Quantification of muscular activity in the shoulder region during monotonous repetitive work.

The purpose of this preliminary study was to evaluate the degree of myoelectric activity (EMG) in the shoulder muscles during repetitive monotonous work cycles in order to assess the load on the shoulder muscles. The tasks were video recorded and the movements performed were analyzed with a direct computer-based observational method (OS-WALD). The amplitude distribution probability function (ADPF) of the surface EMG from the deltoideus lateralis and trapezius pars descendens muscles was analyzed in 6 persons working on assembly lines. For each subject, "on-site" EMG and video recordings were performed over 10-minute periods at four different workstations on the same day. Force/EMG amplitude calibration curves had been previously calculated for the same subjects using a strain-gauge dynamometer. The results obtained during the working periods were the following: the mean static contraction level, as defined by Jonsson (13), was 1.3% MVC in the trapezius muscle and 0.5% MVC in the deltoid muscle; the medium contraction level was 5.2% MVC in the trapezius and 1.7% MVC in the deltoid; the peak contraction level was 17% MVC in the trapezius and 6.2% MVC in the deltoid. When compared to previously suggested upper limits of ADPF levels, all the contraction levels monitored were below the recommended limits. The assessment of musculoskeletal load in the shoulder region via EMG and video recordings led us to conclude that the work cycles examined were characterized by low-repetitive monotonous work and were well organized in order to avoid the occurrence of musculoskeletal injuries.