In an earlier occupational study the EMG was found to be unchanged during a whole working day. The purpose of this study was therefore to examine EMG changes during experiments in a laboratory environment comparable to occupational work. The surface EMG of the quadriceps muscle of 16 subjects was analysed during 3 experiments: (I) 10% maximal voluntary contraction (MVC) sustained for 1 h, and with a constant work intensity of the muscle; (II) 20% MVC for 5 sec alternating with a rest of 5 sec for 1 h; (III) 20% MVC was sustained until exhaustion. In the latter the endurance time decreased with increasing MVC. The results indicated that the methods used in this study can be trusted to reveal changes in EMG due to fatigue in a group of subjects but not in the individual subject. The decrease in mean power frequency (MPF) and the increase in root mean square amplitude (RMS) were most pronounced during the first 10 min of the 10% sustained contraction, indicating a decrease in the conduction velocity along the muscle fibres or synchronization of motor unit firing. In contrast, at 20% intermittent contraction the MPF was unchanged. Between 10 and 60 min of contraction time the increase in RMS amplitudes associated with none or only slight changes in MPF indicated recruitment of new motor units for both types of contraction. The lack of EMG fatigue during a whole day in an earlier occupational study may be because the work performed required varying degrees of muscle force.(ABSTRACT TRUNCATED AT 250 WORDS)