The sustained unfused tetanus of the human flexor digitorum sublimis was studied in isometrical conditions during electrical stimulation of the muscle. The frequency of stimulation was 8-12 p.p.s., train duration 20-60 s. 3 distinct successive phases of tetanus were revealed. These phases differ in the ratio of the mean tension to the amplitude of its oscillations, in relaxation rate and the efficiency of the duble pulse action. The duration of phases depends on muscle length, stimulation frequency and previous activity. It is supposed that phase II is adopted for the quick movements and muscle properties in phase III for the efficient mantaining of sustained tension.