At sustained maximal activity most investigators have observed a progressive fall in the total surface electromyogram (EMG), expressed either as the root-mean-square amplitude or as the rectified integral. It is accepted thatt the powerful central motor drive involves most of the motor units (MUs) in force generation. At submaximal sustained contractions the EMG recorded from the skin surface shows a progressive increase in the total activity as the muscle fatigues. Two mechanisms may underlie that findings: recruitment of additional MUs and an increase in MU discharge rates of already active units.The purpose of the present study was to assess the changes of the surface EMG recorded by branched bipolar electrodes using a level-trigger averaging technique.