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Tuesday, 08 March 2011 21:13

Postures at Work

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A person’s posture at work—the mutual organization of the trunk, head and extremities—can be analysed and understood from several points of view. Postures aim at advancing the work; thus, they have a finality which influences their nature, their time relation and their cost (physiological or otherwise) to the person in question. There is a close interaction between the body’s physiological capacities and characteristics and the requirement of the work.

Musculoskeletal load is a necessary element in body functions and indispensable in well-being. From the standpoint of the design of the work, the question is to find the optimal balance between the necessary and the excessive.

Postures have interested researchers and practitioners for at least the following reasons:

    1. A posture is the source of musculoskeletal load. Except for relaxed standing, sitting and lying horizontally, muscles have to create forces to balance the posture and/or control movements. In classical heavy tasks, for example in the construction industry or in the manual handling of heavy materials, external forces, both dynamic and static, add to the internal forces in the body, sometimes creating high loads which may exceed the capacity of the tissues. (See figure 1) Even in relaxed postures, when muscle work approaches zero, tendons and joints may be loaded and show signs of fatigue. A job with low apparent loading—an example being that of a microscopist—may become tedious and strenuous when it is carried out over a long period of time.
    2. Posture is closely related to balance and stability. In fact, posture is controlled by several neural reflexes where input from tactile sensations and visual cues from the surroundings play an important role. Some postures, like reaching objects from a distance, are inherently unstable. Loss of balance is a common immediate cause of work accidents. Some work tasks are performed in an environment where stability cannot always be guaranteed, for example, in the construction industry.
    3. Posture is the basis of skilled movements and visual observation. Many tasks require fine, skilled hand movements and close observation of the object of the work. In such cases, posture becomes the platform of these actions. Attention is directed to the task, and the postural elements are enlisted to support the tasks: the posture becomes motionless, the mus-cular load increases and becomes more static. A French research group showed in their classical study that immobility and musculoskeletal load increased when the rate of work increased (Teiger, Laville and Duraffourg 1974).
    4. Posture is a source of information on the events taking place at work. Observing posture may be intentional or unconscious. Skilful supervisors and workers are known to use postural observations as indicators of the work process. Often, observing postural information is not conscious. For example, on an oil drilling derrick, postural cues have been used to communicate messages between team members during different phases of a task. This takes place under conditions where other means of communication are not possible.

     

    Figure 1. Too high hand positions or forward bending are amont the most commom ways  of creating “static” load

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          Safety, Health and Working Postures

          From a safety and health point of view, all the aspects of posture described above may be important. However, postures as a source of musculoskeletal illnesses such as low back diseases have attracted the most attention. Musculoskeletal problems related to repetitive work are also connected to postures.

          Low back pain (LBP) is a generic term for various low back diseases. It has many causes and posture is one possible causal element. Epidemiological studies have shown that physically heavy work is conducive to LBP and that postures are one element in this process. There are several possible mechanisms which explain why certain postures may cause LBP. Forward bending postures increase the load on the spine and ligaments, which are especially vulnerable to loads in a twisted posture. External loads, especially dynamic ones, such as those imposed by jerks and slipping, may increase the loads on the back by a large factor.

          From a safety and health standpoint, it is important to identify bad postures and other postural elements as part of the safety and health analysis of work in general.

          Recording and Measuring Working Postures

          Postures can be recorded and measured objectively by the use of visual observation or more or less sophisticated measuring techniques. They can also be recorded by using self-rating schemes. Most methods consider posture as one of the elements in a larger context, for example, as part of the job content—as do the AET and Renault’s Les profils des postes (Landau and Rohmert 1981; RNUR 1976)—or as a starting point for biomechanical calculations that also take into account other components.

          In spite of the advancements in measuring technology, visual observation remains, under field conditions, the only practicable means of systematically recording postures. However, the precision of such measurements remains low. In spite of this, postural observations can be a rich source of information on work in general.

          The following short list of measuring methods and techniques presents selected examples:

            1. Self-reporting questionnaires and diaries. Self-reporting questionnaires and diaries are an economical means of collecting postural information. Self-reporting is based on the perception of the subject and usually deviates greatly from “objectively” observed postures, but may still convey important information about the tediousness of the work.
            2. Observation of postures. The observation of postures includes the purely visual recording of the postures and their components as well as methods in which an interview completes the information. Computer support is usually available for these methods. Many methods are available for visual observations. The method may simply contain a catalogue of actions, including postures of the trunk and limbs (e.g., Keyserling 1986; Van der Beek, Van Gaalen and Frings-Dresen 1992) .The OWAS method proposes a structured scheme for the analysis, rating and evaluation of trunk and limb postures designed for field conditions (Karhu, Kansi and Kuorinka 1977). The recording and analysis method may contain notation schemes, some of them quite detailed (as with the posture targeting method, by Corlett and Bishop 1976), and they may provide a notation for the position of many anatomical elements for each element of the task (Drury 1987).
            3. Computer-aided postural analyses. Computers have aided postural analyses in many ways. Portable computers and special programs allow easy recording and fast analysis of postures. Persson and Kilbom (1983) have developed the program VIRA for upper-limb study; Kerguelen (1986) has produced a complete recording and analysis package for work tasks; Kivi and Mattila (1991) have designed a computerized OWAS version for recording and analysis.

                 

                Video is usually an integral part of the recording and analysis process. The US National Institute for Occupational Safety and Health (NIOSH) has presented guidelines for using video methods in hazard analysis (NIOSH 1990).

                Biomechanical and anthropometrical computer programs offer specialized tools for analysing some postural elements in the work activity and in the laboratory (e.g., Chaffin 1969).

                Factors Affecting Working Postures

                Working postures serve a goal, a finality outside themselves. That is why they are related to external working conditions. Postural analysis that does not take into account the work environment and the task itself is of limited interest to ergonomists.

                The dimensional characteristics of the workplace largely define the postures (as in the case of a sitting task), even for dynamic tasks (for example, the handling of material in a confined space). The loads to be handled force the body into a certain posture, as does the weight and nature of the working tool. Some tasks require that body weight be used to support a tool or to apply force on the object of the work, as shown, for example in figure 2.

                Figure 2. Ergonomic aspects of standing

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                Individual differences, age and sex influence postures. In fact, it has been found that a “typical” or “best” posture, for example in manual handling, is largely fiction. For each individual and each working situation, there are a number of alternative “best” postures from the standpoint of different criteria.

                 

                 

                 

                 

                 

                 

                 

                 

                 

                 

                 

                 

                 

                Job Aids and Supports for Working Postures

                Belts, lumbar supports and orthotics have been recommended for tasks with a risk of low back pain or upper-limb musculoskeletal injuries. It has been assumed that these devices give support to muscles, for example, by controlling intra-abdominal pressure or hand movements. They are also expected to limit the range of movement of the elbow, wrist or fingers. There is no evidence that modifying postural elements with these devices would help to avoid musculoskeletal problems.

                Postural supports in the workplace and on machinery, such as handles, supporting pads for kneeling, and seating aids, may be useful in alleviating postural loads and pain.

                Safety and Health Regulations concerning Postural Elements

                Postures or postural elements have not been subject to regulatory activities per se. However, several documents either contain statements which have a bearing on postures or include the issue of postures as an integral element of a regulation. A complete picture of the existing regulatory material is not available. The following references are presented as examples.

                  1. The International Labour Organization published a Recommendation in 1967 on maximum loads to be handled. Although the Recommendation does not regulate postural elements as such, it has a significant bearing on postural strain. The Recommendation is now outdated but has served an important purpose in focusing attention on problems in manual material handling.
                  2. The NIOSH lifting guidelines (NIOSH 1981), as such, are not regulations either, but they have attained that status. The guidelines derive weight limits for loads using the location of the load—a postural element—as a basis.
                  3. In the International Organization for Standardization as well as in the European Community, ergonomics standards and directives exist which contain matter relating to postural elements (CEN 1990 and 1991).

                   

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