Work-Related Musculoskeletal Disorders Report, Workshop Summary, and Workshop Papers (1999) / Chapter Skim
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Work Factors, Personal Factors, and Internal Loads: Biomechanics of Work Stressors
Work Factors, Personal Factors, and Internal Loads: Biomechanics of Work Stressors
Pages 116-151
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The objective of this manuscript is to review the state of available scientific evidence concerning the relationships between work factors, including host factors, ant! the resulting internal tissue loads.
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Tendons are the connective tissues that attach muscle to bone and therefore transmit muscle forces to the skeletal system to produce voluntary movements and exertion.s A consequence of force exerted by the bo(ly, or acting a~ain.st the hociv is that ..
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Mechanical relationships between external forces, postures and internal tendon loading were demonstrated by Armstrong and Chaffin (1979) for the carpal tunnel ofthe wrist using the analogy of a pulley and a belt.
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While the effective distance between the fulcrum and the point of insertion for a specific muscle varies depending on the angle of the joint, the leverage of the muscles is almost always very small relative to the load application point, hence the internal muscle forces are usually several times larger than the external forces. As a result, most of the loads experienced by the joints within the .
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within the muscle. Because the recruitment sequence of muscle fibers during exertions works from smaller to larger fibers, the same small slow twitch fibers are repeatedly used and fatigued even during low level contractions (Sjogaard, 1996 )
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. In asymmetric tasks, for example lateral bending or resisted twisting, the lateral shear forces ranged exceeded 150 N
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Electrophysiological and tactile deficits consistent with carpal tunnel syndrome has been observed under experimentally induced compression of the median nerve (Gelberman, et al., 1981; Gelberman, Szabo, and Williamson, 1983~. The biomechanical model of the wrist developed by Armstrong and Chaffin (1979)
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Internal forces produce torques or rotation about the joints, and tension, compression, torsion, or shear within the anatomical structures of the body. External forces act against the human body, and may be produced by an external object or in reaction to the voluntary exertion of force against an external object.
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Hand-arm, or segmental vibration may be introduced when using power hand tools or operating controls such as steering wheels on off-road vehicles. Physiological responses to human-transmitted vibration include endocrine and metabolic, vasodilatation/constriction, motor, sensory, central nervous system and skeletal responses.
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External physical stress factors described in Sections 3. ~ and 3.2 relates to distinct internal physical stress factors.
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More detailed job analysis methods analyze the job at the element or micro level using by direct physical measurements. These analyses involve breaking down the job into component actions, measuring and quantifying physical stress factors.
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Keyserling (1986) combined time study with posture classification for estimating the time that specific postures are assumed.
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(1990) to study the trunk postures in assembly line workers performing highly repetitive work.
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. The relative effects of hand-transmitted vibration and other physical stress factors are often difficult to separate because many jobs using vibrating hand tools also involve considerable use of the upper limb.
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Workplace Layout The previous discussion has shown that the postures assumed and forces exerted affect internal tissue loading. Given that the design of the workplace affects the forces exerted and postures and motions exhibited by an individual, it is reasonable to expect that the tissue loads are, in part, a function of the way in which the work and the workplace is designed.
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It is not simply the use of a particular tool, but the way the tool is used that imposes physical stress on the tool operator. The relative effects of various physical stress factors involving work with hand tools are difficult to separate because many jobs using hand tools also involve extensive use of the upper limbs.
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Variation in the anthropometric characteristics between individuals will have an impact on the biomechanical properties of the musculoskeletal system, which in turn will impact the internal loads that anatomical structures experience. Sources of anthropometric variation that impact tissue loading include: I
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predicts that wrist size affects wrist loading such that greater tendon load per unit length is inversely proportional to the radius of curvature of the pulley. Variations in work methods will result in different tissue loading due to variations in the postures, motions, and magnitude of the external forces experienced.
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While survey and production time data may have less accuracy and precision, the methods are more practical for observing a greater numbers of subjects over longer observation periods. Based on the relationships between external physical stress factors and properties described, Table 6 considers selected external measurement methodologies and the physical stresses that each is capable of measuring.
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are loa(led during the performance of work tasks. Second we examined the relationship between external physical stresses and internal tissue loading.
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(1988~. Influence of dynamic factors and external loads on the moment at the lumbar spine in lifting.
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(1992~. Biomechanical factors affecting upper extremity cumulative trauma disorders in sign language interpreters.
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. A checklist for evaluation ergonomic risk factors associated with upper extremity cumulative trauma disorders.
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. A myoelectrically based dynamic three-dimensional model to predict loads on lumbar spine tissues during lateral bending.
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(1995~. Musculoskeletal loading and carpal tunnel pressure, Repetitive Motion Disorders of the Upper Extremity, American Academy of Orthopedic Surgeons, Rosemont, IL, 123-132.
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(1992~. Influence of body segment dynamics on loads at the lumbar spine during lifting.
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42 WORK-RELATED MUSCULOSKELETAL DISORDERS Table I: Examples of Physical Stress Factors Cited in The Literature Reference Factors Considered Armstrong, et al., 1981 Armstrong, et al., 1986 Arndt, 1997 Bernard, et al., 1994 Bovenzi, et al., 1991 Chiang, et al., 1990 Derksen, et al.,1994 Feuerstein and Fitzgerald, 1992 Keyserling, 1986 Keyserling, et al., 1993 Marras and Schoenmarklin, 1993 McAtamney, and Corlett, 1993 Silverstein, et al., 1986 Wieslander, et al., 1989 Repeated exertions with certain postures Stressful exertions High forces Repetitive and sustained exertions Certain postures Vibration Low temperatures Mechanical stresses Work pace Working time Time pressure Hours of computer use Vibration acceleration Vibration exposure Local exposure to cold Poor working postures Rest-break frequency Deviations from neutral Work envelope excursions High-impact hand contacts Pace of movements Intensity of muscular tension Smoothness of movements Awkward working postures Repetitiveness Local mechanical contact stress Forceful manual exertions Awkward posture Hand tool use Angular velocity Angular acceleration Posture Muscle use (repetitive or static) Force or load Repetitive motion Forceful exertions Exposure to vibration Heavy loads on the wrist
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WORKSHOP PAPERS: RADWIN AND LAVENDER 143 Table 2. Theoretical Framework for the Relationship Between External Physical Stress Factors and Properties as Typically Descnbed in the Scientific Literature Property Physical Magnitude Repetition Rate Stress Force | Forceful Exertions | Repetitive Exertions Motion Extreme Postures Repetitive Motions and Motions Vibration High Vibration Repeated Vibration Level Exposure Cold Cold Repeated Cold Temperatures Exposure Duration Sustained Exertions Sustained Postures Long Vibration Exposure Long Cold Exposure
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Time of Vibration exposure Time of Cold Exposure
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Cumulative tissue loads · Muscle fiber recruitment and muscle fatigue rate Energy expenditure, fatigue and metabolite production · Cumulative tissue loads · Cumulative vibrational energy exposure · Cumulative thermal energy loss
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:S · o u ~n a' ed ~ o o · .
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High High electromagnetic) Biomechanical Models Using Reach Force (Magnitude)
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High High Electrogoniometers/Motion Analysis and Forces Opposed Accelerometers Attached to Objects Contacted Vibration (Magnitude, Repetition, Duration) Medium High Ambient Temperature Cold (Magnitude)
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150 Physical Stress WORK-RELATED MUSCULOSKELETAL DISORDERS 1 Frequency L Magnitude Duration Ti Ime Figure I: Representation of Magnitude, Duration and Repetition for Physical Stress-Time Record.
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Self-Reports damask Description i T~ oo A Cycle Time PI Goniometer Accelerometers Distances nt me StancJards oduction Rates Pa reclslon Figure 2: Relationship between accuracy and precision for different measurement methodololgies 151
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