The use of surface electromyography (EMG) for studying the effect of mechanical compression of occupational origin on muscle fatigue has been the subject of poor attention in ergonomic research. This study examined the effect of backpack carrying on fatigue of two shoulder muscles during sustained low force static contraction: the middle deltoid (MD) muscle and the upper trapezius (UT) muscle on which the backpack strap exerted direct compressive force. EMG activities of MD and UT muscles, of the dominant and non-dominant sides, were studied on eight subjects during two tasks, a maximal and an exhausting submaximal bilateral isometric 90 degrees arm abduction, which were performed while carrying a backpack load of 0, 10, and 20 kg, respectively. EMG amplitude (root mean square, RMS) and spectral (mean power frequency, MPF) parameters were computed from the recorded signals. No significant differences between the dominant and non-dominant sides were found for none of the parameters whatever the load-carrying mass. Load-carrying masses which were tested did not influence significantly the maximal bilateral arm abduction performance contrary to the time to exhaustion during the submaximal task, which decreased significantly with increasing load-carrying mass. A significant increase in fatigability, defined by the slope of MPF decrease, was observed for both muscles when load-carrying mass increased; but only UT presented a significant increase in muscle fatigue level, defined by the MPF value with respect to its initial value, at the end of the exhausting submaximal task. Furthermore, the increase in muscle activation of UT, quantified by RMS, during the exhausting task was not significantly higher with increasing load-carrying mass. So, the increased signs of local fatigue of UT may be interpreted by a localised blood flow impairment resulting from the direct compressive force exerted by backpack on this muscle.