Ergonomics - Science Related to the Effect of Work Posture, Body Movements and Brain Use on Human Body, Brain and Mind.
Manual packaging, storage and retrieval operations frequently involve the task characteristics that have been associated with the development of disorders of the upper extremities and lower back. When risk factors that cause these disorders are recognized, they can often be reduced or even
eliminated with very little effort and no capital expense. In the original process design and installation itself these risk factor can be eliminated provide science of the disorders that are developed due working methods is developed. There are some methods, which operational plant personnel, who have had little or no training in ergonomics, with an analysis tool to both evaluate and improve current workstations and work methods, as well as to avoid the hazards of poorly designed tools and equipment, prior to their purchase and installation.
The costs that are most generally addressed related to musculo-sceletal disorders are the direct expense of high worker's compensation insurance and the potential for OSHA fines. These costs, although high, are not the only costs associated with the muscular skeletal disorders. Absenteeism and turn-over rates, as well as reductions in product quality and productivity, can also be important consequences that can have indirect costs that are much higher than the direct financial costs.
Some of the disorders are explained below.
1 Tendinitis and Tenosynovitis: A tendon is made of tough, fibrous connective tissue in which muscle fibers end and muscles are attached to bones. A tendon may or may not be surrounded by a sheath. A sheath protects and lubricates tendons. Tendons of the wrist, shoulder, and hand are surrounded by tendon sheaths; however, tendons of the elbow and forearm are not. Tenosynovitis can occur when a tendon that is surrounded by a sheath becomes inflamed because of excessive or insufficient amounts of synovial fluid in the sheath. Stenosing tenosynovitis is caused by the narrowing of the tendon sheath and triggering movements of the digits which is known as trigger finger. Tendinitis occurs when a tendon becomes inflamed because the tendon has been stretched beyond its strength or when the tendon is stretched across the bone when the wrist is deviated.
2 Thoracic Outlet Syndrome
Thoracic Outlet Syndrome is a neurovascular condition which occurs when there is
compression of the neurovascular components (nerves of the brachial plexus and the brachial artery
and its branches) or compression between the muscles of the neck and shoulder. The result is
numbness and pain in the distal upper extremity. Most of the cases are attributed to poor muscle
condition. The syndrome is a result of reaching above shoulder level or carrying heavy loads at the
side of the body. Overhead work causes the muscles of the shoulder region to become tight or
compressed
3 Rotator Cuff Tendinitis
The rotator cuff consists of four tendons that fuse over the joint in the shoulder. The rotator
tendons rotate the arm inward and outward, as well as move the arm away from the side of the body.
Rotator cuff tendinitis is a common shoulder tendon disorder which occurs when the arm tendons
push against the bony structure in the shoulder, the acromion. Pain occurs when the tendon becomes
inflamed. The condition occurs in tasks that require the shoulder to be abducted (arm is away from
the center of the body) and the elbow is extended.
4 Epicondylitis
Epicondylitis is a form of tendinitis that occurs when the tendons from the elbow tear or
degenerate. These tears are a result of repeated mechanical overload. There are two forms of
epicondylitis: “tennis elbow” (medial) and “golfer’s elbow” (lateral). Both result from forceful
forearm rotation with a bent wrist or repeated gripping, twisting, and wrist
extension or flexion against resistance According to Sinclair's [1965] study almost 50% of the subjects that had tennis elbow symptoms worked on jobs involving gripping tools with contractions of the extensor muscles or forearm rotation.
2.5 De Quervain’s Disease
This disorder is a form of tenosynovitis which occurs when the extensor tendons of the
thumb become inflamed due to thumb movement to the extreme of its range of motion.. According
to Guidotti (1992) this disorder tends to occur in occupations which use hand tools that require
deviation of the wrist either toward the thumb (radial deviation) or toward the little finger (ulnar
devaition).
2.6 Carpal Tunnel Syndrome
Carpal tunnel syndrome (CTS) is the most common nerve entrapment disorder which occurs
when the median nerve is compressed between the carpal ligament and structures in the narrow
carpal canal. Within this area are the median nerve and the
flexor tendons that connect the fingers with the muscles that provide grip strength. Compression of
the median nerve causes tingling, pain and numbness to the hands and fingers, and can cause loss of
feeling and loss of grip. CTS is usually a result of forceful hand movements or
repeated grasping, pinching and gripping.
2.7 Raynaud’s Syndrome
This disorder is associated with occupational tasks which require the handling of vibration
tools and working in cold temperatures (Lahey, 1984). Vibration delivered to the hand and arm by
pneumatic tools causes injury to the small blood vessels in the hand resulting in a lack of circulation.
This lack of circulation results in whiteness of the fingers.
Several occupations subject operators to biomechanical stress and operators in them have been found to be suffering from these disorders. The biomechanical stressors are referred to as occupational risk factors that cause musculo-sceletal disorders. A risk factor is defined as any attribute, experience or
exposure that increases the probability of a disorder.
Research studies have found that the following risk factors can contribute to musculoskeletal disorders:
1) awkward postures, 2) wrist deviations, 3) forceful exertions, 4) pinch grips, 5) cold, 6) vibration, 7) high grip forces, 8) direct mechanical impact, 9) wrist accelerations, and 10) repetitive motions.
1 Awkward Posture
Shoulder Abduction. The side-ways movement of the upper arm away from
the center of the body.
Shoulder Adduction. The movement of the upper arm toward or across the
center of the body.
Shoulder Flexion. The forward movement of the upper arm away from the
center of the body.
Shoulder Extension. The backward movement of the upper arm (reaching
behind the back).
Forearm Pronation. Rotating the forearm so that the palm is down.
Forearm Supination. Rotating the forearm so that the palm is up.
Radial Wrist Deviation. Bending the hand at the wrist in the direction of the
thumb.
Ulnar Wrist Deviation. Bending the hand at the wrist in the direction of the
little finger.
Wrist Flexion. Bending the hand toward the palm at the wrist.
Wrist Extension. Bending the hand backward at the wrist (dorisflexion).
Awkward postures that result in fatigue and discomfort are usually a result of poor work
station design, layout, or poor work methods that require excessive horizontal or vertical reach
distances. Examples of awkward postures include shoulder elevation, awkward elbow postures
(elbow above mid-torso), deviated wrist postures (flexion, extension with force, radial deviation and
ulnar deviation), and pinch grips.
Research Studies
According to Tichauer [1966], tasks, such as tightening screws, are particularly harmful because the
forearm is extended and turned through supination (palms-up) at the same time. In a study of 154
workers, the number of complaints of pain in the extensors of the forearm increased as the included
angle between the forearm and upper arm increased. The most comfortable posture is when the arm
is flexed so that the included angle between the forearm and the upper arm is 90 degrees.
In a study of 1979 of two matched female populations, one group having problem of carpel tunnel syndrome and the other not having the problem. The work methods of both groups were analyzed with respect to hand and wrist positions and hand force. It was found that the diseased group tended to use a wrist position deviated from neutral more frequently than the non-diseased group. Furthermore, the diseased group exerted more force than the non-diseased group.
Fernandez, Dahalan, and Halpern [1991] performed a study to determine the effect of wrist posture (neutral, 1/2 of maximum extension, 1/2 of maximum flexion, maximum extension, and maximum flexion) on seven pinch grip styles (five pulp pinches, a lateral and a chuck pinch). The pulp pinches were defined by the finger that was used with the thumb to pinch. They found that as the wrist deviated from the neutral position, the pinch strength decreased by up to 34 percent for the maximum flexion using the ring finger and thumb.
DeCaro, Feuerstein, and Hurwitz [1992] found that the more the hand and wrist deviated from the neutral position, the more the subjects experienced pain and
fatigue.
Harber, Bloswick, Beck, Pena, Baker, and Lee [1993] compared 50 supermarket checkers’ motions and symptoms from questionnaires. They found relationships between wrist flexion and extension and carpal tunnel syndrome symptoms. They also found that forearm pronation showed a
relationship with hand,wrist, and lower arm symptoms.
Loslever and Ranaivosoa [1993] investigated both biomechanical and epidemiological data on seventeen high risk jobs. They found the prevalence of carpal tunnel syndrome highly correlated with the frequency of flexion and the use of high forces. They also found that high or low flexions were a greater risk than high or low extensions.
The studies indicate that having the wrist in an awkward posture is definitely a contributing factor to the incidence of CTDs.
2. Forceful Exertions
Forceful exertions such as using knives and other tools can cause inflammation of joints, muscles, and tendons. Poorly balanced tools and poorly maintained tools (e.g., dull knives or scissors) can increase the force required by an operator. Gloves can also increase the muscular effort required to perform a gripping task because they can reduce tactile feedback, as well as reduce the friction between the hand and the object being grasped.
According to Welch [1972], the use of excessive force was the cause of tenosynovitis in 30% of 500 cases studied.
Kim and Fernandez [1993] conducted a study for a simulated drilling task at different
applied forces and angles of wrist flexion. They found that the maximum acceptable frequency for
gripping was significantly reduced as the required drilling force and the angle of wrist flexion
increased. Therefore, they concluded that the task frequency for a drilling task should be lowered as
force and wrist flexion angle increase.
Vanderpool, Friis, Smith, and Harris [1993] performed a study with cardiac sonographers. Based on a questionnaire to identify the possible causes of musculoskeletal disorders (the response rate was low) they found that high-pressure hand grip correlated significantly with carpal tunnel syndrome symptom.
3. Cold Temperatures
Working in cold temperatures and handling cold parts can reduce manual dexterity and tactile sensitivity and this demands use of increased force to do the job. This creates the potential for muscle strains and sprains. Gloves when used to avoid the cold temperature, also increase the force required to perform a task.
Schiefer, Kok, Lewis, and Meese [1984] studied the relationship between finger skin temperature
and performance with four manual dexterity tasks. They found that cooling the finger skin temperature to 0-20 degrees Celsius affected strength and dexterity and therefore performance. Hence a person operates at a higher proportion of his maximum strength in cold temperature.
Reference
A COMPUTER-BASED JOB ANALYSIS SYSTEM TO REDUCE
CUMULATIVE TRAUMA DISORDERS
Steven L. Johnson
M. Michelle Dime
Sherry A. Brown
Jeffrey B. Hardcastle
Department of Industrial Engineering
University of Arkansas
Fayetteville, Arkansas 72701
January, 1996
http://comp.uark.edu/~sjohnson/EJA-Users-Manual.pdf
Industrial Engineering Knowledge Revision Plan - One Year Plan
January - February - March - April - May - June
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