Tension lag time (TLT), a latency from the onset of electromyographic activities of prime mover muscle to the rise of tension, of knee extensor muscle was measured at twitch, tetanic, and fast voluntary contraction in three normal subjects. Twitch and tetanic contractions were evoked by four different strengths of electrical stimuli, and the peak tensions attained at fast voluntary contraction were within the range of tensions evoked by electrical stimulation. In each mode of contraction, the relationship between TLT and peak tension (Fmax) was approximated by a hyperbolic function of Fmax (TLT-a) = b. TLT was influenced by three factors: (1) Fmax, the greater Fmax, the shorter TLT; (2) force detection level to point out timing of the rise of tension, the higher the level, the longer TLT was; and (3) the mode of contraction, shortest at the twitch, longest at the voluntary, and intermediate in the tetanic contraction.