Postural angles as an indicator of postural load and muscular injury in occupational work situations

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Abstract
This paper explores the use of information on the position of the upper arm and head as an indicator of load on the shoulder and of risk of shoulder injury for workers performing electromechanical assembly work. Two tests were used in the evaluation of the method, examining whether or not: (1) the development of musculo-skeletal injuries among groups of workers could be related to postural angles of the upper arm, and (2) there was a correlation between the two indicators of shoulder load, position of upper arm and upper trapezius EMG. Postural angles of flexion/extension and abduction/adduction of the right upper arm in the shoulder joint as well as flexion/extension of the head and back were measured using potentiometer-sensed pendulums. In most subjects, electromyography (EMG) was simultaneously recorded from upper trapezius muscles. The magnitude of the postural angles of the shoulder joint influenced the shoulder load. However, several parameters not quantified by postural angle measurements also increase the shoulder load independently of arm position, and must be taken into account in order to use postural angles alone as an indicator of shoulder load. This was supported by finding a significant positive correlation between the median arm flexion and the median trapezius load for a well-defined work task, a correlation which was weakened or disappeared when other work tasks with different body movements or external loads were included in the analysis. A group of female workers adopting a posture, with a median arm flexion of less than 15°, a median arm abduction of less than 10° and using a light (0.35 kg) hand tool recorded a 20% incidence of sick leaves due to shoulder injuries for workers employed 2-5 years, and.a 30% incidence for those employed more than 5 years. This is a significantly lower incidence than for other groups working with higher arm flexion.