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What is Ergonomics? Describe how human-machine interaction can be improved? Which undesirable aspects of the system could be altered with ergonomic design?
According to Cost- Benefit model, which conditions should be satisfied to make preventive measures cost effective?
Discuss the functions of muscular and skeletal system.
What is Anthropometry? Explain various types of anthropometric data used in Ergonomics.

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Chapter 5
Repetitive Tasks: Risk Assessment
and Task Design
Classification of Work -related Pain
Category 1. The work exposure is a necessary cause of the disorder (as in
occupational diseases such as silicosis or lead poisoning).
Category 2. The work exposure is a contributory causal factor but not a
necessary one.
Category 3. The work exposure provokes reaction by a latent weakness or
aggravates an existing disease.
Category 4. The work exposes the worker to potential dangers which may
increase the likelihood of a disease developing (such as alcoholism in liquor
industry workers).
Some Basic Concepts Used in
Exposure: The condition of being subjected to something be it an infectious
agent, extremes of weather, or mechanical loading.
Outcome: A change in personal condition, normally as a result of one or more
Case: An individual who has developed the outcome of interest.
Prevalence: The number of cases of a disorder at any particular time.
Incidence: The number of new cases over a time period.
Odds ratio (OR): A statistic that describes the likelihood of an outcome given
exposure to a risk factor.
The main occupational factors associated with musculoskeletal
conditions at work are:

Most WMSDs are associated with one or more of the above factors.
In several cases, exposure to more than one factor causes a large
increase in the prevalence of the disorder.
Injuries to the upper body at work
The most clear-cut work-related upper body injuries occur as a result of
accidents at work and many occur when hand tools are being used.
The hand-powered tools most commonly involved in injury were knives,
hammers, wrenches, shovels, and ropes and chains. The powered tools
most commonly involved were saws, drills, grinders, hammers, and
welding tools.
Of the main incidents which precipitated an injury, the majority involved
the tool striking the user. This was the case with both powered and nonpowered tools. However, a significant minority of injuries were caused by
overexertion (approximately 25%–30%). The upper extremities were the
body area most commonly injured and the most common injuries were
cuts and lacerations followed by strains and sprains.
A‘‘strain’’ may be defined as overexercise or overexertion of some part
of the musculature whereas a ‘‘sprain’’ is a joint injury in which some of
the fibers of a supporting ligament are ruptured although the ligament
itself remains intact.
Common injuries and their prevention
It appears that there are several different classes of hand tool-related injury and
that several different approaches for prevention may be needed.
1.The most common injury would seem to be of a catastrophic nature in which
the tool itself suddenly strikes the user causing a laceration, bruise, or sprain.
2. A second, more pernicious, type of injury involves sprains or strains which
appear to result from the handling of the tool itself over longer periods of time.
3. A third type of injury occurs to the skin in the form of blisters due to pressure
‘‘hot spots’’ caused by poor handle design.
1. Attempts to prevent the first type of injury might emphasize training workers
safe tool-handling techniques and to think ahead—to recognize potentially
dangerous situations and to prepare the workplace to minimize the likelihood of
unforeseen events.
2. Attempts to prevent the second type of injury might concentrate on the
redesign of the tool itself and training workers to recognize the onset of fatigue
and avoid stressful work postures.
3.Handle redesign can prevent the third type of injury as can increasing the
task variety of the job.
Control of Neck Problems at Work
Grandjean (1987) concluded that the head and neck should not be flexed
forward by more than 150 if undue postural stress is to be avoided.
There is considerable evidence that frequent or sustained flexion of the
head and neck beyond this is related to chronic neck and shoulder pain.
This is exacerbated if the flexion is accompanied by rotation of the head and
if the shoulders and arms have to be held in an elevated position at the
same time (as is common in certain occupations such as dentistry and
Wrist Posture and Tendon Function
Carpal Tunnel Syndrome
Carpal tunnel. Section through the wrist showing (A) carpal bones, (B) tendons, and (C)
carpal tunnel containing finger flexor tendons, blood vessels, and nerves. If the wrist is held
in extreme postures, the tendons rub against bone when the fingers move and friction is
increased. (Courtesy of Dr. Douglas A. Bauk of IBM Brazil. With permission.)
Carpal Tunnel Syndrome
Carpal tunnel syndrome (CTS) is associated with forceful and repetitive work
alone or in combination with other factors. Vibration of the hand and wrist is
also associated with the condition but extreme postures on their own are not.
Combined stressors such as force and extreme posture or repetition are
strongly associated with CTS. As with tennis elbow, it seems that CTS is
more common in ‘‘hand-intensive jobs’’ in fish processing, supermarkets
(checkout workers), etc.
Tennis Elbow
General view of the forearm, wrist, and hand showing injury sites.
Tennis Elbow
Overexertion of the extensor muscles of the wrist can lead to a condition
known as ‘‘tennis elbow’’ (lateral humeral epicondylitis)
Elbow problems are found among mechanics, butchers, and construction
Haahr and Anderson (2003) found that new cases of tennis elbow were
related to non-neutral hand/arm postures at work and the use of heavy
handheld tools in physically demanding jobs.
Low social support was an additional risk factor in females.
Disorders of the shoulder
That the shoulder joint requires muscle activity to be held in place might alert
the ergonomist to its likely susceptibility to rapid fatigue and damage when
exposed to static loads or repetitive actions.
One of the simplest ways to reduce occupationally induced shoulder stress
in many jobs is to provide armrests, slings, or other means of supporting the
weight of the arms to enable the shoulder muscles to relax.
Whenever the hands or arms are used, muscle activity is necessary to keep
the humerus in its socket and to hold the scapula in place on the thorax. The
stabilizer muscles of the scapula are at a great mechanical disadvantage
when the arms are held forward of the body (or cantilevered) and static
muscle contractions are needed to resist the resulting moments.
One of these muscles, serratus anterior, acts to pull the scapula into the
thorax. Excessive load or fatigue of this muscle may cause pain in the upper
back. Damage to serratus anterior can result in a condition known as a
‘‘winged scapula’’ where the scapula protrudes because it is no longer held
close to the thorax due to muscle weakness. Carrying heavy rucksacks can
damage the nerve supply to this muscle resulting in a condition known as
‘‘rucksack palsy.’’
Methods of Reducing Shoulder Stress
1. If possible work with the hands near waist level and close to the body
2. If the hands have to be positioned above shoulder level, their
elevation above the shoulders should be no more than 358. Hand loads
should not exceed 0.4 kg and the posture should be held for no more
than 20 s for each minute of work
3. Avoid shoulder flexion=abduction >908 for >10% of cycle time 4.
Select taller workers for workplaces which cannot be modified
5. Take regular rest breaks 6. Minimize handheld weight
7. Provide external support for the weight of the arms (slings, ledges,
8. Confine work objects within the zone of convenient reach
9. Provide wrist rests for keyboard workers
Strain Index
The strain index (SI) is an observational tool for the assessment of jobs for
risk of upper extremity WMSDs (Moore and Garg, 1995). Note that index
measures the risk associated with a job, not the risk to particular individuals
performing the job.
It requires an assessor to measure or estimate six task variables:
Intensity of exertion
Duration of exertion per task cycle
Number of efforts per minute
Wrist posture
Speed of exertion
Duration of task/day
Precision vs Power Grips
Screwdriver Handle Designs
Some innovative screwdriver handle designs for one- and two-handed
operation. (After the Ergonomic Design Group, Stockholm, Sweden.)
Handle Design
A useful rule of thumb for evaluating handle diameters is that the handle
should be of such a size that it permits slight overlap of the thumb and
fingers of a worker with small hands.
The larger the handle diameter, the bigger the torque that can be applied
to it, in principle, but people with small hands must be able to enclose the
handle with their fingers.
Cylindrical handles are better than handles with finger grooves since these
cause pressure hot spots and blistering of the skin of hands they do not fit.
Tools Redesigning
Redesigned tools with improved hand–handle interfaces: (a) socket wrench with T-bar for
greater mechanical advantage, (b) pliers with bent handles to maintain neutral wrist posture,
(c) drill with handle at appropriate wrist angle, and (d) paint scraper with thumb- stall to relieve
pressure on the palm of the hand and prevent blistering of the skin on the palm.
Keyboard Design
Conventional Keyboards cause ulnar deviation
Separate banks of keys for each hand may
relieve wrist strain.
Keyboard Design
poor motor skill leads to excess co-contraction of muscles
and temporary muscle aches which may be mistaken for
tenosynovitis (which is rare among typists).
Several researchers have focused on the design of
keyboards as a means of reducing musculoskeletal
problems in keyboard operators.
Zipp et al. (1983) investigated the posture of the hands and
wrists noting marked ulnar variation and fatigue.
They concluded that keyboards should be designed with
separate banks of keys (one for each hand) each bank being
inclined and contoured to be compatible with the functional
anatomy of the hand.
Chapter 4
Standing and Sitting at Work
TABLE 4.1 Some Advantages of the Standing
Work Position
1. Reach is greater in standing than in sitting
2. Body weight can be used to exert forces
3. Standing workers require less legroom than seated workers
4. Legs are very effective at damping vibration
5. Lumbar disc pressures are lower
6. It can be maintained with little muscular activity and requires
no attention
7. Trunk muscle power is twice as large in standing than in semistanding or sitting
Dimensions for sit–stand workstations
Health risks increase with highly static and highly repetitive work
Neutral PostureThe joints are close to the mid-point of their ranges of motion and there is a slight
lumbar lordosis.
Posture of an astronaut resting in zero gravity showing the
neutral positions of the joints in the sagittal plane.
A range of standing and sitting postures varying in both lumbar angle
and degree of constraint
(a) Angles of pelvic tilt in different body positions. On the left, posterior pelvic tilt with flattened lumbar
curve. On the right, anterior pelvic tilt with exaggerated lumbar lordosis. Neutral postures in the center.
(b–f) Posterior tilting of the pelvis and flattening of the lumbar curve during the transition from standing
to squatting (i.e., increasing hip flexion in the upright position).
The relationship between lumbar curvature, hip flexion, and knee flexion
Lumbar spine posture as a percentage of maximum
lumbar flexion for different hip and knee angles.
Lumbar disc pressures in different positions of the body
The pelvis is held in an anteriorly tilted position by the iliopsoas muscles and the hip joint is free to
extend as happens during the stance phase of gait. The trunk and head are rotated until these are
vertically above the legs. This is achieved by extension of the lumbar and cervical spines and this is
why the vertically held spine is S shaped in humans and is ‘‘C’’ shaped and held horizontal to the
ground in quadrupeds. Bones and Joints In the erect posture, the line of gravity of superincumbent
body parts passes through the lumbar, the sacral and hip joints, and in front of the knee and ankle
joints. This places an extension torque around the knee joint, which is resisted because the joint is
already fully extended. The flexion torque around the ankle is resisted by the plantar flexors.
Muscles and Ligaments
A person standing erect under the influence of gravity is never in a state of passive equilibrium. The
body can be conceived of as a pillar of segments stacked one on top of the other and linked by
joints. It is momentarily balanced when the resultant of all forces acting on it is zero. The system is
designed to minimize any displacement of the line of action beyond the base of support described
by the position of the feet and compensatory mechanisms come into play to maintain balance
immediately this happens. Muscles and ligaments play a stabilizing role by means of the active and
passive torques. These exert around joints to correct small, fleeting displacements of the lines of
action away from the joints. A good posture may be defined as one in which the destabilizing
moments are minimized and the posture is maintained by the resistance of the relatively
incompressible bones (as well as interleaved soft tissues such as the intervertebral discs).
When the body is pulled off-balance by the requirements of badly designed jobs or workspaces, the
antigravity muscles come into play and a new equilibrium position is established but with the
associated cost of isometric muscle activity.
Erector Spinae Muscles
These are the main extensors of the trunk and are also used to control flexion. During relaxed
standing, very little muscle activity occurs since the lumbar lordosis minimizes the trunk flexion
moment. When the trunk is flexed even slightly forward or when a weight is held in front of the body,
the erector spinae muscles come into play. Leg Muscles The soleus and gastrocnemius muscles are
true postural muscles in the sense that these are always switched-on when standing. When leaning
forward, the activity of the gastrocnemius muscle increases. Prolonged standing causes significant
localized leg muscle fatigue and is one of the causes of leg discomfort. Abdominal Muscles There is
very little abdominal muscle activity in standing and even less in sitting (Burdorf et al., 1993) postures.
These muscles may help to maintain a proper relationship between the thorax and pelvis by
preventing excessive anterior pelvic tilt and hyperlordosis. The abdominals can prevent trunk
extension caused, for example, by loads placed high on the back (or when putting on a backpack, for
example) or when walking down steep hills.
Hamstring and Gluteal Muscles
The hamstring and gluteal muscles are hip extensors. The gluteal muscles exhibit hypertrophy in
humans and their function is to stop the trunk from jackknifing forward over the legs—unlike in
quadrupeds where the trunk is already jackknifed and the gluteals are used for locomotion. The
gluteals are, however, used for locomotion in ladder or stairs climbing. Activity in the hamstrings is less
in the standing position but increases when the stander leans forward, holds a weight, or pulls.
Iliopsoas Muscles Psoas major and iliacus are hip flexors and are constantly active in normal standing
as these prevent extension of the hip joint (the trunk jackknifing backward over the legs or the loss of
lumbar lordosis if the head position is maintained).
Iliopsoas Muscles
Psoas major and iliacus are hip flexors and are constantly active in normal standing as these prevent
extension of the hip joint (the trunk jackknifing backward over the legs or the loss of lumbar lordosis if
the head position is maintained). The iliopsoas muscles act against the hip extensors.
Adductors and Abductors of the Hip
When standing on two feet, these muscles provide lateral stability, preventing translation of the pelvis
in the frontal plane. When standing on one foot (and also during the stance phase of gait), the pelvis
tends to tilt in the direction of the unsupported side. The hip abductors on the side of the supporting leg
contract to maintain the pelvis level.
Physiology of standing
Prolonged standing causes physiological changes including peripheral
pooling of blood, a decrease in stroke volume and increases in heart rate,
diastolic and mean arterial pressure, peripheral resistance, and thoracic
impedance. Standing up from a supine position is accompanied by an
increase in the dimensions of the nasal passages).
Constrained standing is particularly troublesome for older workers or those
with peripheral vascular disease because the venous muscle pump, which
returns blood to the heart, ceases to function. Fidgeting is a preconscious
defense against the postural stresses of constrained standing or sitting. Its
purpose is to redistribute and relieve loading on bones and soft tissues and
to rest muscles.
Physiology of standing
Varicose veins are superficial veins, often in the legs, in which the valves
function ineffectively, resulting in pooling of blood and painful swelling. With
deep veins, the problem is more serious and can cause blood to return along
abnormal pathways resulting in long-term health problems, including chronic
edema and leg ulcers. Risk factors include obesity, cigarette smoking, high
blood pressure, and lack of exercise.
Anterior wedging of the intervertebral disc occurs in the slumped sitting position (A 1⁄4 posterior
ligaments). Soft tissues between the anterior and posterior elements of the spine may be
pressurized resulting in pain.
Working Posture
A good working posture is one in which the spine is toward the mid-point of
its range of movement and the trunk is unconstrained—free to move anteriorly
and posteriorly.
Spinal posture when standing is affected by workspace design: (a) unconstrained relaxed standing posture, (b)
no toe space, causing compensatory lumbar and hip extension, (c) lumbar extension caused upward reaching,
(d) poor foot position causing extension, and (e) relaxed standing with a footrest to prevent excessive lumbar
The postural triangle. A person’s working posture is a result of the requirements of
the task, the design of the workspace, and personal characteristics such as body
size and shape and eyesight. Consideration of all three components is needed in
posture analysis and workspace design.
Critical dimensions for user-seat-desk fit.
One of the most basic considerations in workstation design is the
anthropometric fit between users and furniture. Designers typically design to
ensure that 90% of users will be accommodated. Problems can therefore occur
with extremely tall, short, or obese individuals and special arrangements may
need to be made to accommodate them. Much office furniture is designed
around a desk height of ~73 cm and assumes provision of a height adjustable
chair. Immediately, it can be seen that with fixed desk heights, there are
conflicting requirements in trying to accommodate both tall and short users.
Anthropometric mismatches can have serious consequences for health and
efficiency because of the way these increase the postural load on the body.
Height adjustable desks are available and were proposed many years ago for
use with height adjustable chairs to increase the range of users accommodated
by a workstation.
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