The loads on the shoulder muscles of building workers were investigated by applying a new fatigue index method, for evaluation of localized muscle fatigue during work. Based on a power spectrum analysis of myoelectric signals, and calculations of spectral moments, the method enables the monitoring of changes in the propagation velocity of muscle fibre action potentials. The propagation velocity alters with the integrated response to the strain of the load on muscle metabolism and circulation Myoelectric signals from four shoulder muscles as well as the ECG were recorded on magnetic tape at the building sites during regular daily work. For analysis, specially developed computerized procedures were used, including procedures for automatic checks of signal quality Significant localized muscle fatigue was found in one or more muscles during a spell of work. The fatigue was particularly marked in the supraspinatus and trapezius muscles, and was seen in static work tasks even when the force exerted was small. T...