PHYSICAL TASKS:WORK-RELATED MUSCULOSKELETAL DISORDERS OF THE UPPER EXTREMITY

WORK-RELATED MUSCULOSKELETAL DISORDERS OF THE UPPER EXTREMITY

Characteristics of Musculoskeletal Disorders

The National Institute of Occupational Safety and Health (NIOSH 1997) states that musculoskeletal disorders, which include disorders of the back, trunk, upper extremity, neck, and lower extremity are one of the 10 leading work-related illnesses and injuries in the United States. Praemer et al, (1992) report that work-related upper-extremity disorders (WUEDs), which are formally defined by the Bureau of Labor Statistics (BLS) as cumulative trauma illnesses, account for 11.0 % of all work- related musculoskeletal disorders (illnesses). For comparison, occupational low-back disorders ac- count for more than 51.0% of all WRMDs. According to BLS (1995), the cumulative trauma illnesses of upper extremity accounted for more than 60% of the occupational illnesses reported in 1993. These work-related illnesses, which include hearing impairments due to occupational noise exposure, rep- resent 6.0% of all reportable work-related injuries and illnesses (Marras 1996).

As reviewed by Karwowski and Marras (1997), work-related musculoskeletal disorders currently account for one-third of all occupational injuries and illnesses reported to the Bureau of Labor Statistics (BLS) by employers every year. These disorders thus constitute the largest job-related injury and illness problem in the United States today. According to OSHA (1999), in 1997 employers reported a total of 626,000 lost workday disorders to the BLS, and these disorders accounted for $1 of every $3 spent for workers’ compensation in that year. Employers pay more than $15–20 billion in workers’ compensation costs for these disorders every year, and other expenses associated with MSDs may increase this total to $45–54 billion a year.

Such statistics can be linked to several occupational risk factors, including the increased produc- tion rates leading to thousands of repetitive movements every day, widespread use of computer keyboards, higher percentage of women and older workers in the workforce, better record keeping of reportable illnesses and injuries on the job by employers, greater employee awareness of WUEDs and their relation to the working conditions, and a marked shift in social policy regarding recognition and compensation of the occupational injuries and ilnesses.

Definitions

Work-related musculoskeletal disorders (WRMDs) are those disorders and diseases of the musculo- skeletal system which have a proven or hypothetical work related causal component (Kuorinka and Forcier 1995). Musculoskeletal disorders are pathological entities in which the functions of the mus- culoskeletal system are disturbed or abnormal, while diseases are pathological entities with observable impairments in body configuration and function. Although WUEDs are a heterogeneous group of disorders, and the current state of knowledge does not allow for a general description of the course of these disorders, it is possible nevertheless to identify a group of so-called generic risk factors, including biomechanical factors, such as static and dynamic loading on the body and posture, cog- nitive demands, and organizational and psychosocial factors, for which there is an ample evidence of work-relatedness and higher risk of developing the WUEDs.

Generic risk factors, which typically interact and cumulate to form cascading cycles, are assumed to be directly responsible for the pathophysiological phenonmena that depend on location, intensity, temporal variation, duration, and repetitivness of the generic risk factors (Kuorinka and Forcier 1995). It is also proposed that both insufficient and exccessive loading on the musculoskeletal system have deleterious effects and that the pathophysiological process is dependent upon individual characteristics with respect to body responses, coping mechanisms, and adaptation to risk factors.

Musculoskeletal disorders can be defined by combining the separate meanings for each word (Putz-Anderson 1993). Cumulative indicates that these disorders develop gradually over periods of time as a result of repeated stresses. The cumulative concept is based on the assumption that each repetition of an activity produces some trauma or wear and tear on the tissues and joints of the particular body part. The term trauma indicates bodily injury from mechanical stresses, while dis- orders refer to physical ailments. The above definition also stipulates a simple cause-and-effect model for CTD development. According to such a model, because the human body needs sufficient intervals of rest time between episodes of repeated strains to repair itself, if the recovery time is insufficient, combined with high repetition of forceful and awkward postures, the worker is at higher risk of developing a CTD. In the context of the generic model for prevention shown in Figure 15, the above

asbestosis). Work-related diseases are defined as multifactorial when the work environment and the performance of work contribute significantly to the causation of disease (WHO 1985). Work-related diseases can be partially caused by adverse work conditions. However, personal characteristics, en- vironmental, and sociocultural factors are also recognized as risk factors for these diseases.

The scientific evidence of work-relatedness of musculoskeletal disorders has been firmly estab- lished by numerous epidemiologic studies conducted over the last 25 years of research in the field (NIOSH 1997). It has also been noted that the incidence and prevalence of musculoskeletal disorders in the reference populations were low, but not zero, most likely indicating the nonwork-related causes of these disorders. It was also documented that such variables as cultural differences, psychosocial and economic factors, which may influence one’s perception and tolerance of pain and consequently affect the willingness to report musculoskeletal problems, may have significant impact on the pro- gressions from disorder to work disability (WHO 1985; Leino 1989).

Armstrong et al. (1993) developed a conceptual model for the pathogenesis of work-related mus- culoskeletal disorders. The model is based on the set of four cascading and interacting state variables of exposure, dose, capacity, and response, which are measures of the system state at any given time. The response at one level can act as dose at the next level (see Figure 15). Furthermore, it is assumed that a response to one or more doses can diminish or increase the capacity for responding to suc- cessive doses. This conceptual model for development of WRMDs reflects the multifactorial nature of work-related upper-extremity disorders and the complex nature of the interactions between ex- posure, dose, capacity, and response variables. The proposed model also reflects the complexity of interactions among the physiological, mechanical, individual, and psychosocial risk factors.

In the proposed model, exposure refers to the external factors (i.e., work requirements) that produce the internal dose (i.e., tissue loads and metabolic demands and factors). Workplace organi- zation and hand tool design characteristics are examples of such external factors that can determine work postures and define loads on the affected tissues or velocity of muscular contractions. Dose is defined by a set of mechanical, physiological, or psychological factors that in some way disturb an internal state of the affected worker. Mechanical disturbance factors may include tissue forces and deformations produced as a result of exertion or movement of the body.

Physiological disturbances are such factors as consumption of metabolic substrates or tissue dam- age, while the psychological disturbance factors are those related to, for example, anxiety about work or inadequate social support. Changes in the state variables of the worker are defined by the model as responses. A response is an effect of the dose caused by exposure. For example, hand exertion can cause elastic deformation of tendons and changes in tissue composition and / or shape, which in turn may result in hand discomfort. The dose–response time relationship implies that the effect of a dose can be immediate or the response may be delayed for a long periods of time.

The proposed model stipulates that system changes (responses) can also result in either increased dose tolerance (adaptation) or reduced dose tolerance lowering the system capacity. Capacity is defined as the worker’s ability (physical or psychological) to resist system destabilization due to various doses. While capacity can be reduced or enhanced by previous doses and responses, it is assumed that most individuals are able to adapt to certain types and levels of physical activity.

Muscles, for example, can develop increased aerobic or anaerobic metabolic capacity. Furthermore, muscular responses are characterized in the model as a series of cascading mechanical and physio- logical events. The local changes (system responses), such as deformation and the yielding of con- nective tissues within the muscle, are conveyed to the central nervous system by sensory afferent nerves and cause corresponding sensations to effort and discomfort, often referred to as perceived fatigue.

The main purpose of the dose-response model is to account for the factors and processes that result in WRMDs in order to specify acceptable limits with respect to work design parameters for a given individual. The proposed model should be useful in the design of studies on the etiology and pathomechanisms of work-related musculoskeletal disorders, as well as in the planning and evaluation of preventive programs. The model should complement the epidemiologic studies, which focus on associations between the top and bottom of the cascade with physical workload, psychological de- mands, and environmental risk factors of work at one end and the manifestations of symptoms, diseases, or disabilities at the other.

Recently, Tanaka and McGlothlin (1999) updated their 3D heuristic dose–response model for repetitive manual work risk factors using the epidemiologic finding. Their earlier model for the postulated relationships between the risk factors for carpal tunnel syndrome (CTS) (for description see Karwowski and Marras 1997) was modified by including the time exposure factor. This was based on examination of prevalence of CTS data for 1988 from the National Health Review Survey (NHIS) and the Occupational Health Supplement (OHS). The authors found that compared to the nonexposed population, the prevalence (P) of CTS among the people exposed to bending / twisting of the hands / wrists many times an hour increased by several times regardless of the length of daily hours exposed. The prevalence of CTS was then defined as follows:

The proposed model indicates that high-intensity work (involving high force, high repetition, and/ or high joint deviation) should not be performed for a long period of time, but low-intensity work may be performed for a longer period. The 3D representation of the relationships between the risk factors for CTS is illustrated in Figures 16 and 17.

Causal Mechanism for Development of WUEDs

As reviewed by Armstrong et al. (1993), work-related muscle disorders are likely to occur when a muscle is fatigued repeatedly without sufficient allowance for recovery. An important factor in de- velopment of such disorders is motor control of the working muscle. Ha¨gg (1991) postulates that the recruitment pattern of the motor neurons can occur according to the size principle, where the small units are activated at low forces. Given that the same units can be recruited continuously during a given work task, even if the relative load on the muscle is low, the active low-threshold motor units can work close to their maximal capacity and consequently maybe at a high risk of being damaged. It has also been shown that muscle tension due to excessive mental load can cause an overload on some specific muscle fibers (Westgaard and Bjørkland 1987). Karwowski et al. (1994) showed that cognitive aspects of computer-related task design affect the postural dynamics of the operators and the related levels of perceived postural discomfort. Finally, Edwards (1988) hypothesizes that occu- pational mucle pain might be a consequence of a conflict between motor control of the postural activity and control needed for rhythmic movement or skilled manipulations. In other words, the primary cause of work-related muscular pain and injury may be altered motor control, resulting in imbalance between harmonious motor unit recruitment relaxation of mucles not directly involved in the activity.

As discussed by Armstrong et al. (1993), poor ergonomic design of tools with respect to weight, shape, and size can impose extreme wrist positions and high forces on the worker’s musculoskeletal system. Holding heavier objects requires an increased power grip and high tension in the finger flexor tendons, causing increased pressure in the carpal tunnel. Furthermore, the tasks that induce hand and arm vibration cause an involuntary increase in power grip through a reflex of the strength receptors. Vibration can also cause protein leakage from the blood vessels in the nerve trunks and result in

edema and increased pressure in the nerve trunks and therefore also result in edema and increased pressure in the nerve (Lundborg et al. 1987).

Musculoskeletal Disorders: Occupational Risk Factors

A risk factor is defined as an attribute or exposure that increases the probability of a disease or disorder (Putz-Anderson, 1988). Biomechanical risk factors for musculoskeletal disorders include repetitive and sustained exertions, awkward postures, and application of high mechanical forces. Vibration and cold environments may also accelerate the development of musculoskeletal disorders. Typical tools that can be used to identify the potential for development of musculoskeletal disorders include conducting work-methods analyses and checklists designed to itemize undesirable work site conditions or worker activities that contribute to injury. Since most of manual work requires the active use of the arms and hands, the structures of the upper extremities are particularly vulnerable to soft tissue injury. WUEDs are typically associated with repetitive manual tasks with forceful exertions, such as those performed at assembly lines, or when using hand tools, computer keyboards and other devices, or operating machinery. These tasks impose repeated stresses to the upper body, that is, the muscles, tendons, ligaments, nerve tissues, and neurovascular structures. There are three basic types of WRDs to the upper extremity: tendon disorder (such as tendonitis), nerve disorder (such as carpal tunnel syndrome), and neurovascular disorder (such as thoracic outlet syndrome or vibration–Raynaud’s syndrome). The main biomechanical risk factors of musculoskeletal disorders are presented in Table 22.