Postural load and the development of musculo-skeletal illness.

Early in the 1970s, high rates of sick-leave due to musculo-skeletal complaints were frequently recorded among workers at Standard Telefon and Kabelfabrik's (STK's) factory in Norway. Workstations were redesigned according to ergonomics principles that allowed workers a wider choice of working postures and following their introduction in 1975, there was a marked reduction in sickness absence. Postural load was studied in groups of female workers in well defined assembly tasks. Trapezius load was recorded by electromyography (EMG). Simultaneously, postural angles of the upper arm in the shoulder joint and flexion/extension of head/neck and back were measured by using pendulum potentiometers. A quantitative relationship was found for the group between its median value of static trapezius load and the development of musculo-skeletal sick-leave, as a function of length of employment. Further support for a relationship between musculo-skeletal injury and trapezius load was found for the same subjects who suffered less musculo-skeletal sick-leave, consistent with the reduced trapezius load when working at the redesigned work stands. The relationship between postural load and musculo-skeletal injury was studied in comparable groups of the female workers with respect to age, working hours per day and time of employment. Psychosocial problems, spare time activities and living habits of workers did not show any significant difference across the groups. Postural load, both in terms of the magnitude of the flexion angle of the upper arm in the shoulder joint and the distribution of the work load between flexors and extensors, appeared to influence the incidence of load-related musculo-skeletal illness in the upper part of the body. The incidence of musculo-skeletal sick-leave in a group of workers with a median static trapezius load of about 1 to 2% MVC (Maximum Voluntary Contraction) for most of the work day, was approximately the same as for a group of comparable female workers without continuous work load. This suggests that a static trapezius load level of about 1% MVC is acceptable for the major part of the work day if adequate breaks in the load pattern are allowed when needed. At the same time, a median arm flexion of 15 degrees and a median arm abduction less than 10 degrees indicate the amplitude of these angles for 50% of the recording time. No details about the work-pause pattern was obtained, therefore these limits are only a rough indication of an acceptable arm position.