Biomechanical and ergonomic considerations on shoulder muscle load. Experimental studies

Abstract

Work-related shoulder disorders represent a significant subset of work-related musculoskeletal disorders. Shoulder tendinitis has a high prevalence in major populations of employees. There is an urgent need for hints and directions on how to evaluate risks for developing chronic work-related shoulder disorders, as well as for methods of assessing hazardous working conditions.The aims of this work were to assess usability and accuracy in techniques and methods for exploring arm positions and shoulder muscle activity and to investigate the pattern of shoulder muscle interplay regarding voluntary control. The aims were also to elucidate the relations between the external physical demands originating from working tasks and internal load on different structures in the shoulder and to propose a model for estimating the risk of developing acute and chronic work-related musculoskeletal disorders of the shoulder, by using observable data which characterize the exposure.Electromyography (EMG) and intramuscular pressure (IMP) measurements were used to assess shoulder muscle activity. Bipolar electrodes were used for acquisition of EMG from the superficial muscles and for deep lying muscles indwelling wire electrodes were used. Acquisition of IMP was accomplished by microcapillary infusion technique. A 3D motion analysis system (MacReflex) has been used for control and registration of arm postures.The results indicate that IMP and EMG measurements produce similar estimates of muscle load and that there were substantial differences in IMP generation between shoulder muscles. Extremely high IMPs were recorded in the supraspinatus muscle. The MacReflex motion analysis system was found to produce position data with an angle error less than 1.8( in biomechanical and ergonomic application. It was demonstrated that the activity of the descending part of the trapezius muscle could be voluntarily reduced to 56% of the initial activity, without altering external conditions, and that the main part of the trapezius load was transferred to the rhomboid major and minor and the transverse part of the trapezius. IMP measurements showed that accumulated muscle fatigue might occur in the infra- and/or supraspinatus, if the upper arm exceeds 30( of arm elevation and that impaired muscle blood flow might occur if the upper arm exceeds 50(. In the validation of the biomechanical shoulder model, it was established that the shoulder muscle forces estimated by the model acceptably agree with the shoulder muscle forces estimated by the EMG measurements, except for the levator scapulae and the supraspinatus, where estimations from EMG measurements were 53% and 207%, respectively, of the activity calculated by the model. In conclusion: feedback assisted reduction of trapezius activity will affect other shoulder muscles, MacReflex motion analysis system possesses adequate accuracy for biomechanical and ergonomic purposes and IMP recordings from the infra- and supraspinatus indicate the necessity of restricted arm positions in static or repetitive work.