Literature Summary
POSTURES & MOTIONS OF CONCERN
Work is considered repetitive when the duration of the work cycle is less than 30
seconds, or when one fundamental work cycle constitutes more than 50% of the total
cycle, independent of its length (Eastman Kodak Company, 1986; Kilbom, 1994;
Hagberg et al., 1995).
Movement of body segments about the joints is normal and required for healthy function.
However some types of motion appear to be contraindicated and could differentiate
between safe and unsafe work techniques. Postures with potential to cause MSD
generally have three characteristics, all of which may be present simultaneously: (1)
extreme postures requiring muscle activity and that load passive tissues, (2) non-extreme
postures creating loads acting about the joint, and (3) non-extreme postures changing the
mechanical link system of body segments and which put large stresses on tendons,
muscles or other supporting tissues or reduce the tolerance of the tissue.
Epidemiology and work physiology studies give us plausible postural data upon which to
base guidelines that support a two-level classification of posture: recommended and nonrecommended. As we move from larger joints to smaller joints, such as from trunk to
wrist, the effect of other risk factors, such as force, becomes more important.
The following body areas are discussed with respect to postures and motions of concern,
as cited in the ergonomics literature.
BACK
Musculoskeletal pain and disorders of the back include disc bulges and herniations,
muscle strains and ligament sprains, and degenerative disc disease which often is
associated with pain.
Common muscle imbalances occur in the lower back, hip and pelvic region that can
contribute to injury and make back pain chronic. Tight muscles in the hips, such as the
Hamstring muscles that cross the back of the hip and the Psoas muscle that crosses the
front of the hip, can pull on the lower back and lead to pain. Long and weak abdominal
muscles combined with short and tight low back muscles is also common.
It is also well known that after back pain, two core stabilizing muscles may become
inhibited and will not become active unless specific exercises are performed.
For some occupations, such as mechanic work, where there is frequent reaching with the
dominant hand (such as when using a tool), spinal rotation flexibility can become
imbalanced. This means when you reach forward with your right hand (dominant hand)
you spine is more flexible twisting to the left, compared to reaching with your left hand
(non dominant) and the spine is less flexible twisting to the right.
Literature Summary
Bending habits are very important. People who bend through the spine look like a “C”
curve in the spine, whereas we know that the spine should remain in it’s neutral “S”
curve. That means bending should happen through the hips not the spine, To accomplish
that you need to have adequate flexibility of the hip muscles, so stretching these muscles
is often recommended.
Occupational MSD hazards for back pain and injury are well established. They are:
1.
2.
3.
4.
Heavy physical work where the spine is loaded with high compressive forces
Awkward or static back postures such as bending and twisting
Forceful manual material handling such as heavy, awkward or frequent lifting
Whole body vibration or shock when seated, such as when driving heavy
equipment over rough terrain
Evidence demonstrates increasing risk with increasing exposure (intensity or duration or
both) to awkward back postures, specifically forward bending (flexion) of the torso.
There is consistently strong risk at two hours of exposure with higher risk at longer
durations and some risks at shorter durations. There is also consistently higher risk with
higher degrees of flexion, particularly flexion exceeding 45 degrees.
Trunk Postures and Motions of Concern
Trunk flexion angles greater than 20° forward are considered awkward (Punnett
et. al, 1991). As well, axial rotation in combination with flexion, lateral flexion
are harmful combinations (McGill, 2002).
Marras et al. (1993) examined low back injury in 400 industrial workers and found that
they were able to discriminate between jobs with high risk versus a low risk of back
injury on the basis of five risk factors. The factors included the maximum moment of
force at the lumbar spine, maximum lateral velocity, average twisting velocity, frequency
of repetition of the activity and maximum sagittal trunk angle.
McGill (2002) lists the following risk factors for low back disorders:
 Static trunk flexion and a twisted or laterally bent trunk posture
 Frequent torso motion, higher spine rotational velocity, and spine rotational
deviations
 Peak and cumulative low back shear force, compressive force, and extensor
moment
 Repeated full lumbar flexion
 Excessive magnitude and repetition of compressive loads, shear loads, and
torsional displacement and moments.
The cumulative effect of continuous loading on the spine is an important consideration in
reducing and preventing back pain and injury. We need to recognize when the spine is
loaded. For example driving a truck with a bench seat loads the spine as much as bending
forward in a stooped posture. Some sitting postures are just as hard on the back as
awkward back postures seen in manual labour tasks. These postures flex or flatten the
lumbar spine, out of neutral. This not only increases compressive forces on the spine, but
also adds a shearing stress where one disc shears on the one above or below. These
Literature Summary
forces push the disc posteriorly and is a major contributing factor to disc herniation or
rupture.
Creep is also a problem with long periods of sitting or bending forward. After bending
forward more than 50 degrees, it has been shown that the supporting muscle stop
working, and therefore our spine is being supported by ligaments only. Ligaments creep
under continual tension and can leave our spine quite unstable. Performing a lift or
forceful motion when the spine is in this state may result in injury or back pain.
NECK
Musculoskeletal pain and disorders of the neck are closely related to proper posture.
Proper posture is defined by dropping a plumb line and viewing the body.
The plumb line should fall:
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Earlobe
Tip of front shoulder
Through hip
Front of knee
Through the ankle
A common postural fault is poking the chin forward, technically called “cervical
protraction”. To balance this poor posture, a “chin tuck” exercise can be done. This can
be practised anytime a person finds him or herself poking their chin. For me, I do it when
I drive. For others, it may occur during working tasks with intense visual requirements, or
when the lighting is too bright or dark.
To alleviate accumulated neck tension and discomfort, a person can wrap a bath towel
(rolled up three times) around their neck, tucking the ends of the towel under their
armpits.
Occupational MSD hazards for neck pain include awkward postures, forceful arm or
hand movements that generate loads to the neck and shoulder area and repetitive work
activities involving continuous arm or hand movements that affect the neck/shoulder
musculature or head positions.
Risk control strategies includes locating displays, information and objects within an
ergonomic visual range that minimizes awkward neck posture. Persons performing tasks
can practice looking with their eyes to minimize awkward neck postures. Micro-break
strategy includes performing a chin tuck exercise.
Literature Summary
SHOULDER
Musculoskeletal pain and disorders of the shoulder include tendonitis, bursitis and muscle
strain. Rotator cuff impingement, pain and tendonitis are common. It is well accepted
that the muscles that control and position the shoulder blade involved in the development
of shoulder pain and disorders.
Rotator cuff pain is recognized as aching over the top or front of the shoulder, shoulder or
neck pain when the arm is raised, hand falling asleep when the arm is raised slightly or
arm going asleep at night. As well numbness, pins and needles in the hand or recurring
chest or mid-back muscle spasms.
Shoulder problems are closely related to proper posture. Proper posture of the shoulder is
defined as having the palms face inwards when your arms are hanging loose by the sides
of your body. To find your proper shoulder posture, follow these 4 motions when
standing:
1.
2.
3.
4.
Raise arms to the front of the body (palms face in)
Keeping arms raised, move your arms out to the side of the body
Keeping arms out to the side of the body, squeeze shoulder blades down and in
Lower arms down to the sides of your body, moving only through the shoulder
joint.
If you find your shoulders are squeezed back, then you have achieved proper shoulder
posture. Because o all our daily activities involving reaching and holding our arms
forward and away from our body, we tend to get repetitive strain in the muscle that pulls
the shoulder blades back. This muscle is called the Trapezius muscle. In fact, the lower
fibers of this large diamond shape muscle, are well known to become weak or inhibited.
As well, our chest muscles, known as the Pectoralis muscles, can become short and tight,
especially if you perform strengthening exercises such as bench presses or chest flyes,
without adequate stretching. Exercises that strengthen the lower trapezius and stretch the
chest are often recommended to prevent shoulder pain and injury.
In a NIOSH review of evidence for work related musculoskeletal disorders, Bernard
(1997) points out that occupational MSD hazards for shoulder pain are related to posture.
Repeated or sustained shoulder posture more than 60 degrees away from the side or front
of the body and especially overhead work is the main hazard.
Kilbom (1994) has argued that repetitive shoulder movement (more than 2.5 times per
minute)lasting more than one hour, particularly combined with high force, should be
avoided. Repetitive shoulder motion is usually not a stand-alone hazard, in the absence of
awkward shoulder postures.
Downward heavy force on the shoulder, such as carrying items on one shoulder can also
lead to musculoskeletal problems in the shoulder.
Literature Summary
Numerous studies demonstrate that duration of continuous or repetitive work above
shoulder height increases the risk of shoulder and neck disorders, and the greater the
duration or frequency of exposure, the greater the risk. There is also some evidence for
risks with exposures of less duration. Those exposed to shoulder abduction/flexion of 30
degrees for more than four hours are at higher risk of developing neck/shoulder disorders.
WorksafeBC sets the moderate risk level at 2 hours and the high risk level at 4 hours of
cumulative static exposure or repetitive movements of the shoulder.
According to Washington State Department of Labour and Industries’ Concise
Explanatory Statement (RCW 34.05.325.6a) WAC 296-62-051, Ergonomics, Bernard
(1997) found a high prevalence of shoulder (rotator cuff) tendinitis in occupations
involving overhead work of long duration, such as shipyard welding, but did not find
other strong evidence for postural stress and repetition related to shoulder disorders.
Since that review, Punnett, Fine et al. (2000) reported a strong association between severe
shoulder abduction/flexion of more than 90 degrees and shoulder disorders in a casecontrol study in automobile workers. An association between neck-shoulder disorders
and arm abduction has also been seen for less extreme postures, in the range of 0-30º
abduction, especially if the work is static (Kilbom, Persson et al., 1986; Viikari-Juntura,
Martikainen et al. 2000; Frost, Andersen 1999). There are at least four studies identifying
an increased risk of shoulder disorders with shoulder abduction/flexion more than 1-2
hours (Punnett comments). Holmstrom, Lindell et al. (1992) have also reported severe
shoulder pain with overhead work.
Shoulder Postures of Concern
Shoulder joint angles greater than 60° in flexion or abduction (measured from the
trunk) are considered harmful because the rotator cuff is impinged when the bony
protrusion on the head of the humerus contacts the rotator cuff tendons, especially
the supraspinatus (Chaffin & Andersson, 1991). Shoulder extension is considered
harmful because it stretches the front of the shoulder joint capsule. Shoulder
motions greater than 2.5 per minute are considered repetitive (Kilbom, 1994).
Risk control strategies include locating objects within arm’s reach in order to minimize
awkward shoulder posture. Persons performing reaching tasks can practice reaching
while keeping the shoulder blades back. Micro-break strategies include opening up the
chest and practicing proper shoulder position.
LOWER ARM
Musculoskeletal pain and disorders of the lower include epicondylitis, carpal tunnel
syndrome, hand-wrist tendinitis, (or tendinosis if it is chronic) and hand-arm vibration
syndrome.
Carpal tunnel syndrome is a well known Repetitive Strain Injury. Symptoms of carpal
tunnel are classically numbness or pain in the thumb, index and middle fingers. The
hazards for carpal tunnel problems include highly repetitive work, forceful gripping, hand
Literature Summary
arm vibration, concurrent cold exposure, or a combination of the above. Awkward wrist
posture, without other risk factors, is not enough to lead to problems according to
research.
Epicondylitis is commonly called tennis elbow or golfer’s elbow. The main hazard is
forceful repetitive hand and elbow motions. A combination of both awkward posture and
force is more associated with this condition than repetition only. Speaking of posture, did
you know that elbow posture is safest when bent more than 90 degrees but less than 180
degrees (when gripping)?
DeQuervain’s tendonitis (as the base of the thumb) is a common injury that is caused by
repetitive stretching of thumb tendons. The hazards for tendinitis injury in the lower arm
include forceful gripping, awkward wrist posture, repetitive gripping or wrist motions.
Combination of factors increases risk. Interestingly, personal factors for tendinitis are not
as well established. This means anybody could develop tendinitis when sufficiently
exposed to these hazards.
Hand-arm vibration syndrome can occur with long time exposure to harmful vibration,
such as from holding some power tools. Vibration coming from a tool is measured by the
frequency and acceleration levels of the vibration. Although 6 weeks from exposure to
onset of symptoms is the shortest recorded latency time, this disease (which can be
permanent if left unattended) often takes years before symptoms are noticed. Metabolic
disorders, alcoholism and peripheral neuropathies also contribute to this disease. This
disease occurs in some people who do not have vibration exposure. In these cases it is
known as Raynaud’s Disease.
Wrist Postures and Motions of Concern
Wrist flexion postures greater than 30° are considered harmful as well as
extension more than 45°, ulnar deviation more than 30°, or radial deviation
(Hagberg et. al, 1995; Bernard, 1997; Washington State, 2000).
For the smaller joints such as the wrist and forearm motions that are not
recommended include repetitive wrist flexion or extension, flicking, jerking or
other motions with high wrist accelerations or rapid eccentric muscular
contractions. As well some postures are not recommended in combination with
force generation. For example, Hagberg et. al (1995) note it is not recommended
to:
 move wrist or pronate/supinate while exerting force,
 pronate with wrist and/or fingers flexed,
 supinate and extend wrist,
 be in wrist extension and elbow flexion at the same time, or
 repetitively supinate and pronate.
Marras and Schoenmarklin (1993) found wrist velocity and acceleration in a
flexion-extension motion discriminated between jobs with a high versus a low
Literature Summary
injury risk of carpal tunnel syndrome (based on increased tendon forces from fast
motions).
Forearm Postures and Motions of Concern
Muscles develop a maximal contraction power at an ideal length. We have
decreased static grip strength when the forearm is in full pronation compared to
neutral or supinated forearm posture. This is due to the shortening of the flexor
muscles which originate on the ulnar epicondyle (Chaffin & Andersson, 1991).
Repetitive forearm supination and pronation motions are not recommended.
Other occupational hazards for lower arm MSDs include awkward and repetitive thumb
postures, contact stress in the palm or wrist, and concurrent cold temperatures.
A common muscle imbalance in the lower arm includes tight wrist flexor muscles (travel
along the bottom of the forearm and wrist) and long, weak wrist extensor muscles (travel
along the top of the forearm and hand). When flexing the elbow against resistance – such
as when lifting a load with the arms, the extensor muscles may be overused as the biceps
muscle becomes under-used. This is a common muscle fault. Another problem may
develop if the thumb muscles become imbalanced. It is common to have the bottom of
the thumb strong and tight, while the muscles on the top of the thumb are weak.
Tightness at the bottom of the wrist can contribute to symptoms of carpal tunnel
syndrome.
Literature Summary
Ergonomics Journal and Textbook Reference List
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Armstrong, T.J., Foulke, J.A., Joseph, B.S., Goldstein, S.A. (1982). Investigation of
cumulative trauma disorders in a poultry processing plant. American Industrial
Hygiene Association, 43, 103-115.
Astrand, P-O, Rodahl, K. Textbook of Work Physiology: Physiological bases of
exercise. New York: McGraw-Hill, Inc, 1986.
Ayoub, M.M., Presti, P.L. (1971). The determination of an optimum size cylindrical
handle by use of electromyography, Ergonomics, 14/4, p. 509-518.
Bernard, T (1997). Work Related Musculoskeletal Disorders. NIOSH Publication
Report 97-117.
Chaffin, Don B. and Andersson, Gunnar B.J. Occupational Biomechanics, 2nd
Edition. New York: John Wiley & Sons, Inc., 1991.
Eastman Kodak Company. Ergonomic Design for People at Work, volume 1. New
York: Van Nostrand Renhold, 1983.
Eastman Kodak Company. Ergonomic Design for People at Work, volume 2. New
York: Van Nostrand Renhold, 1986.
Fransson, C. Winkel, J. (1991). Hand strength: the influence of grip span and grip
type, Ergonomics, 34/7, p. 881-892.
Grant, K.A., Habes, D.J., Steward, L.L. (1992). An analysis of handle designs for
reducing manual effort: The influence of grip diameter, International Journal of
Industrial Ergonomics, 10, p.199-206.
Hagberg, M., Silverstein, B., Wells, R., Smith, M.J., Hendrick, H.W., Carayon, P.,
Perusse, M. Work Related Musculoskeletal Disorders (WMSDs): A Reference Book
of Prevention. Kuorinka, I. and Forcier, L. (eds.), London: Taylor & Francis, 1995.
Kilbom, A. (1994). Repetitive work of the upper extremity: Part I-Guidelines for the
practitioner. International Journal of Industrial Ergonomics,14, p.51-57.
Konz, S. (1990). Work design: Industrial Ergonomics. Publishing Horizons,
Worthington.
Kroemer, K.H.E. and Grandjean,E. Fitting the Task to the Human, 5th Edition: A
textbook of Occupational Ergonomics. London: Taylor & Francis, 1997.
Marras, W.S. and Schoenmarklin, R.W. (1993), Wrist Motions in Industry,
Ergonomics, 36/4, 341-451.
Marras, W.S., Lavender, S.A., Leurgans, S.E. Rajulu, S.L., Allread, W.G., Fathallah,
F.A., Ferguson, S.A. (1993). The role of 3-D trunk motion in occupational-related
low back disorders: the effects of workplace factors trunk position and trunk motion
characteristics on risk of injury, Spine, 18/5, 617-628.
McGill, S. Low Back Disorders: Evidence Based Prevention and Rehabilitation.
Windsor, ON: Human Kinetics, 2002.
Punnet, L., Fine, L.J., Keyserling, W.M., Herrin, G.D. and Chaffin, D.B. (1991).
Back disorders and non-neutral trunk postures of automobile assembly workers,
Scandinavian Journal of Work Environment and Health, 17/5), 337-346.
Washington State. WAC 296-62-05174, “Appendix B: Criteria for analyzing and
reducing WMSD hazards for employers who choose the Specific Performance
Approach,” Washington State Department of Labour and Industries, May 2000.
Literature Summary
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Department
of
Labour
and
Industries,
Olympia,
Washington.
http://www.lni.wa.gov/wisha/.
Workers’ Compensation Board of British Columbia. WCB Rehabilitation Services
and Claims Manual.