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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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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
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