; force; working environment; psychosocial factors and individual differences. It provides prompts for specific issues, control options and an action plan.
ii) Complex Techniques
More complex tools include RULA (all discussed in Section 4.1.4), REBA, QEC, and OWAS and the tool used for detailed risk assessment in the international standard, ISO 11228-3 Ergonomics – Manual Handling – Part 3: Handling of low loads at high frequency, OCRA (occupational repetitive action). OCRA has been selected for use in the standard, as it is based on epidemiological data, considers the known risk factors and can assess ‘multitask jobs’. (ISO 11228-3:2007 (E), p. 8). Ergonomists will select the tool most appropriate for the task(s) under examination.
4.2.3 Principles of Control, Preventive and Protective Measures
The principles of control, prevention and protective measures reflect the risk factors that have been discussed in Section 4.2. This may entail some complex changes to work organisation as well as modification of actual work stations and areas. Colombini et al. (2002, p. 137) categorises the types of changes required for prevention as structural, organisational, and training and retraining. The suggestions by Colombini have been combined with those of Bridger (2003) and Helander (2006) to provide the following guidelines and suggestions:
a) Structural Modifications
• Use of ergonomic work tools (appropriately designed tools, handles, grips, etc)
- Bend tool handles to 5-10º (to avoid wrist deviation)
- Avoid extreme ulnar and radial deviation (utilise a neutral wrist posture)
- Use low forces when rotating or flexing wrist
- For finger pinch movements, keep forces below 10N, as this represents 20% of weakest operator maximal pinch strength
• Optimal/ergonomically appropriate workstation layout, equipment, etc.
- Angle parts to be assembled for optimal postures
- Ensure appropriate bench heights
- Ensure suitable chair for task
- Ensure adequate work areas
• Reduce identified excessive use of force, awkward postures, and any compression of tissues (eg: wrist permanently resting while hand is working)
- Encourage use of large muscle groups rather than fine, intrinsic muscles of hands
- Design products for gripping with hand rather than requiring a pinch grip
b) Organisational Modifications
• Ergonomically designed job (appropriate pace, rest breaks/pauses, task variation)
- Rotate workers between high-repetition and low-repetition tasks/jobs
- Use machinery for repetitive jobs and use workers for variable tasks
- Allow workers to set own pace to minimise time or pacing pressure
- Use ergonomic criteria when planning work systems and purchasing equipment
• Reduce duration of frequent and repetitive movements
- Design out tasks requiring rapid movements
- Limit repetitive movements to 2000 per hour or less
- Eliminate very repetitive tasks/jobs (< 30 second cycle time)
• Ensure adequate recovery time
- Eliminate unnecessary overtime
- Avoid repetitive work in extreme temperatures
- Build in pauses in work cycles
c) Training and Retraining (in addition to above strategies)
• Strategies for task variation, rest breaks, work pauses
- Train management/those responsible for work system
• Provide appropriate information on specific risks and injuries
- Inform of risks and damage associated with repetitive tasks
• Provide techniques for task performance in line with ergonomics principles
- Train the workers to perform tasks in → Required order
→ Use both limbs whenever possible
→ Avoid unnecessary movements/actions
→ Grip objects correctly
→ Communicate with Supervisor regarding changes in work, or signs of discomfort
THE QUANTIFICATION OF HUMAN EFFORT AND MOTION FOR THE UPPER LIMBS
J. D. RAMSEY
International Journal of Production Research, Volume 7, 1968 - Issue 1, Pages 47-59
https://www.tandfonline.com/doi/abs/10.1080/00207546808929796
No comments:
Post a Comment