Ranking systems for evaluation of joint and joint motion stressfulness based on perceived discomforts
Introduction
Body discomfort is associated with biomechanical factors such as joint angles, muscle contractions, pressure distribution that produce feelings of pain, muscle soreness, numbness, or stiffness. Minimization of perceived discomfort by eliminating physical constraints can contribute to reduction of the risk for musculoskeletal disorders (Dul et al., 1994; Milner, 1985; Nag, 1991; Putz-Anderson and Galinsky, 1993; Zhang et al., 1996). The warning provided by body discomforts can be seen as an indicator of the mismatches between the person and the task, calling for job redesign (Corlett and Bishop, 1976).
Since Corlett and Bishop (1976) used a diagram of the body to identify the location of an individual's discomfort, discomfort has been largely assessed using the psychophysical methods such as body maps, discomfort scales or questionnaires. Assessment of postural load/discomfort in a given posture is an important step for preventing musculoskeletal disorders and improving work environment. Since the OWAS (Ovako Working Postures Analysing System) method proposed by Karhu et al. (1977), many postural classification schemes have been developed to enable quantitative evaluation of the postural stresses (Genaidy et al., 1994; Juul-Kristensen et al., 1997).
Only a few studies focused on ranking the stressfulness of joint motions, which can be used for understanding adverse effects of working postures on the workers, and as a cost function for predicting human postures. Genaidy and Karwowski (1993) provided a ranking system for the stressfulness of body deviations from neutral postures based on perceived discomfort in non-neutral postures. The ranking showed that hip abduction was the most stressful of all joint motions in the standing posture (rank of 5), hip flexion ranked fourth, and elbow supination, neck lateral bending and hip extension third, etc. Here, a higher rank indicates that the respective joint motion is more stressful. However, the ranking system was confined to the joint motions reaching the limit of range of motions. Genaidy et al. (1995) developed two improved ranking systems for the static, non-neutral postures only, around the joints of the upper extremity and the spine, which were classified by the joint motions and joints, and by the joints. The ranking system by the joint motions and joints revealed that the stressfulness of non-neutral postures was ranked (from highest to lowest): (1) shoulder extension, severe elevation and adduction; (2) wrist severe extension, and elbow supination; (3) wrist severe flexion, shoulder light elevation, and lower back lateral bending, etc. In addition, another ranking by the joints showed that the shoulder scored higher discomfort ratings than other joints, followed by the wrist, elbow, lower back, and finally neck. Here, the same rank of 3 was assigned to the wrist, elbow and lower back. However, these systems were based on the subjective discomforts perceived in limited joint motions of the standing posture according to the existing micro-postural classification schemes, and dealt only with the upper extremities and the spine, not with the whole body.
To overcome these restrictions of the existing ranking systems, in this study, comprehensive ranking systems for evaluation of the stressfulness of varying joint motions in their full range of motion were developed. The rankings were based on perceived discomfort ratings for almost every joint motion in the sitting and standing postures. The ranking systems were classified by the joints and respective joint motions, the joint motions alone, and the joints alone.
Section snippets
Subjects
Twenty male undergraduate and graduate students volunteered to participate in the experiment aimed to measure perceived body discomforts for varying postures. All subjects were reported to be in good health and were without history of any musculoskeletal disorders. Means and standard deviations of their physical characteristics were: (1) age—25.2±2.6 years; (2) stature—172.1±5.7 cm; and (3) body weight—66.8±6.8 kg.
Rating method
Of several psychophysical scaling methods, the magnitude estimation method was
Normalized discomforts and RDIs
Averages of the normalized discomforts in each level of joint motions, and RDIs in each joint motion across 20 subjects for the sitting and standing postures are summarized in Table 2, Table 3, respectively. Perceived discomforts for all joint motions in both postures increased linearly or quadratically as the joints deviated away from neutral positions. Neutral position is the posture that any joint motion is not occurred, i.e., angle of any joint motion is 0°. Furthermore, there were drastic
Discussion and conclusions
Three ranking systems for the stressfulness of the joint motions and joints were proposed, which were based on perceived discomfort ratings obtained using the magnitude estimation method. The results showed that discomfort levels were significantly affected by the types of joint motions, size of joint motions, and joints. On the basis of the discomfort levels for varying joint motions, several distinct classes of the joint motions and joints were assigned different ranks of postural stress with
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