The aim of this study was to clarify the effects of tendinous tissue properties on origin of greater force output at short inter-pulse intervals in the 2-pulse trains compared to those at longer inter-pulse intervals. Thus, this study investigated the contributions of the second stimulus (C2) in 2-pulse trains with different inter-pulse intervals on the torque response and tendinous tissue properties of human skeletal muscle in vivo. The torque response and tendinous tissue elongation following single pulses and 2-pulse trains at different inter-pulse intervals (5, 10, 20, 30, 40, 50, 80, 100, 150, and 200 ms) were recorded in the tibialis anterior muscle using real-time ultrasonography. C2 with inter-pulse intervals of 5-100 ms invoked significantly greater torque responses than single pulses. In contrast, the elongation and compliance of tendinous tissue for C2 with inter-pulse intervals from 5-80 ms were significantly lower than those of the single-pulse response. A significant negative relationship between torque response and tendinous tissue compliance was observed in C2 with different inter-pulse intervals. The torque response as a result of C2 is greater at short inter-pulse intervals in which the force summation due to second stimulus coincides with the period of decreased tendinous tissue compliance due to the first stimulus.