Effects of tendinous tissue properties on force output evoked by 2-pulse trains at different inter-pulse intervals in the human tibialis anterior muscle.

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.