The degree of shortening or lengthening of muscles during joint actions has not been clarified in humans, although such information is essential in understanding human muscle functions. In this study, the tendinous movement of a muscle was determined by real-time ultrasonography during voluntary contractions. The tibialis anterior muscle (TA) was tested in five healthy men who performed dorsi- and plantar flexion movements (shortening and lengthening of TA) at two frequencies (0.1 and 1.5 Hz). The insertion point (η) of fascicles onto the aponeurosis was clearly visualized on the ultrasonogram, and its position relative to a fixed marker moved proximally and distally according to dorsi- and plantar flexion of ankle joint. The movement of η occurred in phase with the angular change of ankle joint, giving high correlations (r = 0.93 to 0.97) between the displacement of η and the angle. The displacement of η for one radian of joint angle change, 46.5 ± 1.7 (SD) mm, was comparable to the reported moment arm of TA. The present method has many potential applications in the field of muscle physiology and biomechanics in humans.
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