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Page 219 6 Biomechanics This chapter provides a review of the biomechanics literature on the low back and upper extremities. Biomechanics is the study of forces acting on and generated within the body and of the effects of these forces on the tissues, fluids, or materials used for diagnosis, treatment, or research purposes. The discussion begins with an overview of basic concepts and methods. This is followed by the two literature reviews. The study selection criteria are presented at the beginning of each review. The two bodies of literatures differ in maturity; the research on the low back is more substantial. 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. These stresses include kinetic (motion), kinematic (force), oscillatory (vibration), and thermal (temperature) energy sources. Loads can originate from the external environment (such as the force generated by a power hand tool) or they may result from voluntary or involuntary actions of the individual (for example, lifting objects). The term “tolerance” is used to describe the capacity of physical and physiological responses of the body to loading. Acute Trauma Load-Tolerance Injury Model Acute trauma injuries refer to those arising from a single identifiable event. Examples of acute injuries include fractures, lacerations, and contusions. Disorders resulting from acute trauma may occur when transient
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Page 220external loads, which are transmitted through biomechanical loading of the body, exceed internal tolerances of the affected tissues for mechanical strain, resulting in pain, discomfort, impairment, or disability. These factors may be affected by individual and organizational factors and by the social context in which the individual is operating. Cumulative Trauma Load-Tolerance Model Work-related musculoskeletal disorders arise from a complex interaction of events that may accumulate over time. In contrast to the acute trauma model, the cumulative trauma model assumes injury may result from the accumulated effect of transient external loads that may, in isolation, be insufficient to exceed internal tolerances of tissues. It is when this loading accumulates by repeated exposures, or exposures of sufficiently long duration, that the internal tolerances of tissues are eventually exceeded. The cumulative trauma model therefore explains why many musculoskeletal disorders are associated with work, because individuals often repeat actions (often many thousands of times) throughout the workday, or spend long periods of time (as much as eight hours or more daily) performing work activities in many occupations. Internal mechanical tolerance represents the ability of a structure to withstand loading. It is clearly multidimensional and is not considered a threshold but rather the capacity of tissues to prolong mechanical strain or fatigue. Internal tissue tolerances may themselves become lowered through repetitive or sustained loading. A schematic diagram useful for elaborating the factors that can cause pain, discomfort, impairment, and disability is illustrated in Figure 1.2. External loads are produced in the physical work environment. These loads are transmitted through the biomechanics of the limbs and body to create internal loads on tissues and anatomical structures. Biomechanical factors include body position, exertions, forces, and motions. External loading also includes environmental factors whereby thermal or vibrational energy is transmitted to the body. 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. External kinetic measurements, for example, include physical properties of the exertions (forces actually applied or created) that individuals make. These measurements have the
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Page 221most direct correspondence to internal loads because they are physically and biomechanically related to specific anatomical structures of the body. When external measurements cannot be obtained, quantities that describe the physical characteristics of the work are often used as indirect measures. These include (a) the loads handled, (b) the forces that must be overcome in performing a task, (c) the geometric aspects of the workplace that govern posture, (d) the characteristics of the equipment used, and (e) the environmental stressors (e.g., vibration and cold) produced by the workplace conditions or the objects handled. Alternatively, less directly correlated aspects of the work, such as production and time standards, classifications of tasks performed, and incentive systems, are sometimes used as surrogate measures to quantify the relationship between work and physical stress. The literature contains numerous methodologies for measuring physical stress in manual work. Studies from different disciplines and research groups have concentrated on diverse external factors, workplaces, and jobs. Factors most often cited include forceful exertions, repetitive motions, sustained postures, strong vibration, and cold temperatures. Although the literature reports a great diversity of such factors, it is possible to group these methodologies into a coherent body of scientific inquiry. 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. These basic quantities constitute the external and internal loading aspects of work and energy produced by, or acting on, the human in the workplace. Kinetic (Force) Measurements Force is the mechanical effort for accomplishing an action. Voluntary motions and exertions are produced when internal forces are generated from active muscle contraction in combination with passive action of the connective tissues. Muscles transmit loads through tendons, ligaments, and bone to the external environment when the body generates forces through voluntary exertions and motions. Internal forces produce torques about the joints and tension, compression, torsion, or shear within the anatomical structures of the body. External forces act against the human body and can be produced by an external object or in reaction to the voluntary exertion of force against an external object. Force is transmitted back to the body and its internal
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Page 222structures when opposing external forces are applied against the surface of the body. Localized pressure against the body can transmit forces through the skin to underlying structures, such as tendons and nerves. Pressure increases directly with contact force over a given area and decreases when the contact area is proportionally increased. Contact stress is produced when forces compress the soft tissues between anatomical structures and external objects. This may occur when grasping tools or parts or making contact with the workstation. Contact stress may be quantified by considering contact pressure (force per unit area). An increase in contact force or a decrease in contact area will result in greater contact stress. Pounding with the hands or striking an object will give rise to stress over the portion of body contact. Reaction forces from these stress concentrations are transmitted through the skin to underlying anatomical structures. Kinematics (Motion) Measurements Motion describes the displacement of a specific articulation or the position of adjacent body parts. Motion of one body segment relative to another is most commonly quantified by angular displacement, velocity, or acceleration of the included joint. Motion is specific to each joint and therefore motions of the body are fully described when each individual body segment is considered together. Motions create internal stress by imposing loads on the involved muscles and tendons in order to maintain the position, transmitting loads to underlying nerves and blood vessels, or creating pressure between adjacent structures within or around a joint. Oscillatory (Vibration) Measurements Vibration occurs when an object undergoes oscillatory or impulsive motion. Human vibration occurs when the acceleration of external objects acts against the human body. Vibration is transmitted to the body through physical contact, either from the seat or the feet (whole-body vibration) or when grasping a vibrating object (hand-arm vibration). Whole-body vibration is associated with vibration when riding in a vehicle or standing on a moving platform. 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. External vibration is transmitted from the distal point of contact to proximal locations on the body, which sets into motion the musculoskeletal system, receptor organs, tissues, and other anatomical structures.
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Page 223Vibration transmission is dependent on vibration magnitude, frequency, and direction. Dynamic mechanical models of the human body describe the transmission characteristics of vibration to various body parts and organs. Such models consider the passive elemental properties of body segments, such as mass, compliance, and viscous damping. Vibration transmission is affected by these passive elements and is modified by the degree of coupling between the vibration source and the body. The force used for gripping a vibrating handle and the posture of the body will directly affect vibration transmission. Thermal (Temperature) Measurements Heat loss occurs at the extremities when working outdoors, working in indoor cold environments such as food processing facilities, handling cold materials, or exposing the hands to cold compressed air exhausts. Local peripheral cooling inhibits biomechanical, physiological, and neurological functions of the hand. Exposure to localized cooling has been associated with decrements in manual performance and dexterity, tactility and sensibility, and strength. These effects are attributable to various physiological mechanisms. Physical Stress Exposure Properties The physical stresses described above may be present at varying levels. These variations can be characterized by three properties: magnitude, repetition, and duration. The relationship between physical stresses and their exposure properties is illustrated in Figure 6.1. Magnitude is the ~ enlarge ~ FIGURE 6.1 Representation of magnitude, duration, and repetition for physical stress-time.
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Page 224 extent to which a physical stress factor is involved. Magnitude quantifies the amplitude of the force, motion, vibration, or temperature time-varying record and has the physical units of the corresponding physical measure (e.g., Newtons of force, degrees of rotation, m/s2 of vibration acceleration, or degrees Celsius of temperature). Repetition is the frequency or rate at which a physical stress factor repeats. Duration corresponds to the time that one is exposed to a physical stress factor and is quantified in physical units of time. Force is quantified by its magnitude, the repetition rate, and duration of force application at a given location of the human body. Measures of motion include the magnitude of joint angular displacement, velocity, or acceleration; the repetition rate of the motion; and the duration time that the motion is sustained. Vibration is quantified by the magnitude of the acceleration of a body, the repetition rate at which vibration occurs, and the duration time the vibration is sustained. 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. Combinations of different physical stresses and exposure properties can be used to describe factors that are commonly reported for quantifying exposure. These relationships are summarized in Table 6.1. Physical stresses are correspondingly quantified as described in Table 6.2. This organization is useful because it provides a construct for comparing and combining studies using different measurements and methodologies, as represented in Table 6.1, into a TABLE 6.1 Theoretical Framework for the Relationship Between External Physical Stress Factors and Properties as Typically Described in the Scientific Literature Property Physical Stress Magnitude Repetition Rate Duration Force Forceful exertions Repetitive exertions Sustained exertions Motion Extreme postures and motions Repetitive motions Sustained postures Vibration High vibration level Repeated vibration exposure Long vibration exposure Cold Cold temperatures Repeated cold exposure Long cold exposure
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Page 225 TABLE 6.2 Relationship Between External Physical Stress Factors and Their Properties as They are Typically Measured Property Physical Stress Magnitude Repetition Rate Duration Force Force generated or applied Frequency with which force is applied Time that force is applied Motion Joint angle, velocity, acceleration Frequency of motion Time to complete motion Vibration Acceleration Frequency with which vibration occurs Time of vibration exposure Cold Temperature Frequency of cold exposure Time of cold exposure common framework. For example, physical stress measurements using a survey methodology that simply assesses the presence or absence of highly repetitive wrist motions can therefore be compared with a study that measures the frequency of motions using an electrogoniometer. This is possible because both studies have quantified the repetition property of wrist motion. Similarly, a study that considers the weight of objects lifted can be compared with a study that assesses muscle force using electromyography because both studies quantify the magnitude of force. A body of scientific knowledge from diverse investigations thus emerges. The external physical stress factors described above relate to distinct internal physical stress factors. This relationship is summarized in Table 6.3. For example, force magnitude is directly related to the loading of tissues, joints, and adjacent anatomical structures, as are the metabolic and fatigue processes of contracting muscles. The strength of these relationships depends on the particular measurement and the type of stress. Biomechanical and physiological mathematical models have been developed to quantitatively describe some of these relationships. Moore, Wells, and Ranney (1991) and Armstrong et al. (1993) have recognized similar relationships between external and internal factors. Internal Loads The musculoskeletal system is the load bearing structure within vertebrate animals. Bony structures bear gravitational forces and internal forces of skeletal muscle contraction in maintaining the body posture. As such, bones are the primary load-bearing tissue within the body. Forces applied to the body, including gravity, compress or bend the bones. Liga-
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Page 226 TABLE 6.3 Relationships Between External and Internal Physical Stress Property Physical Stress Magnitude Repetition Duration Force Tissue loads and stress Muscle tension and contraction Muscle fiber recruitment Energy expenditure, fatigue, and metabolite production Joint loads Adjacent anatomical structure loads and compartment pressure Transmission of vibrational energy Tissue loading rate and energy storage Tissue strain recovery Muscle fiber recruitment and muscle fatigue rate Energy expenditure, fatigue, and elimination of metabolites Cartilage or disc rehydration Cumulative tissue loads Muscle fiber recruitment and muscle fatigue rate Energy expenditure, fatigue, and metabolite production Motion Tissue loads and stress Adjacent anatomical structure loads and compartment pressure Transmission of vibrational energy* Tissue loading rate and energy storage Tissue strain recovery Cumulative tissue loads Vibration Transmission of vibrational energy to musculoskeletal system Transmission of vibrational energy to somatic and autonomic sensory receptors and nerves Transmission of energy to muscle spindles* Recovery from vibrational energy exposure Cumulative vibrational energy exposure Cold Thermal energy loss from the extremities Cooling of tissues and bodily fluids Somatic and autonomic receptor stimulus Recovery from thermal energy loss Cumulative thermal energy loss Note: * Indicates internal stress.
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Page 227 ments hold together the bony structure by crossing articulations where bones interconnect. Retinacula share similar structural and biomechanical properties to ligaments that act as pulley systems by guiding tendons around articulations. 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. These include ligaments and connective tissue, tendon, muscle, intervertebral discs, and nerves. A detailed examination of how each of these tissues is subjected to internal loading follows. Ligaments and Connective Tissue By their nature, as the connective tissues linking bones within the skeletal system, ligaments are primarily exposed to tensile loads. A typical stress-strain curve for ligamentous tissue reveals that the tissue initially offers little resistance to elongation as it is stretched; however, once the resistance to elongation begins to increase, it does so very rapidly. Thus, the ligaments, while loosely linking the skeletal system, begin to resist motion as a joint's full range of motion is approached. By severing ligaments in cadaveric lumbar motion segments, Adams et al. (1980) showed that the supraspinous-interspinous ligaments segments are the first ligamentous tissues to become stressed with forward bending of the lumbar spine. Stability and movement of the spine or any other articulation within the low tensile region of the ligamentous stress-strain curve must be accomplished using muscular contraction. This is not to say that ligaments do not contribute to joint loading. Several authors have shown that with extreme flexion (forward bending) of the torso, there is an electrical silence in the spinal musculature (Floyd and Silver, 1955; Golding, 1952; Kippers and Parker, 1984; Toussaint et al., 1995). This finding suggests that at times ligaments are used to resist the bending moments acting on the spine. The degree of ligamentous contribution to the forces placed on the intervertebral disc during manual material handling tasks has been debated in the scientific literature (Cholewicki and McGill, 1992; Dolan, Earley, and Adams, 1994; Potvin, Norman, and McGill, 1991). Nevertheless, there is consensus that ligaments are subjected to tensile stress with extreme movements and hence can contribute to the mechanical loads placed on the body's articulations, including the intervertebral disc. When ligaments act as a turning point for tendons (pulleys), they are exposed to shear forces and contact stresses. For example, the transverse
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Page 228carpal ligament, in bridging the carpal bones in the wrist, forms a pulley by which the path of the finger flexor tendons is altered when the wrist is flexed. Similarly, the palmar ligaments maintain the path of the tendons from the finger flexor muscles to the distal phalanges. Goldstein et al. (1987) showed that the tendon strain on the proximal side of the transverse carpal ligament was greater than the strain on the distal side of the ligament. This finding indicates that the friction between the tendon and the ligament results in the ligament being exposed to shear loads in addition to normal loads. Goldstein et al. (1987) also demonstrated that the magnitude of shear was dependent on an interaction between tensile load and posture. Tendons Tendons are a collagenous tissue that forms the link between muscle and bone. The orientation of the collagen fibers in tendons is in the form of parallel bundles. This arrangement of fibers minimizes the stretch or creep in these tissues when subjected to tensile loading (Abrahams, 1967). With repeated loading of synovial tissues, surrounding tendons can become inflamed, particularly where the tendons wrap around bony or ligamentous structures. In more severe cases, the collagen fibers of the supraspinatus tendon can become separated and eventually degraded, wherein debris containing calcium salts creates further swelling and pain (Schechtman and Bader, 1997). Muscles Skeletal muscles provide locomotion and maintenance of posture through the transfer of tension by their attachment to the skeletal system via tendons. Tension is developed through active contraction and passive stretch of contractile units, or muscle fibers. The musculoskeletal system uses simple mechanics, such as levers, to produce large angular changes in adjoining body segments. Consequently, the amount of muscular force required to produce a desired exertion or movement depends on the external force characteristics (resistance or load dynamics handled) and the relative distance from the fulcrum to the point of external force application and from the fulcrum to the point of muscular insertion. 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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Page 229tions result from the internal muscle forces as they work in opposition to the external forces. Intervertebral Disc The intervertebral disc serves as a joint since it permits rotation and translation of one vertebra relative to another. It also maintains the space between vertebrae so that spinal nerves remain unimpinged and protects the upper body and head from the large peak forces experienced in the lower extremities. Anatomically, the disc is comprised of two parts: the nucleus pulposus and the annulus fibrosus. The nucleus pulposus is in the central region of the disc and is comprised of a gelatinous mixture of water, collagen, and proteoglycans. The annulus fibrosus is comprised of alternating bands of angled fibers oriented approximately 60 degrees relative to the vertical (White and Panjabi, 1990). 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. Hutton and Adams (1982) found that cadaver discs from males between the ages of 22 and 46 could, on average, withstand single loads of over 10,000 N before failure occurred. In most cases, the failure was in the thin bony membrane that forms the boundary between the disc and the vertebral body (vertebral endplate) rather than through nuclear prolapse. Since the disc is an avascular structure, the health of the endplate is critical for nutrient exchange, and even small failures may hasten the degenerative process. Researchers have found that prolapsed discs occurred more frequently when the vertebral segments were wedged to simulate extreme forward bending of the spine (Adams and Hutton, 1982). In this position, the anterior portion of the annulus fibrosis undergoes compression while the posterior portion is under tensile stress. Over 40 percent of the cadaver discs tested by Adams and Hutton (1982) prolapsed when tested in this hyperflex posture, and with an average of only 5,400 N of compression force applied. This finding shows that the disc is particularly susceptible to bending stresses. In a later study in which Adams and Hutton (1985) simulated repetitive loading of the disc, previously healthy discs failed at 3,800 N, again mostly through trabecular fractures of the vertebral bodies. Taken together, these studies show that the disc, especially the vertebral endplate, is susceptible to damage when loading is repetitive or when exposed to large compressive forces while in a severely flexed posture. Since in vitro studies of lumbar motion segment failure may not fully represent the state of affairs in vivo, additional factors have been considered. It should be clear from earlier discussions of muscle that the internal
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Page 276justable workstations were built to simulate repetitive wrist flexion with a power grip, wrist flexion with a pinch grip, and wrist extension with a power grip. In general, maximum acceptable torque decreased as the exertion frequency increased for the three types of exertions. Maximum acceptable torque was greatest for power grip flexion and least for power grip extension. Maximum acceptable torque decreased over the seven hours of testing. There were no significant differences in maximum acceptable torque from day to day; however, the average maximum acceptable torque for a 5 days per week exposure was 36.3 percent lower than for the same task performed 2 days per week. In an experiment similar to the one for wrist flexion-extension, Snook et al. (1997) quantified maximum acceptable torques for ulnar deviation motions of the wrist similar to a knife-cutting task at various repetition rates using the psychophysical method. The subject adjusted the resistance on the handle while the experimenter manipulated or controlled all other variables. The subjects were instructed to work as if they were paid on an incentive basis. Maximum acceptable torque decreased over the 7 hours of testing in both series. Maximum acceptable torque decreased with increasing frequency in both series, but the change was not statistically significant. Snook, Ciriello, and Webster (1999) employed the same method to determine maximum acceptable torque for extension motions of the wrist performed with a pinch grip. 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. Psychophysical measures were used by Kim and Fernandez (1993) to investigate simulated repetitive drilling tasks. Maximum acceptable frequency decreased with greater drilling force and with greater wrist flexion. Ratings of perceived exertion increased with force and with wrist flexion angle. Marley and Fernandez (1995) used the method of adjustment to determine the maximum acceptable frequency for a simulated drilling task. The psychophysically adjusted task frequency was significantly lower when wrist deviation was required, particularly wrist flexion. A similar laboratory study investigated the maximum acceptable frequency for a simulated gripping task (Dahalan, Jalaluddin, and Fernandez, 1993). Maximum acceptable frequency decreased significantly as grip force magnitude and exertion duration increased. Ratings of perceived exertion increased with higher grip force. Davis and Fernandez (1994) found that the acceptable frequency for a simulated drilling task was maximum with a neutral wrist position and decreased with increased angles of wrist flexion, extension, and radial
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Page 277deviation. Marley and Fernandez (1995) showed that maximum acceptable frequency for a simulated drilling task decreased as a function of wrist flexion angle. Klein and Fernandez (1997) evaluated the effects of wrist posture on maximum acceptable frequency for a simulated drilling task. Wrist flexion (10 and 20 deg), extension (20 and 40 deg), and radial deviation (10 and 20 deg) all produced significant decreases in maximum acceptable frequency compared with the neutral posture. Another area in which psychophysical measures have been used is to study lifting, positioning, and pinching tasks. Work duration for limiting shoulder-girdle fatigue during lifting-positioning tasks decreased, as with force and with repetition rate (Putz-Anderson and Galinsky, 1993). When repetition and reach height were varied, task duration decreased, as with required working height and with required repetition rate. Males tended to engage in longer work trials than females, despite controlling for upper body strength. Klein and Fernandez (1997) used the psychophysical approach to determine maximum acceptable frequency for pinching using a lateral pinch posture. Maximum acceptable frequency was reduced as wrist flexion angle, force magnitude, and task duration increased. 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. Following each treatment, subjects rated exertion level and discomfort using three psychophysical scales (the Borg 10-point ratio rating scale and two 10-centimeter visual analog scales used to rate comfort and ease of work). Subjects were instructed to imagine that they were assembly line workers performing the task for an 8-hour day. In 1990, Ulin et al. determined the preferred work location for driving screws with a pistol-shaped screwdriver to be 114 to 140 cm for a mixed male-female subject pool. In a 1992 study, Ulin et al. demonstrated how work location, work orientation, and tool selection affected perceived exertion when using pneumatic hand tools. Lowest exertions were observed when working in neutral postures. Ulin and colleagues found (1) that perceived exertion was lowest when the horizontal reach distance was small and when working at mid-thigh or elbow height (1993b) and (2) that perceived exertion increases as a function of work pace (1993a). In addition, perceived exertion is affected by work location, work orientation, and tool type. Both work location and task frequency were significant factors in determining the Borg rating. As work pace increased, so did the Borg rating of perceived exertion for each work location. Driving screws at elbow height on the vertical surface and with the lower arm close to the body on the horizontal surface was the work location that produced the lowest ratings of
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Page 278perceived exertion. Differences in local discomfort were found for the vertical work locations. While driving screws at knee height, the torso was most stressed, at elbow height the wrist-hand were most stressed, and at shoulder height the shoulder and upper arm were the most stressed. Schoenmarklin and Marras (1989b) demonstrated that hammer handle angle did not significantly affect forearm muscle fatigue based on a shift in EMG mean power frequency, but wall hammering produced marginally greater muscle fatigue than did bench hammering. Linqvist (1993) observed a correlation between power hand tool handle displacement and subjective strain ratings. This laboratory study investigated responses to power tool spindle torque reaction forces during the final stages of tightening threaded fasteners with a right-angled nut runner. A distinctive feature of nut runners during the torque reaction phase is that the handle is rapidly displaced as torque builds up, causing a movement of the upper extremity. Subject ratings of strain increased monotonically as a function of torque level. Ratings of strain were higher for medium-hard joints compared with hard joints, and ratings of strain were higher for slow-shutoff tools compared with high-shutoff tools. Ratings of strain were positively correlated with the handle displacement; correlations were strongest for the slow-shutoff tool used on a hard joint. Oh and Radwin (1998) evaluated the relative effects of power hand tool process parameters (target torque, torque buildup time, and workstation orientation) on subjective ratings of perceived exertion. Increasing the torque reaction force resulted in higher ratings of perceived exertion. Subjective ratings of perceived exertion were lowest when torque buildup time was 35 ms; however, greater peak torque variance was associated with this condition. Radwin and Ruffalo (1999) investigated the effects of key-switch design parameters on short-term localized muscle fatigue in the forearm and hand. Subject reports of fatigue were reduced with the lower key make-force. Self-reported fatigue occurred in all cases (keying rate decreased over the duration of the test session), but no significant differences were observed based on RMS EMG for low-level exertions in repetitive keying. A laboratory study evaluated the effects of muscle, tendon, or skin vibration on the early and late components of polyphasic cutaneous responses elicited in the flexor carpi radialis muscle by electrical stimulation of the radial nerve at the wrist (Martin, Roll, and Hugon, 1990). Palm skin vibration depressed both components of the flexor reflex, while skin vibration on the back of the hand induced either a facilitation or an inhibition. In addition, this kind of vibration modified the location of the
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Page 279sensation evoked by the electrical stimulation of the nerve. In all cases, the vibration stimulus attenuated the perceived intensity of the electrical stimulus. 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. The studies that are reviewed report short-term impairments of function observed in the laboratory or in the field, rather than long-term impairments or disabilities. Studies dealing with longer-term effects are reviewed in the epidemiology literature covered in Chapter 4. Rather these studies reveal relationships between workplace exposures and short-term outcomes such as pain, discomfort, and level of function. A summary of these articles is presented in Table 6.8. Pain and Discomfort Due to External Loading In some studies, pain and discomfort have been examined in relation to work posture and force. For example, Schoenmarklin and Marras (1989b) found that hammering on a vertical wall resulted in significantly greater discomfort than hammering on a bench. Gerard et al. (1999) found that subjective discomfort increased as a function of key make-force with rubber dome key switches. Lin, Radwin, and Snook (1997) found that force, wrist flexion angle, and repetition are all significant factors in determining discomfort. They developed a subjective model of discomfort on a 10-cm analog scale. The continuous model was compared with and agrees with discrete psychophysical data from other published studies. Other studies have demonstrated the effects of pace and work schedule on perceived pain, discomfort and exertion. For example, a study by Hagberg and Sundelin (1986) found that pain and discomfort reports increased with longer durations of work time. The increase was smallest when the work-rest cycle included short rest periods. In a laboratory experiment, Snook et al. (1997) found that the rate of pain and discomfort reports increased with longer duration of work in which subjects adjusted the resistance of a handle while grasping it with a power grip and repetitively moving it through 80 deg ulnar deviation wrist motion, similar to a knife-cutting task (Snook et al., 1997). In a study by Ulin et al. (1993b) both work location and task frequency were found to be significant factors—
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Page 280 TABLE 6.8 Summary Table of Articles Measuring Adverse Outcomes (Pain, Discomfort, Impairment, or Disability) Due to External Loads (Force, Motion, Vibration, and Cold) and Their Properties (Magnitude, Frequency, and Duration) External Load Force Motion Vibration Temperature Reference Adverse Outcome External Load Mag Rep Dur Mag Rep Dur Mag Rep Dur Mag Rep Dur Hand and Wrist Imrhan, 1991 Pinch strength Wrist and pinch posture • Snook et al., 1997 General discomfort Work duration in repetitive ulnar deviation task • • • Radwin and Jeng, 1997 Keying rate Keyboard force • Radwin and Ruffalo, 1999 Keying rate and localized discomfort Keyboard force • Ulin et al., 1990 General discomfort Vertical height in power screwdriver operation • Schoenmarklin and Marras, 1989b General discomfort Orientation for hammering • Ulin et al., 1993a General discomfort Power hand tool shape, work orientation, and work location • • • • Batra et al., 1994 General discomfort and grip strength Glove thickness • • Fleming, Jansen, and Hasson, 1997 Endurance time Gripping with and without gloves • •
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Page 281 Schiefer et al., 1984 Manual performance in block threading, knot tying, peg test, and threading screws Cold environment • Gerard et al., 1999 Localized discomfort Keyboard key mechanism • Gerard and Martin, 1999 Manual performance in visual-manual tracking Handle vibration and recovery time • • Riley and Cochran, 1984 Manual performance in pegboard, pencil tapping, tweezers manipulation, and assembly Cold environment • Malchaire, Piette, and Rodriguez-Diaz, 1998 Vibration perception threshold Handle vibration • Lin, Radwin, and Snook, 1997 General discomfort Force and posture repetition frequency • • • • Lin and Radwin, 1998 General discomfort Force and posture and repetition frequency • • • • O'Driscoll et al., 1992 General discomfort Force and posture and repetition frequency • • • • Knowlton and Gilbert, 1983 Grip strength Following hammering tasks using hammers with angled handles • • Mital, Kuo, and Faard, 1994 Torque strength Use of common hand tools and gloves • continues
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Page 282 TABLE 6.8 Continued External Load Force Motion Vibration Temperature Reference Adverse Outcome External Load Mag Rep Dur Mag Rep Dur Mag Rep Dur Mag Rep Dur Holewijn and Heus, 1992 Grip strength, maximum rhythmic frequency and endurance time Cooling the hands • Dempsey and Ayoub, 1996 Pinch strength Wrist flexion/extension, ulnar/radial deviation, and pinch separation • Pryce, 1980 Grip strength Wrist flexion/extension and ulnar/radial deviation • O'Driscoll et al., 1992 Grip strength Wrist flexion/extension and ulnar/radial deviation and handle size • Blackwell, Kornatz, and Heath, 1999 Grip strength Handle size • Hallbeck and McMullin, 1993 Grip strength Wearing gloves while varying wrist flexion/extension • • Jeng, Radwin, and Rodriquez, 1994 Maximum pinch rate Pinch force •
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Page 283 Neck, Shoulder, and Upper Arm Oberg, Sandsjo, and Kadefors, 1994 Localized discomfort in the trapezius muscle region Duration of holding a load with the shoulder abducted • • Putz-Anderson and Galinsky, 1993 Time for localized discomfort for the shoulder girdle Load in the hand and repetition rate in repetitive lifting and positioning tasks • • •
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Page 284 that is, as work pace increased, so did the Borg rating of perceived exertion for each work location. Differences in local discomfort were found for the vertical work locations. Functional Limitations Due to External Loading Marshall, Mozrall, and Shealy (1999) demonstrated that the combination of wrist-forearm posture had significant effects on wrist range of motion. Pryce (1980) studied the effect of wrist posture (neutral and ulnar deviation, and 15 deg each side of neutral in volar and dorsiflexion) 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. Strength was also affected by extension-flexion angle. Knowlton and Gilbert (1983) investigated the effects of ulnar deviation on strength decrements when using hammers to drive nails in a standard task. Grip strength was measured before and after performing the hammering task. Peak grip strength was reduced by an average 67 N with a conventional claw hammer compared with 33 N with a curved-handle ripping hammer. Average grip strength was reduced by 84 N with the claw hammer compared with 49 N with the ripping hammer. There was no significant difference between the number of strikes required to complete the task with the two tools. Imrhan (1991) examined the effects of different wrist positions on maximum pinch force. The results showed that all of the deviated wrist positions reduced the observed pinch strength, with palmar flexion having the greatest effect and radial deviation having the least. O'Driscoll et al. (1992) also investigated the effects of posture on grip strength. Grip strength was reduced in any deviation from a self-selected position. Measured strength and the degree of wrist extension was inversely related to the handle separation distance on the Jamar dynamometer. This was true regardless of hand size, although the effects were more pronounced for small hands. A laboratory study by Hallbeck and McMullin (1993) found that gender, glove type, hand dominance, and wrist position had a significant effect on the magnitude of power grasp. Force was maximized with a bare hand in a neutral wrist posture. Dempsey and Ayoub (1996) reported that gender, wrist posture (neutral, maximum flexion, maximum extension, maximum radial deviation, and maximum ulnar deviation), pinch type (pulp2, pulp3, chuck, and lateral), and pinch width (1, 3, 5, and 7 cm) all had significant effects on strength. Maximum values were obtained with a neutral wrist, a separation distance of 5 cm, and a lateral grasp. Female strength was on average 62.9 percent of male strength.
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Page 285 Blackwell, Kornatz, and Heath (1999) investigated the effect of grip span on isometric grip force. An optimal grip size allowed for the greatest forces. Batra et al. (1994) demonstrated that reduction in grip strength was positively correlated with glove thickness but not with glove size. In a subsequent analysis, the following selected glove attributes were correlated to reductions in demonstrated strength: (1) tenacity—friction between the glove and a standard piece of plastic, (2) snugness—hand volume versus glove volume, (3) suppleness—a measure of pliability, and (4) thickness. A decrease in grip force was significantly affected by glove type—asbestos and leather gloves reduced grip strength to approximately 82.5 percent of bare-handed levels, while surgical gloves reduced grip strength to 96.3 percent of bare-handed levels. Mital and colleagues studied the influence of a variety of commercially available gloves on the force-torque exertion capability of workers when using wrenches and screwdrivers in routine maintenance and repair tasks (Mital, Kuo, and Faard, 1994). Subjects exerted a maximum volitional torque during a simulated task. The results indicated that tool type was a predictor of volitional torque. Gloves also affected volitional torque; torque was greater with the use of gloves. Temperature can be an important moderating variable. Riley and Cochran (1984) studied manual dexterity performance at different ambient temperatures. Subjects wore typical industrial worker apparel without gloves during manual dexterity tests. Results indicated that after 15 minutes of cold exposure, there was no difference between performance at 12.8 and 23.9 degrees Celsius, but there was a difference between performance at 1.7 and 12.8 degrees as well as between performance at 1.7 and 23.9 degrees. Holewijn and Heus (1992) found that isometric grip strength was significantly reduced by cooling. The rate of force buildup was also influenced by temperature, with slower buildup under conditions of cooling. Cooling reduced the maximum grip frequency by 50 percent compared with the reference condition. The endurance time for the sustained contraction at 15 percent MVC was reduced by 50 percent with warming compared with the reference condition. A psychomotor task was developed by Jeng, Radwin, and Rodriquez (1994) for investigating functional deficits associated with carpal tunnel syndrome. A rapid pinch and release psychomotor task utilizing muscles innervated by the median nerve was administered. Subjects were instructed to pinch the dynamometer above an upper force level and then release below a lower force level as quickly as possible. Average pinch rate decreased from 5.4 pinches/sec to 3.7 pinches/sec as the upper force increased from 5 to 50 percent MVC. Pinch rate was significantly faster and overshoot force was less for the dominant hand. Control subjects
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Page 286performed 25 to 82 percent better than subjects with carpal tunnel syndrome. Age contributed 6 percent of the total variance for pinch rate and 7 percent of the total variance for the time below the lower force level. The results suggest that patients with carpal tunnel syndrome may experience similar functional psychomotor deficits in daily living and manual work activities. Schiefer et al. (1984) demonstrated that finger skin temperature and performance on manual dexterity tests decreased as the ambient air temperature decreases. Upper Limb Summary Overall, the literature reveals that there are strong relationships between physical loads in the workplace and biomechanical loading, internal tolerances, and pain, impairment, and disability. Although many of these relationships are complex for the upper limb, the associations are clear. 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). Relationships between external loading and internal tolerances (i.e., mechanical strain or fatigue) have also been demonstrated. Finally, relationships have been shown between external loading and pain, discomfort, impairment, or disability. Although the relationships exist, the picture is far from complete. Individual studies have for the most part not fully considered the characteristic properties of physical work and external loading (i.e., magnitude, repetition, or duration). Few studies have considered multiple physical stress factors or their interactions. The absence of these relationships, however, does not detract from the basic theoretical construct of the load-tolerance model. In fact, it suggests the need for additional research. When considered together, a broader picture emerges. The existence of relationships together supports the load-tolerance model presented in this report. Furthermore, biomechanics forms the basis to reduce external loading. The relationships that are established indicate appropriate interventions for reducing exposure to external loads in the work environment through ergonomics and work design. Future research efforts targeting the missing relationships may help provide additional workplace interventions for preventing and reducing the risk of work-related disorders.
Representative terms from entire chapter: