Risk factors for musculoskeletal disorders — working postures
Danuta Roman-Liu, Central Institute for Labour Protection - National Research Institute, Poland
- 1 Introduction
- 2 Definition of working posture
- 3 Evidence-based risk factors for the development of musculoskeletal disorders
- 4 Population of workers at risk of awkward postures
- 5 Prevention of awkward working postures
- 6 Optimal and hazardous working postures
- 7 Emerging risks
- 8 References
- 9 Links for future reading
The article illustrates how the working posture influences musculoskeletal load, which determines musculoskeletal disorders development. It shows how dimensions of the working area can be adjusted in order to facilitate an optimal working posture. It also mentions two other factors, exerted force and time, which can strengthen or minimize the negative influence of awkward postures. The article gives also brief overview of exposure of workers in different sectors to painful and/or tiring positions as well as emerging risks related mostly to physical inactivity.
Definition of working posture
Working posture is the posture adopted by an employee while performing work tasks. It can be altered often, or a single posture can be sustained for an extended time. The human body can be represented by segments, such as an arm, forearm, thigh or trunk, connected to other segments by joints. Working posture can be described by angles between body segments. The number of angles defining body posture depends on the level of accuracy. For example, the human upper limb may be defined most accurately as having 37 separate angles. However, it can also be defined by the adoption of a minimum of seven angles. As a minimum, the hand is represented by one rigid segment.
The location and movement of the individual segments of the human body are usually described with respect to the three cross-sectional anatomical planes (Figure 1).
The vertical plane, which passes from front to rear, dividing the body into right and left sections, is called the sagittal plane. It can be specified that this plane passes directly through the midline of nose. When the trunk is considered, movements of this segment of the body in the sagittal plane are referred to as bending forward or backward (Figure 2). When considering the upper limb, forward movement from the natural position in the sagittal plane is called flexion, and backward movement is called extension.
The movement of rigid links of the body is also defined by angles in planes parallel to the sagittal plane. The frontal plane divides the human body into belly and back sections, and passes through the front of the face. Movements performed in the frontal plane are called abduction and adduction, or when referring to the trunk — bending sideways. Abduction is a movement that draws a segment away from the sagittal plane of the body. Adduction is a movement that brings it closer to the sagittal plane of the body.
The transverse plane runs perpendicular to the frontal and sagittal planes, and is parallel to the ground. It divides the body into upper and lower parts. Movement in this plane is defined as rotation or twisting.
When the adopted posture is standing upright with upper limbs hanging down naturally, a so-called natural posture, all angles defining the posture equal zero. Postures close to the natural one are recommended.
Evidence-based risk factors for the development of musculoskeletal disorders
Musculoskeletal load related to occupational work is very often a cause in the development of work-related musculoskeletal disorders (MSDs). MSDs may be the result of an injury arising due to sudden overload or the result of cumulative trauma. The risk of developing musculoskeletal disorders is closely linked to biomechanical and psychosocial factors related to work tasks, as well as to individual factors associated with the employee (i.e. age, previous musculoskeletal disorders).
Biomechanical risk factors, besides posture, also take into consideration force exertion and time. Posture duration and pattern of postural loading (exertion of external forces) and unloading (recovery periods) determine the character of the load (static or repetitive). These two load types are especially strenuous and can trigger MSDs development.
Heavy physical work, lifting and carrying, static muscular strains, short, monotonous and repetitive work, awkward postures, exposure to vibrations and lack of activity are regarded as classic physical factors leading to the development of work-related MSDs. Manual handling tasks play a special role in MSDs development.
Numerous studies confirm the link between the risk of musculoskeletal pain development and awkward body posture. There was a proven increase in the risk of developing neck and shoulder pain symptoms when neck twisting or bent postures were adopted often during work compared to a situation in which they were seldom adopted. The risk of neck pain increases when the neck is rotated more than 45 degrees during more than 25% of the work time. An increase in risk also occurs when the neck is flexed more than 45 degrees from the natural neck position during more than 5% of working time. However, even if the neck is flexed at 20 degrees for more than 40% of work time, the risk increases rapidly with time.
Risk for development of shoulder pain increases substantially when work is performed with hands above shoulder level, or when work tasks require reaching below knee level. Arm posture may be defined as belonging to one of four zones in the transverse plane and one of four zones in the frontal plane. Zone borders are 20 degrees and 60 degrees. The forth zone is backward flexion of the arm. Comfortable working posture requires an arm angle of less than 20 degrees in both the sagittal and frontal planes.
Many pain complaints relate to the lower back, which is mostly due to the bending or twisting of the body. Working in bent or twisted body positions for more than two hours a day is a strong risk factor for developing back pain. Bending and twisting of the trunk of more than 30 degrees and sustaining it longer than 15 minutes significantly increases the risk when compared with sustaining the posture for less than five minutes. Awkward trunk posture (twisting or bending) is also a strong risk factor in absenteeism due to back pain.
Bending the trunk forward/backward may be classified in terms of one of four load zones. The zone, relating to optimal working posture, refers to bending up to 20 degree, the second — from 20 degree to 60 degree. When the trunk is bend foreword above 60 degree or when the trunk is bent backwards the risk of developing MSDs is growing rapidly. For the trunk bending sideways (in frontal plane) or twisting the upper part of trunk with respect to lower part (transverse plane), comfortable zones are determined with criteria of 10° (Figure 2).
Upper limbs are usually involved in performing work tasks. However, legs can also be exposed to awkward positions. This happens when there is insufficient room for the legs and feet and in cases when a foot pedal is used. The foot pedal should be located at ground level in such a way as to avoid uncomfortable foot and leg positions. Other hazardous situations occur during kneeling or squatting. This is especially hazardous when it occurs frequently or is prolonged.
Population of workers at risk of awkward postures
Different occupations and patterns of static load are cited for disorders in different parts of the body. However, musculoskeletal disorders in the group of workers exposed to static work occur significantly more often than in other worker groups. For example in the group of dentists, which is strongly exposed to static load, 87.2 % reported having experienced at least one MSD symptom in the past 12 months, with the most prevalent MSD reported at the neck (57.5%), lower back (53.7%) and shoulder (53.3 %). Self-reported musculoskeletal symptoms in sewing machine operators compared with symptoms in other occupations showed a higher occurrence of a cumulative deleterious effect on static load in the neck and shoulder regions.
Exposure due to work sector and type of job was examined during a broad study of all European countries in the Forth European Working Conditions Survey (2007). According to the results of the survey, the highest rates of tiring or painful working postures can be found in the “Agriculture and fisheries” sector, followed by “Construction” and “Hotels and Restaurants” (Figure 3). The lowest rate is for “financial intermediation”. The highest rates for tiring and painful posture exposure can be found in skilled agricultural and fishery workers, followed by craft and related trades workers. The lowest is for technicians and professionals (Figure 4). Data from the most recent survey (2015) suggests that these relative rankings remain valid, with 70.2% of workers in Agriculture, forestry and fishing and 66.4% of those working in Construction reporting working in tiring or painful positions for at least a quarter of the time, compared to only 25.2% of those working in Financial and insurance activities.
The survey concludes that exposure to painful or tiring positions is higher among men than among women. Men are exposed about 20% more than women and exposure to painful or tiring positions in the workplace is similar in all four age groups. Results of the survey showed large differences in exposure to awkward working postures between the various sectors and occupations. Differences according to age and gender are less significant. It should be kept in mind, however, that this research was focused on postures only, and it did not take into account the posture duration.
Prevention of awkward working postures
Working postures are determined by the relationship between the workstation dimensions, the materials or tools used, the anthropometric dimensions of an employee and the task demands (to perform a specific operation).
The dimensions of the segments of the employee’s body in combination with the spatial structure of the workstation determine the working posture of the employee. The best situation is when the workstation dimensions can be adjusted in order to facilitate an optimal working posture.
Points of the workstation that are in direct contact with the employee (contact points) are especially important. Examples of these would be steering elements and computer keyboards, as well as computer screens, which dictates gaze direction. The structure described by contact points determines the workspace, and, thus, the working posture. Working posture is one of the main factors determining the musculoskeletal load of the employee. Therefore, to obtain optimal working posture, machinery or workstations dimensions and equipment should be adjusted to the dimensions of the general employee population.
For the design and determination of workstation dimensions suitable to users, population anthropometric data are usually values represented by the 5th percentile and the 95th percentile. In accordance with the distribution of the variable (for example length of forearm), the 5th percentile is the value of the variable below which 5% of the population fall. The 95th percentile represents the value below which 95% of the population falls.
In regard to the determination of functional spatial characteristics of workstations, are applied criteria relating to the areas referred to the horizontal plane (transverse) as well as the vertical planes (sagittal and frontal). In the horizontal plane, the recommended (normal range) should be determined by subsequent positions of the hand and forearm rotation relative to the elbow (Figure 5). A less optimal area (maximum range) would be created by means of successive positions of hand movement across a straight limb in relation to the shoulder joint. Work carried out outside the maximum range is connected with movement of the trunk and introduces awkward postures.
The height of the work plane is of vital importance for the organization of the workstation and should be adjusted on the basis of the type of work activity. The height of the work plane is usually determined relative to the height of elbow. Work not requiring particular accuracy (precision work) should be performed 75 mm below the elbow (when arms are hanging naturally down). When manual handling tasks are performed, an employee’s elbows should not be lifted to a height of more than 100 mm above the elbow level when upper limb is hanging naturally down (Figure 6). Specific work requiring visual inspection and accuracy of performed tasks requires higher work planes in comparison to the normal position of work.
Optimal and hazardous working postures
The optimum, least strenuous body posture is natural, meaning a posture with a rigid trunk and upper limbs hanging down naturally along the body. In this position, all angles between body segments equal zero. The higher the deviation from the natural body posture, the higher the musculoskeletal load.
Performing each work task requires the involvement of the entire body, though the various segments of the body are involved in different ways. There are, for example, activities only involving upper limbs, in which case lower limbs and the back are loaded statically. With respect to the upper limbs, special attention should be paid to wrist posture. Strong flexion or extension, often in conjunction with high repetitiveness, often leads to carpal tunnel syndrome.
Postures that are awkward or limited may cause muscular overload of ligaments and tendons. These working postures cause large values of forces and torques in the spine and joints of upper and lower limbs.
Time factors play an important role. The most strenuous work situations happen when postures are sustained for long periods of time without alteration (static postures) or are repeated numerous times (repetitive tasks). To make the situation of prolonged sitting less hazardous, not only should the least strenuous posture be adopted, but it also should be varied from time to time. The third important factor having influence on MSD development is exerted force. These three factors should always be considered jointly in musculoskeletal load assessment.
There is a relationship between posture duration and incidence of MSDs. Static postures result in the diminishing of blood circulation through the muscles. If an awkward working posture must be adopted for an employee to perform work tasks, it should be sustained for a very short time. The more strenuous the working posture, the shorter time it should be sustained. Maximum holding time (MHT) for various working postures is presented in Figure 7.
When considering sitting posture, attention should be paid to trunk posture, which should be slightly inclined towards the back from a vertical trunk position. Angles in the knees and elbows should be close to 90° degree angle.
Musculoskeletal disorders are largely the result of mechanical loads resulting from excessive burdens related to work technique. Work-related physical load is mostly associated with the exertion of large forces. However, also work with low but long lasting load (work at computer, assembling small objects, etc.) may lead to MSD development.
Work involving only activity of the upper limbs poses a severe risk of MSDs development. Usually in this case, the spine and limbs remain in a static position over a long period of time, which may particularly result in MSDs development in the lumbar spine and neck. There are also operations involving movement of only certain parts of the upper limbs, like for example hand with immobile shoulder and forearm. This creates a musculoskeletal load for some of body segments with repetitive load, since other parts of the body are involved in the maintenance of the posture (static load).
Static load in any posture, even at comparably low levels, results in the reduction of blood circulation due to muscle inactivity. This is associated with health risks, such as coronary diseases, type II diabetes, obesity and even certain types of cancers. The evaluation of risks and these potential emerging risks are forecasted in the EU-OSHA, 2011 (Risk Observatory). Evidence suggests that static postures may be associated with increase in risk for lower back disorders, which may even lead to permanent working incapacity in a number of cases.
Complementary to static load during work is the inactivity during leisure time and increased commute time. An increase in sedentary or prolonged standing work postures, resulting in physical inactivity at work, is caused by the growing use of computers and automated systems. Computer use leads to increased time spent in a fixed posture with eyes fixed on the computer screen, meaning that computer users have an increased risk for experiencing musculoskeletal disorders. One of the most serious emerging risks is associated with laptop use. More and more common in both home and work environment, working with laptops imposes a strenuous work posture with a low possibility of altering it in accordance with musculoskeletal load requirements. However, occupations at risk not only include office workers at computers, but also call centre agents, crane operators, straddle-carrier drivers, truck and bus drivers, workers in semiconductor factories and workers operating automated systems and machines.
Inactivity during work and leisure time is also closely related to obesity, which significantly increases the risk factor for developing back pain. Many obese employees also suffer from high blood pressure and diabetes.
Also, workers affected by prolonged standing in the workplace can be regarded at risk for the apparition of oedema in the legs. Prolonged standing also increases the risk of varicose veins and deep-vein thrombosis.
Both prolonged sitting or standing at workplaces and occupations with very little physical activity, as well as high movement repetitions, imply an increased prevalence for musculoskeletal diseases (MSDs) mostly in the back, neck and shoulder regions. Unfortunately, more and more employees are at such risk. This means that it is very crucial that adopted working postures are optimal from a musculoskeletal load point of view. Also, measures supporting the alternation of body positions, as well as on‑site health programmes aimed at preventing the risk, are necessary.
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