Musculoskeletal Disorders and the Workplace Low Back and Upper Extremities (2001) / Chapter Skim
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Biomechanics
Biomechanics
Pages 219-286
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The number of studies reviewed is 196 for the low back and 109 for the upper extremities. CONCEPTS OF LOAD TOLERANCE The term "load" describes physical stresses acting on the body or on anatomical structures within the body.
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Biomechanical loading is further affected by individual factors, such as anthropometry, strength, agility, dexterity, and other factors mediating the transmission of external loads to internal loads on anatomical structures of the body. Measures of External Loads External loads are physical quantities that can be directly measured using various methodologies.
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A conceptual framework is presented below for organizing the physical parameters in manual work. Physical Stresses Physical stress can be described in terms of fundamental physical quantities of kinetic, kinematic, oscillatory, and thermal energy.
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Hand-arm vibration, or segmental vibration, is introduced by using power hand tools or when grasping vehicular controls. Physiological reactions to human-transmitted vibration include responses of the endocrine, metabolic, vascular, nervous, and musculoskeletal systems.
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The relationship between physical stresses and their exposure properties is illustrated in Figure 6.1. Magnitude is the I _ ·_> _ FIGURE 6.1 Representation of magnitude, duration, and repetition for physical stress-time.
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Similarly, temperature level and associated repetition rate and duration quantify cold exposure. Interactions The characteristic exposure properties of physical stresses together quantify external loads acting against the body.
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A body of scientific knowledge from diverse investigations thus emerges. The external physical stress factors described above relate to distinct internal physical stress factors.
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Cold · Thermal energy loss from · Recovery from · Cumulative the extremities thermal energy thermal energy · Cooling of tissues and loss loss bodily fluids · Somatic and autonomic receptor stimulus Note: * Indicates internal stress.
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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 exertions. A consequence of force exerted by the body or acting against the body, motions produced by the body, oscillatory energy transmitted to the body, or thermal energy released from the body, is that adjacent tissues are subjected to mechanical and thermal loads.
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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 body during exer
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In essence, the disc behaves as a pressure vessel and transmits force radially and uniformly. Thus, the disc is capable of withstanding the large compressive forces that result from muscular recruitment.
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Carpal tunnel syndrome is believed to result from a combination of ischemia and mechanical compression of the median nerve within the carpal canal of the wrist. Evidence of compression of the median nerve by adjacent tendons has been reported by direct pressure measurements (Tanzer, 1959; Smith,
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Measurements of Internal Loading Physical stress imparted to internal tissues, organs, and anatomical structures in manual work is rarely measured directly. Due to the obvious complexities and risks associated with invasive internal physical stress measurements, investigations often employ indirect internal measures or external measurements that are physically related to internal loading of the body.
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Therefore, work activities in which cocontraction is more common impose greater loads on the tissues of the musculoskeletal system. Localized Muscle Fatigue As muscles fatigue, the loadings experienced by the musculoskeletal system change.
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Grip force increases observed for sinusoidal vibration at 40 Hz was comparable to grip force when handling a load twice as great. This effect was not observed for 160 Hz vibration.
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Biomechanical loading is also affected by individual characteristics, such as anthropometry, strength, agility, dexterity, and other factors mediating the transmission of external loads
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It is important to realize that individual factors as well as organizational factors and social context can influence biomechanical loading and structure tolerance, as well as the risk of suffering a disorder; these issues are covered in other sections of this report. The objective of this section is to explore the evidence, in this context, that external loads can trigger the pathway to low back disorders.
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This is consistent with the logic described in Figure 1.2. Biomechanical Risk Factors Measured in the Workplace The panel reviewed the industrial observation literature for information relating biomechanical loading of the body and reports of low back disorder.
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The job demand definition considered load location relative to the worker, as well as frequency of lift and exposure time. Demands were considered for all tasks associated with a material handling job.
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This study identified 16 trunk kinematic variables resulting in statistically significant odds ratios associated with risk of low back disorder reporting in the workplace. While none of the single variables was as strong a predictor as load moment, when load moment was combined with three kinematic variables (relating to the three dimensions of trunk motion)
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The evaluation considered factors expected to be associated with spine loading, including load location measures. These measures defined an expected worker tolerance (identified by biomechanical, physiological, strength, or psychophysical limits)
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240 u o cry 5o V)
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Second, increased low back disorder reporting can be identified well when the location of the load relative to the body (load moment or load location) is quantified in some way.
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The early models of spine loading made assumptions about which trunk muscles supported the external load during a lifting task (Chaffin and Baker, 1970; Chaffin et al., 1977~. These models assumed that a single muscle vector could be used to summarize the load supporting (and spine loading)
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As indicated in the review of quantitative biomechanical surveillance studies, most spine loading estimates performed at the workplace employed twodimensional, single-equivalent muscle models. Thus, one would expect that in these studies, the spinal compression was underestimated and shear force estimates would not be realistic.
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Under controlled laboratory conditions, these authors employed biologically assisted models to assess the biomechanical significance of exposure to these "field documented" safe or risky exposure levels for all five risk factors. In a series of studies, they showed that exposure to higher load moments and forward flexion (Marras and Sommerich, 1991a, l991b; Granata and Marras, 1993, 1995a)
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Both the field surveillance as well as the biomechanical interpretation of the risk factors in this study agree well with field surveillance and biomechanical interpretation of risk factors described earlier by Marras and colleagues. Hence, it is clear that unless sufficiently sensitive and robust biomechanical analyses are performed at the worksite, the relationship between factors associated with workplace observations of risk and biomechanical loading may not be apparent or this relationship may be underestimated.
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These increased internal forces resulted in greater spine loading in both compression and shear. These findings are consistent with the observations of the importance of load moment noted in Table 6.4.
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How these relate operationally to clinical syndromes is less certain. Spine Tissue Tolerance Biomechanical logic dictates that loads imposed on a structure must exceed a mechanical tolerance limit for damage to occur.
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. Thus, proper biomechanical assessments of low back risk at work can be performed only when the posture of the trunk is considered.
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All of the industrial surveillance studies shown in Table 6.4 indicate that load location (known to affect trunk posture) , observed trunk posture, or both are associated with an increased risk of low back pain at work.
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(1993, 1995) , who found that exposure to external load moments of 73.6 Nm was associated with high risk of occupationally related low back pain reporting.
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Torsion can also cause the facet joints to fail (Adams and Hutton, 1981~. More rapid twisting motions have been associated with high-risk jobs and laboratory investigations have explained how increases in twisting motion can lead to increases in spine loading in compression as well as shear (McGill, 1991; Marras and Granata, 1995~.
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This laboratory study has demonstrated how individual factors such as personality can interact with perception of psychosocial stress to increase trunk muscle coactivation and subsequent spine loading. Hence, it appears that psychosocial stress may influence risk through a biomechanical pathway.
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but a small number of "in-plant" studies were also considered, in which laboratory methods were followed in the field. While most studies were performed in viva in a true laboratory setting, we also considered some cadaver studies and biomechanical models in which strain was measured or computed while systematically manipulating external physical stress.
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external loads and pain, discomfort, functional limitations, and disability. Physical Stress Factors and External Loading The cumulative trauma model (Figure 1.2)
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A laboratory study investigated the effects of complex wrist-forearm postures on wrist range of motion in the flexion-extension and radial-ulnar deviation planes (Marshall, Mozrall, and Shealy, 1999~. Combinations of wrist-forearm postures had significant effects on wrist range of motion; the largest effects were those of wrist flexion-extension on radial deviation.
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Vibration A study by Radwin, Armstrong, and Chaffin (1987) demonstrated that hand-arm vibration exposure, similar to the vibration associated with the operation of power hand tools, directly affects the force exerted when handling tools.
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Among the five buildup times tested, hand motion was greatest for 150 ms. External Physical Loading and Internal Loads The relationship between external loading and biomechanical loading (internal loads and physiological responses)
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(3) Consequently, the force acting on adjacent anatomical structures, such as ligaments, bones, and the median nerve, depends on the wrist angle.
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Viscoelastic properties were measured under simulated physiological loading conditions by attaching strain gauge transducers on tendons just proximal and distal to an undisrupted carpal tunnel. Shear traction forces were significantly greater in the extended and flexed wrist postures compared with the neutral wrist posture and were significantly greater in flexion than in extension.
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found that although the external load on the finger remained constant, the internal loading, as measured by carpal tunnel pressure, experienced a nearly twofold increase by using a pinch grip. Magnetic resonance images of the wrist in the neutral position, 45degree flexion, and 45-degree extension have been used to measure the distance between confining structures around the median nerve (Skie et al., 1990~.
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A simulated drilling task that controlled applied force and wrist flexion found that EMG activity in the finger flexor and extensor muscles increased with force (Kim and Fernandez, 1993~. Dahalan and colleagues (1993)
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A summary of articles measuring internal tolerances due to external loads appears in Table 6.7. Physiological Measures of Mechanical Strain or Fatigue from External Loads Electromyography and blood flow have been used to measure the effects of work pauses on localized muscle fatigue in the upper limbs.
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found that when subjects maintained a 5.4 kg simulated drilling force for a total duration of three minutes, systolic blood pressure and deltoid EMG activity increased with trial duration and wrist flexion angle, while deltoid median power frequency decreased with wrist angle and trial duration, indicating localized fatigue. Dahalan, lalaluddin, and Fernandez (1993)
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Stover Snook, who pioneered psychophysical methods for lifting, established a procedure to ascertain the maximum acceptable torques for various types and frequencies of repetitive wrist motion (Snook et al., 1995~.
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Maximum acceptable torque and extension duration decreased with increasing task frequency. Maximum acceptable torque during wrist extension with a pinch grip was less than wrist flexion with a pinch grip, wrist flexion with a power grip, or ulnar deviation.
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Perceived exertion increased with force magnitude, wrist flexion angle, and task duration. A number of experiments performed by Ulin employed psychophysical methods for studying power hand tool orientation, location, and shape.
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evaluated the relative effects of power hand tool process parameters (target torque, torque buildup time, and workstation orientation) on subjective ratings of perceived exertion.
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These observations suggested to the authors a possible impairment of the protective withdrawal reflex under vibratory environmental conditions at rest and eventually in active muscles. External Physical Loading and Pain, Discomfort, or Functional Limitations The following section reviews literature that directly investigated pain, discomfort, or functional limitations due to external loading.
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and maximum power grip strength. Strength was affected by ulnar deviation angle, and grip force was greatest in the neutral position and decreased as the deviation angle increased.
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A psychomotor task was developed by leng, Radwin, and Rodriquez (1994) for investigating functional deficits associated with carpal tunnel syndrome.
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The biomechanical literature has identified relationships between physical work attributes and external loads for force, posture, vibration, and temperature. Research has also demonstrated relationships between external loading and biomechanical loading (i.e., internal loads or physiologic responses)
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