In order to examine variations in the recruitment order of motor units, voluntary contraction of m. vastus medialis was studied at various gradients of tension increase. Two inserted electrodes were used for the recording of spike discharges of two or three motor units. Spike discharges were recorded simultaneously with the value of tension of knee extension and with the integrated value of surface electromyograms (EMG) . The threshold value of a motor unit depended on the gradient G (kg/sec) of the tension increase. A similar change was also found in the latency time Tl (sec), where Tl is the time interval from the onset of EMG to the moment of recruitment of a motor unit. The relation Tl =ρG-1+λX was experimentally found, where ρ is a proportional constant. The motor unit with a positive/negative value of λ is characterized to be static/phasic.