Work Factors, Personal Factors, and Internal Loads: Biomechanics of Work Stressors

Robert G. Radwin, Ph.D. University of Wisconsin-Madison Department of Biomedical Engineering and Department of Industrial Engineering

and

Steven A. Lavender, Ph.D. Rush Presbyterian St. Luke's Medical Center Department of Orthopedic Surgery

1. Introduction

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. Indirect internal physical stress measures include electrophysiological measurements such as electromyograms, or non-specific physiological measures such as heart rate, oxygen consumption, substrate consumption, or metabolite production. More commonly, external loads are assessed either from measuring (1) the kinetics and kinematics of the body, (2) the physical and temporal aspects of the work performed, or (3) correlates to physical and temporal characteristics used as surrogate measures of internal load. The strength of the association between these measures and internal loads generally decreases from the former to the later.

External kinetic and kinematic measurements include physical properties of exertions (forces actually applied or created) or the motions that individuals make. These measurements have the most direct correspondence to internal loads because they are physically and biomechanically related to specific anatomical structures of the body. When kinetic and kinematic measures cannot be obtained, quantities that describe the physical characteristics of the work are often used as indirect measures of the kinetics and kinematics including: (a) measures of loads handled, (b) the forces that must be overcome in performing a task, (c) the geometric aspects of the workplace which govern posture, (d) the characteristics of the equipment used, or (e) the environmental stressors produced by the workplace or objects handled. Alternatively less directly correlated aspects of the work, such as production and time standards, classifications of tasks performed, or incentive systems are sometimes used to quantify the relationship between work and physical stress.

The objective of this manuscript is to review the state of available scientific evidence concerning the relationships between work factors, including host factors, and the resulting internal tissue loads. The paper will focus on the biomechanical stresses placed on the tissues and the methodological issues encountered when estimating tissue loads as people perform work tasks.



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--> Work Factors, Personal Factors, and Internal Loads: Biomechanics of Work Stressors Robert G. Radwin, Ph.D. University of Wisconsin-Madison Department of Biomedical Engineering and Department of Industrial Engineering and Steven A. Lavender, Ph.D. Rush Presbyterian St. Luke's Medical Center Department of Orthopedic Surgery 1. Introduction 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. Indirect internal physical stress measures include electrophysiological measurements such as electromyograms, or non-specific physiological measures such as heart rate, oxygen consumption, substrate consumption, or metabolite production. More commonly, external loads are assessed either from measuring (1) the kinetics and kinematics of the body, (2) the physical and temporal aspects of the work performed, or (3) correlates to physical and temporal characteristics used as surrogate measures of internal load. The strength of the association between these measures and internal loads generally decreases from the former to the later. External kinetic and kinematic measurements include physical properties of exertions (forces actually applied or created) or the motions that individuals make. These measurements have the most direct correspondence to internal loads because they are physically and biomechanically related to specific anatomical structures of the body. When kinetic and kinematic measures cannot be obtained, quantities that describe the physical characteristics of the work are often used as indirect measures of the kinetics and kinematics including: (a) measures of loads handled, (b) the forces that must be overcome in performing a task, (c) the geometric aspects of the workplace which govern posture, (d) the characteristics of the equipment used, or (e) the environmental stressors produced by the workplace or objects handled. Alternatively less directly correlated aspects of the work, such as production and time standards, classifications of tasks performed, or incentive systems are sometimes used to quantify the relationship between work and physical stress. The objective of this manuscript is to review the state of available scientific evidence concerning the relationships between work factors, including host factors, and the resulting internal tissue loads. The paper will focus on the biomechanical stresses placed on the tissues and the methodological issues encountered when estimating tissue loads as people perform work tasks.

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--> 2. Internal Tissue Loading The musculoskeletal system is the load bearing structure within vertebrate animals. Boney structures resist gravitational forces and maintain the body's shape. As such, bones are the primary load bearing tissue within the body. Forces applied to the body, including gravity, attempt to compress or bend the bones. Ligaments hold together the bony structure by crossing articulations where bones inter-connect. Ligaments also act as a pulley system 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, is that adjacent tissues are subjected to mechanical loads. These include cartilage, disc, bursa, and nerve. A detailed examination of how each of the tissues is subjected to mechanical loading follows. 2.1. Bone When an individual performs a movement or exertion, forces are generated within the body to initiate and control it. The bones must resist tensile, compressive, shear, and torsional forces, in addition to bending moments. Bone is an adaptable tissue that acts according to Wolfs Law, which states that bone material is added where there is increased stress and bone material is resorbed where stresses on the tissue are reduced. Relatively little emphasis has been placed on the injuries created by the repetitive loading of bone during occupational activities. Although, recent studies have shown that stress fractures in the lower extremities are not uncommon in new military recruits (Linenger and Shwayhat, 1992; Anderson, 1990; Giladi et al, 1985, Jordaan and Schwellnus, 1994). This suggest that the bone remodeling associated with Wolf s Law is a slow process and that the vigorous training that occurs during the initial weeks of boot camp does not allow this adaptation to occur. Others have reported that osteoarthritis (OA) in the hip and knees is more prevalent in individuals employed in occupations that experience greater loading of the lower extremity (Kohatsu and Schurman, 1990; Lindberg and Axmacher, 1988; Lindberg and Montgomery, 1987; Vingard et al., 1991). Anderson and Felson (1988) found a relationship between the frequency of knee bending and OA. These same authors also report that knee strength demands were also predictive of knee OA in women aged 55 to 64 years. Taken together, these studies begin to demonstrate the link between workplace activities and changes in bone tissues. 2.2. Ligaments and Connective Tissues 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. Adams et al. (1980),by severing ligaments in cadaveric lumbar motion

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--> segments, showed 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 don't contribute to joint loading. Several authors have showed 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 inter-vertebral disc during manual material handling tasks has been debated in the scientific literature (Cholewicki and McGill, 1992; Dolan et al., 1994; Potvin et al., 1994). 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 inter-vertebral disc. When ligaments act as a turning point for tendons (pulleys), they are exposed to shear forces and contact stresses. For example the transverse carpal 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 palmer 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. Even though ligaments act as pulleys, the ligaments themselves are rarely the tissues damaged in work related musculoskeletal injuries. Instead, it is the tendons that experience the morphological changes which result in symptoms and injuries. 2.3. 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 (Chaffin and Andersson, 1991). This arrangement of fibers minimizes the stretch or creep in these tissues when subjected to tensile loading. With repeated loading tendons can become inflamed, particularly where the tendons wrap around bony or ligamentous structures. In more severe cases the collagen fibers can become separated and eventually pulverized wherein debris containing calcium salts creates further swelling and pain (Chaffin and Andersson, 1991). Mechanical relationships between external forces, postures and internal tendon loading were demonstrated by Armstrong and Chaffin (1979) for the carpal tunnel of the wrist using the analogy of a pulley and a belt. A tendon sliding over a curved articular surface may be considered analogous to a belt wrapped around a pulley. That model reveals that the force per arc length F1, exerted on the trochlea is a function of the tendon tension Ft, the radius of curvature r, the coefficient of friction between the trochlea and the tendon m, and the included angle of pulley-belt contact q such that: (1)

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--> When the extrinsic finger flexor tendons wrap around the trochlea, the synovial membranes of the radial and ulnar bursas surrounding the tendons are compressed by forces in both flexion and extension. The resulting compressive force is directly proportional to the tension developed in the tendons and the finger flexor muscles, which are related to the external force of exertion by the hand. Normally the coefficient of friction between the tendon and trochlear surface would be expected to be very small. The model predicts that if the supporting synovia became inflamed and the coefficient of friction m increased, F1 would increase (Chaffin and Andersson, 1991). This would also result in increased shearing forces Fs as the tendons attempt to slide through their synovial tunnels, since shear forces are generally proportional to F1 and the coefficient of friction: Fs = F1μ. (2) This gives rise to the concept that repeated compression could aggravate further synovial inflammation and swelling. Armstrong and Chaffin (1979) also showed that the total force transmitted from the belt to a pulley FR, depends on the wrist angle q, and the tendon load Ft as described by the equation: FR = 2Ft sin(θ/2) (3) Consequently the force acting on adjacent anatomical structures such as ligaments, bones, and the median nerve, depends on the wrist angle. The greater the angle is from a straight wrist, the greater the resultant reaction force on the tendons. The same equation also shows that the resultant force transmitted by a tendon to adjacent wrist structures is a function of tendon load. 2.4. Muscles 2.4.1. Force Generation and Biomechanics 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 employs 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

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--> body during exertions result from the internal muscle forces as they work in opposition to the external forces. 2.4.2. Co-contraction The synergistic activation of the muscles controlling an articulation is often referred to as co-contraction. In many cases the co-contraction is between muscles working fully or partially in opposition to one another. From a biomechanical perspective, co-contraction is a way in which joints can be stiffened, stabilized, and moved in a well-controlled manner. Co-contraction, however, also has the potential to substantially increase the mechanical loads (compression, shear, or torsion) or change the nature of the loads placed on the body's articulations during an exertion or motion. This is because any co-contraction of fully or partially antagonistic muscles requires increased activation of the agonistic muscles responsible for generating or resisting the desired external load. Thus, the co-contraction increases the joint loading first by the antagonistic force, and second by the additional agonist force required to overcome this antagonistic force. Therefore, work activities where co-contraction are more common impose greater loads on the tissues of the musculoskeletal system. 2.4.3. Localized Muscle Fatigue As muscles fatigue the loadings experienced by the musculoskeletal system change. In some cases the changes result in alternative muscle recruitment strategies or substitution patterns where-in other secondary muscles, albeit less suited for performing the required exertion, are recruited as replacements for the fatigued tissues. This substitution hypothesis has received experimental support from Parnianpour and colleagues (1988) who showed considerable out of plane motion in a fatiguing trunk flexion/extension exercise. It is believed that the secondary muscles are at greater risk of over-exertion injury in part due to their smaller size or less biomechanically advantageous orientation, and in part due their poorly coordinated actions. Alternatively, larger adaptations may occur which result in visible changes in behavior. For example, changes in lifting behavior have been shown to occur when either quadriceps or erector spinae muscles have be selectively fatigued (Novak et al., 1993; Trafimow et al., 1993; Marras and Granata, 1997). Fatigue may also result in ballistic motions or exertions in which loads are poorly controlled and rapidly accelerated, which in turn, indicates there are large impulse forces within the muscles and connective tissues. Localized muscle fatigue can also occur in very low level contractions, for example those used when supporting the arms in an elevated posture. In this case the fatigue is further localized to the small, low force endurance fibers (slow twitch) 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 ). Murthy et al. (1997), using near infrared spectroscopy to quantify tissue oxygenation as an index of blow flow, found reduced oxygenation within 10 to 40 seconds of initiating sustained contractions at values as low as 10 percent of the muscles maximum capacity, thereby indicating an interference with the metabolic processes.

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--> 2.5. Inter-vertebral 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 fibrosis. The nucleus pulposus is in the central region of the disc and is comprised of a gelataneous mixture of water, collagen, and proteoglycans. The annulus fibrosis is comprised of alternating bands of angled fibers oriented approximately 30 degrees relative to the horizontal (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 which forms the boundary between the disc and the vertebral body (vertebral endplate) rather than through nuclear prolapsed. 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 flexed 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 where Adams and Hutton (1985) simulated repetitive loading of the disc, previously healthy discs failed at 3,800 N, again mostly through endplate fracture. Taken together, these studies show that the disc, especially the vertebral endplate, is susceptible to injury when loading is repetitive or when exposed to large compressive forces while in a severely flexed posture. It should be clear from earlier discussions of muscle that the internal forces created by the muscles can be quite large in response to even modest external loads. When the muscles which support, move, and stabilize the spine are recruited, forces of significant magnitude are placed on the spine. Several investigators have quantified spine loads during lifting and other material handling activities. The earliest attempts to quantify the spinal loads used static sagittal plane analyses (Morris et al., 1961; Chaffin, 1969). Validation for these modeling efforts came from disc pressure and electromyographic studies (Nachemson et al., 1964). More advanced models have been developed to quantify the three dimensional internal loads placed on the spine. Schultz et al. (1982a) developed and validated an optimization model to determine the three dimensional internal spine loads that results from asymmetric lifting activities. Compression estimates ranged from 520 N for upright unloaded standing, to 1560 N for unloaded subjects flexed 30 degrees, and up to 2660 N for the same group of subjects, flexed 30 degrees while holding an 80 N weight with their arms extended (Schultz et al., 1982b). In asymmetric tasks, for example lateral bending or resisted twisting, the lateral shear forces ranged exceeded 150 N, depending in part upon the optimization criteria used (Schultz et al., 1982c). Others have sought to predict the internal

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--> muscle forces directly from electromyographic signals, and then used these muscle forces in conjunction with a geometric models of the torso to compute the resulting forces acting on the spine. Typically these models have been validated using computed moments from a link segment model (McGill and Norman, 1986), or by measured external torque (Marras and Sommerich, 1991). McGill and Norman reported compression values on the L4/L5 disc ranging between 6 and 8 thousand Newtons as their subjects lifted 450 N loads. Anterior shear forces ranged between 200 and 1200 Newtons for the same lifts. During lateral bending exertions McGill (1992) reported compressive loads of 2500 N, lateral shear forces over 80 N, and anterior shear forces as high as 239 N. Marras and Granata (1997) have shown that the compression and shear values during lateral bending (extension) are dependent upon the movement speed. Similar velocity effects were reported for twisting exertions (Marras and Granata, 1995). Others have quantified spine loads indirectly by examining the reaction forces and moments obtained with linked segment models. McGill et al. (1996) have shown that there is a very strong predictive relationship (r2=.94) between the external spine moments and the spine reaction forces generated by their electromyographic assisted model. This indicates that the changes observed in the more readily quantifiable spine reaction moments, due to changes in the modeled task parameters, are representative of the changes in actual spine loading. Increased lifting speed, lower initial lifting heights, and longer reach distances all significantly increase the spine reaction moments, and hence, have a significant impact on the compressive and shear forces acting on the disc (De Looze et al., 1993; Frievalds et al., 1984; Leskinen et al., 1983; McGill and Norman, 1985; Schipplein et al., 1995; Buseck, et al. 1988; De Looze et al., 1994; Dolan et al., 1994; Tsuang et al., 1992). More recently, three-dimensional dynamic linked segment models have been developed to evaluate the spine loading during asymmetric tasks (Gagnon et al., 1993; Gagnon and Gagnon, 1992; Kromodihardjo and Mital, 1987; Lavender et al., 1998). These later models have been useful for documenting the spine loads (indirectly) that stem from lifting activities that involve twisting and lateral bending. 2.6. Nerves Nerves, while not contributing either actively or passively to the internal forces generated by the body, are exposed to forces, vibration, and temperature variations that affect their function. Carpal tunnel syndrome is believed to result from a combination of ischemia and 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, et al., 1977). 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) in Equation 3 predicts that median nerve compression will increase with increased wrist flexion and extension, or finger flexor exertions. Increased intra-carpal canal pressure was observed by Armstrong, et al. (1991) wrist and finger extension and flexion, and for increased grip exertions. Rempel (1995) reports similar findings and for repetitive hand activity and during typing. Environmental stimuli, for example cold temperatures and vibration, have been shown to affect the response of peripheral nerves. Low temperatures, for example, can affect cutaneous

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--> sensory sensitivity and manual dexterity. Vibratory stimuli, with repeated exposure, is believed to cause via a reflex response (nerve) contraction of the smooth muscles of the blood vessels associated with Reynaud's Syndrome (Chaffin and Andersson, 1991). Less severe nerve damage resulting from, vibratory stimuli has been associated with paresthesias and tingling sensations. 3. External Loading Factors 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, long vibration exposure and cold temperatures. An example of the variety of factors cited is contained in Table 1. Although the literature reports such a great diversity of factors, it is possible to group these methodologies into a coherent body of scientific inquiry. A conceptual framework is now presented for organizing the physical parameters in manual work. 3.1. Physical Stress Physical stress can be described in terms of fundamental physical quantities of motion, force, vibration, and temperature. These basic quantities comprise the kinematic, kinetic, oscillatory and thermal aspects of work and energy produced by, or acting on the human in the workplace. 3.1.1. Motion Motion describes the displacement of a specific articulation or the relationship between adjacent body parts. Motion of a body segment relative to another segment 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 in total. 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. 3.1.2. Force 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 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. Force is transmitted back

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--> to the body and its internal structures when opposing external forces 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 the soft tissues are compressed between bone and external objects. This may occur when grasping tools, parts or making contact with the work station. Contact stress may be quantified by considering contact pressure (force/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. 3.1.3. Vibration Vibration occurs when an object undergoes oscillatory motion. Human vibration, the term commonly used, is produced by the acceleration of an external object. Vibration is transmitted to the human body through physical contact either with seat or the feet (whole body vibration), or by grasping a vibrating object (hand-arm vibration). Whole-body vibration is associated with vibration from riding in a vehicle or from standing on a moving platform. 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. External vibration transmits from the distal location of contact to proximal locations of the body and sets into motion the musculoskeletal system, receptor organs, tissues and other anatomical structures. Vibration transmissibility 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. Vibration can introduce disturbances in muscular control by way of a reflex mediated through the response of muscle spindles to the vibration stimulus. This reflex is called the tonic vibration reflex which results in a corresponding change in muscle tension when vibration is transmitted from a vibrating handle to flexor muscles in the forearm (Radwin, Armstrong and Chaffin, 1987). 3.1.4. Temperature Heat loss occurs at the extremities during work in cold environments, such as in food processing, handling cold materials, working outdoors, or exposure to cold air exhaust from pneumatic hand power tools. 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

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--> effects are attributable to various physiological mechanisms. 3.2. Physical Stress Properties The physical stresses described in Section 3.1 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 characteristic properties are illustrated in Figure 1. Magnitude is the 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. Repetition is the frequency or rate that a physical stress factor repeats. The frequency that the physical stress in Figure 1 repeats is the inverse of the period between repeated exertions, motions, vibration, or cold temperature, and the physical units of time-1. Duration refers to the time that one is exposed to a physical stress and is quantified in physical units of time. 3.3. Interactions The characteristic properties of physical stresses together quantify exposure to external stress. Combinations of different physical stresses and properties can be used to represent factors that are commonly reported for quantifying exposure. These relationships are summarized in Table 2. Physical stresses are quantified in a similar manner as shown in Table 3. Force measurements quantify force amplitude, in addition to the rate and time of force application. Motion of individual joints include the magnitude of angular displacement, velocity or acceleration, the frequency of motions and the time the motion is sustained. Vibration magnitude is quantified as acceleration of the vibrating objects, and repetition and duration is a measure of the frequency and time the vibration occurs. Similarly, cold temperature and associated frequencies and amplitudes quantify cold exposure. This organization is useful because it provides a construct for comparing and combining studies using different measurements and methodologies, as represented in Table 1, into a common framework. For example, physical stress measurements using a survey methodology that simply assesses the presence and 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 may be compared with a study that assesses muscle force using electromyography because both studies quantify the magnitude of force. Therefore, a body of scientific knowledge from diverse investigations emerges. External physical stress factors described in Sections 3.1 and 3.2 relates to distinct internal physical stress factors. This relationship is summarized in Table 4. For example, force magnitude is directly related to the loading of tissues, joints and adjacent anatomical structures, as is 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

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--> relationships. Similar relationships between external and internal factors have been recognized by Moore, Wells and Ranney (1991) and Armstrong, et al. (1993). 4. Assessment of Workplace Factors Physical stress factors in the workplace have been evaluated at different levels of detail, depending on the specific research instrument and measurement methodology used. Survey methods involve observational study at the job or task level. Production and time data may be obtained from existing records such as time standards and process planning data, or from measured data using work sampling or time and motion studies. 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. 4.1. Survey Methods Survey methods include interviews, self-reported questionnaires, or observation and checklists. Questionnaires, diaries and interview techniques are easily administered and are commonly used for quantifying physical work load. The method relies on the firsthand observations and experiences of the employee /supervisor. An employee interview consists of asking questions regarding job/task attributes and associated physical stress exposures (Bernard, et al., 1994). One advantage of these methods are their ability to assess exposure over long time intervals, infrequently performed tasks, or multiple methods at performing tasks. which is a feature not usually available for other methodologies. However the method depends on subjective data. Observational methods are the most common method employed. The most developed methodologies are for postures and motions of different articulations and use of the hands. These include posture classification systems like OWAS (Karhu, et al., 1977; Karhu, et al., 1981). Some observational methods integrate posture classification and a checklist of exertions, tool use and assessment of repetition with a breakdown of task elements (Keyseling, et al., 1993; McAtamney, and Corlett, 1993). 4.2. Production and Time Data Many of these methods are rooted in traditional work measurement, which is historically based on time and motion study that is used for quantifying the temporal aspects of work. Time data is important for understanding the duration of work and rest, for quantifying repetition, and for determining the duration that work is performed. Suitable time data may be available to investigators at various levels of detail. The average daily time allotted for performing a job can be estimated from the shift time. The average time for specific tasks may be obtained from a task rotation or production schedule. Cycle time describes the time for completing a single cycle of production, and may be determined from production rate data. The time to perform a specific element may be available from a time and motion study. Although use of production or cycle time data may be convenient and easily obtained, it

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--> 100. Snijders, C.J., Van Riel, M. P. J. M., and Nordin, M. 1987. Continuous measurements of spine movements in normal working situations over periods of 8 hours or more. Ergonomics, 30, 639-653. 101. Tanzer, R. C. 1959. The carpal-tunnel syndrome. The J. Bone and Joint Surgery, 41-A, 626-634. 102. Toussaint, H. M, de Winter, A. F., de Looze, Y. H. M. P, Van Dieen, JH, Kingma, I (1995). Flexion relaxation during lifting: implications for torque production by muscle activity and tissue strain at the lumbo-sacral joint. J. of Biomechanics, 28, 199-210. 103. Trafimow, J. H., Schipplein, O. D., Novak, G. J., Andersson, G. B. J. (1993). The effects of quadriceps fatigue on the technique of lifting. Spine, 18, 364-367. 104. Tsuang, Y. H., Schipplein, O. D., Trophema, J. H., Andersson, G. B. J. (1992). Influence of body segment dynamics on loads at the lumbar spine during lifting. Ergonomics, 35, 437-444. 105. Tveit, P., Daggfeldt, K., Hetland, S., and Thorstensson, A. 1994. Erector spinae lever arm length variations with changes in spinal posture. Spine, 19, 199-204. 106. Vingard, E., Alfredsson, L., Goldie, I., Hogstedt, C. (1991). Occupation and osteoarthrosis of the hip and knee: A register-based cohort study. International J. of Epidemiology, 20, 1025-1031. 107. White, A. A., Panjabi, M. M. (1990). Clinical Biomechanics of the Spine: Second Edition. J.B. Lippincott Company: Philadelphia. 108. Wieslander, G., Norback, D., Gothe C. J., Juhlin, L. (1989). Carpal tunnel syndrome (CTS) and exposure to vibration, repetitive wrist movements, and heavy manual work: a case-referent study. British Journal of Industrial Medicine, 46, 43-47. 109. Winkel, J. and Mathiassen, S. E., (1994). Assessment of physical work load in epidemiological studies: concepts, issues and operational considerations, Ergonomics, 37(6), 979-988.

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--> Table 1: Examples of Physical Stress Factors Cited in The Literature Reference Factors Considered Armstrong, et al., 1981 Repeated exertions with certain postures Stressful exertions High forces Armstrong, et al., 1986 Repetitive and sustained exertions Certain postures Vibration Low temperatures Mechanical stresses Arndt, 1997 Work pace Bernard, et al., 1994 Working time Time pressure Hours of computer use Bovenzi, et al., 1991 Vibration acceleration Vibration exposure Chiang, et al., 1990 Local exposure to cold Derksen, et al., 1994 Poor working postures Feuerstein and Fitzgerald, 1992 Rest-break frequency Deviations from neutral Work envelope excursions High-impact hand contacts Pace of movements Intensity of muscular tension Smoothness of movements Keyserling, 1986 Awkward working postures Keyserling, et al., 1993 Repetitiveness Local mechanical contact stress Forceful manual exertions Awkward posture Hand tool use Marras and Schoenmarklin, 1993 Angular velocity Angular acceleration McAtamney, and Corlett, 1993 Posture Muscle use (repetitive or static) Force or load Silverstein, et al., 1986 Repetitive motion Forceful exertions Wieslander, et al., 1989 Exposure to vibration Heavy loads on the wrist

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--> Table 2. 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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--> Table 3. 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 that Force is Applied Time that Force is Applied Motion Joint Angle, Velocity, Acceleration Frequency of Motion Time to Compete Motion. Vibration Acceleration Frequency that Vibration Occurs Time of Vibration Exposure Cold Temperature Frequency of Cold Exposure Time of Cold Exposure

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--> Table 4. 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 metabolite metabolites Cartilage or disc rehydration   Cumulative tissue loads Muscle fiber recruitment and muscle fatigue rate Energy expenditure, fatigue and elimination of 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

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--> Table 5: Examples of External Physical Stress Measurements Cited in the Literature     Physical Stress Measured Body Region   Individual Factors Reference Method Force Motion Vibration Cold Application Wieslander, et al., 1988 Survey of job classification questionnaire Duration Duration Duration   Wrist CTS cases and other surgical referents   Chiang, et al., 1990 Survey of job activities   Repetition   Magnitude Wrist Frozen food factory   Bernard, et al., 1994 Survey of job activities and work organization conditionquestionnaires Duration Duration     Neck Shoulders Wrist Hand Newspaper employees   Armstrong, Chaffin & Electromyography Magnitude Magnitude     Hand Garment workers   Foulke, 1979 Posture classification Time study from motion picture film   Duration Duration         Armstrong, et al., 1982 Posture Classification Elemental analysis fromvideo tape Magnitude Repetition Duration Magnitude Repetition Duration     Shoulder Elbow Wrist Hand   Poultry processing Silverstein, Fine and Armstrong, 1986 Time study & production rates from video tape Weight of objects handled and electromyography Magnitude Repetition       Wrist Hand Electronics, appliance, investment casting, apparel sewing, iron foundry and bearing manufacturing   Keyserling, 1986 Posture Classification Time and motion study in real-time off video tapes Time spent in each posture Magnitude Repetition Duration       Trunk Shoulder Automobile assembly   Keyserling, et al., 1993 Risk factor checklist No. Occurrences per cycle Cycle Time Magnitude Repetition Duration Magnitude Repetition Duration Magnitude Repetition Duration   Shoulder Elbow Wrist Hand Engine plant, metal stamping plant, part distribution center   Bovenzi, et al., 1991 Manual goniometer Observation checklist ISO 5349 frequency-weighted vibration Survey of personal attributes   Magnitude Magnitude Duration   Neck Shoulder Elbow Forearm Wrist Hand Chainsaw operators, mechanics, electricians and painters   Marras & Schoenmarklin, 1993 Electrogoniometer Angle, velocity and   Magnitude     Wrist Automotive parts & building products  

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-->     Physical Stress Measured Body Region   Individual Factors Reference Method acceleration Force Motion Vibration Cold Application manufacturing Moore, Wells & Ranney, 1991 Electrogoniometer Electromyography Magnitude Repetition Magnitude Repetition     Wrist Hand Simulated pistol grip tool operation   Latco, et al., Observer ratings   Repetition Derksen, et al., 1994 Electrogoniometer   Magnitude Duration     Trunk Parcel shipping   Radwin & Lin, 1993 Electrogoniometer Spectral analysis   Magnitude Frequency     Wrist Laboratory simulation   Radwin, et al., 1994 Electrogoniometer Local discomfort frequency-weighted filters   Magnitude Repetition Duration     Wrist Laboratory simulation   Cemon, Radwin and Henderson, 1995 Thermistors       Magnitude Frequency Duration Hand Poultry processing   Marras et al., 1993 Electrogoniometer Magnitude Repetition Magnitude Repetition     Back Industrial Workers Anthropometry Punnett et al., 1991 Observer ratings Magnitude Magnitude Duration     Back Automotive Workers Age. Back Injury history Andersson et al., 1974 Intradiscal Pressure Magnitude       Back Sitting in a Vehicle Seat   Doormaal et al., 1995 Posture Coding, Biomechanical Model Magnitude Magnitude Duration     Back, Shoulder Hip, Neck Knees Ambulance Assistants   Lee and Chiou Postural Coding Magnitude Magnitude Repetition     Back Nursing Personnel  

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--> Table 6. Selected External Measurement Methodologies and Their Relationship to Internal Stress External Measure Physical Stresses That Can Be Assessed Accuracy of Estimate Precision of Estimate Job or Task Title Indeterminate Task Descriptions Force (Magnitude, Repetition, Duration) Motion (Magnitude, Repetition, Duration) Vibration (Magnitude, Repetition, Duration) Cold (Magnitude, Repetition, Duration) Low Low Employee Self-Reports Force (Magnitude, Repetition, Duration) Motion (Magnitude, Repetition, Duration) Vibration (Magnitude, Repetition, Duration) Cold (Magnitude, Repetition, Duration) Low Low Tools, Materials and Equipment Handled or Operated Vibration (Magnitude) Force (Magnitude) Motion (Magnitude) Cold (Magnitude) Low Medium Observation Force (Magnitude, Repetition, Duration) Motion (Magnitude, Repetition, Duration) Vibration (Magnitude, Repetition, Duration) Cold (Magnitude, Repetition, Duration) Medium Low Production Rates Force (Repetition) Motion (Repetition) Vibration (Repetition) Low High Time Standards Force (Duration) Motion (Duration) Vibration (Duration) Cold (Duration) Low High Measured Cycle Times Force (Duration) Motion (Duration) Vibration (Duration) Cold (Duration) Medium High Time and Motion Study (elemental times) Force (Repetition, Duration) Motion (Repetition, Duration) Vibration (Repetition, Duration) Cold (Repetition, Duration) High High Loads Handled (mass) Force (Magnitude) Medium Medium Forces Opposed (force gages, load cells, force sensors) Force (Magnitude) Medium High Electromyography Force (Magnitude, Repetition, Duration) High Medium Internal Compartmental Pressure Force (Magnitude, Repetition, Duration) High High Posture Classification Motion (Magnitude) Medium Medium Reach Distances and Workstation Layout Motion (Magnitude) Low High Manual Goniometer Motion (Magnitude) Medium High Electrogoniometer Motion (Magnitude, Repetition, Duration) High High Motion Analysis (video, optical, electromagnetic) Motion (Magnitude, Repetition, Duration) High High Biomechanical Models Using Reach Distances, Loads Handled and Time Force (Magnitude) High Medium

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--> External Measure Physical Stresses That Can Be Assessed Accuracy of Estimate Precision of Estimate Study Biomechanical Models Using Electrogoniometers/Motion Analysis and Forces Opposed Force (Magnitude) High High Accelerometers Attached to Objects Contacted Vibration (Magnitude, Repetition, Duration) Medium High Ambient Temperature Cold (Magnitude) Low Medium Temperature of Extremities Cold (Magnitude) Medium High Continuous Monitoring of Extremity Temperature Cold (Magnitude, Repetition, Duration) High High

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--> Figure 1: Representation of Magnitude, Duration and Repetition for Physical Stress-Time Record.

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--> Figure 2: Relationship between accuracy and precision for different measurement methodologies