In vivo behavior of muscle fascicles and tendinous tissues in human tibialis anterior muscle during twitch contraction.

In this study we investigated the time course of length and velocity of muscle fascicles and tendinous tissues (TT) during isometric twitch contraction, and examined how their interaction relates to the time course of external torque and muscle fascicle force generation. From seven males, supra-maximal twitch contractions (singlet) of the tibialis anterior muscle were induced at 30 degrees , 10 degrees and -10 degrees plantar flexed positions. The length and velocity of fascicles and TT were determined from a series of their transverse ultrasound images. The maximal external torque appeared when the shortening velocity of fascicles was zero. The fascicle and TT length, and external torque showed a 10-30 ms delay of each onset, with a significant difference in half relaxation times at -10 degrees . The time course of TT elongation, and fascicle and tendinous velocities did not differ between joint angles. Curvilinear length-force properties, whose slope of quasi-linear part was ranged from -15.0 to -5.9 N/mm for fascicles and 5.4 to 14.3N/mm for TT, and a loop-like pattern of velocity-force properties, in which the mean power was ranged from 0.14 to 0.80 W for fascicles, and 0.14 to 0.81 W for TT were also observed. These results were attributed to the muscle-tendon interaction, depending on the slack and non-linearity of length-force relationship of compliant TT. We conclude that the mechanical interaction between fascicles and TT, are significant determinants of twitch force and time characteristics.