Effect of stool height and holding time on postural load of squatting postures

Abstract

During an entire work shift, many workers, including welders, farmers, etc., work in squatting postures to handle objects on the ground. It has been suspected that such prolonged squatting without any supporting stool would gradually cause musculoskeletal injuries to workers. Previous studies showed that poor working postures are strongly related to musculoskeletal disorders. This study examines the effect of stool height and posture holding time on the physical stress of squatting postures, based on a laboratory experiment measuring subjective discomfort. Eight healthy male student subjects participated in the experiment, in which the free modulus magnitude estimation method was employed to obtain subjective discomfort ratings for squatting postures. The subjects were asked to rate discomfort levels for the whole body, the lower back, the upper and lower legs every 2 min, while holding the squatting postures for 16 min. Squatting postures have four different stool heights: no stool, 10, 15 and 20 cm. The results showed that the squatting posture with a 10-cm height stool was more comfortable than the other stool heights studied, while the posture with a 20-cm. height stool was most discomfortable. The discomfort levels increased almost linearly with the posture holding time. In addition, the differences of discomfort levels between stool conditions increased after 6 min of posture holding time. Based on the findings of this study, it is recommended that a stool with the proper height be provided for reducing the postural load when adopting squatting postures. Relevance to industry Squatting postures are awkward working postures common to the shipbuilding shops, farms, automobile assembly lines, etc. Many workers usually use stools to reduce postural stresses when adopting squatting postures, without empirical evidences. This study provided the ergonomic guidelines regarding the height of stool and posture holding time in squatting postures. (C) 2003 Elsevier Science B.V. All rights reserved.