Purpose] We measured changes of indices of muscle architecture of the lateral head of the gastrocnemius in graded ankle plantar contraction, and examined the reliability of the measurement indices as well as the relationships of the changes of the respective muscle architecture indices with graded ankle plantar isometric contraction. [Subjects and Method] The subjects were 10 healthy males. We measured muscle thickness, pennation angle, superficial aponeurosis displacement, and deep aponeurosis

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... displacement as indices of muscle architecture with ultrasonography analysis equipment. The exercise task was ankle plantar isometric contraction of 5, 10 and 15 kg, in the prone position with knee and ankle angles of 0 degrees. We performed the measurements again under the same conditions after an interval of about one week. [Results] By the test-retest method the intraclass correlation coefficients (1.1) were 0.77~0.91 for superficial aponeurosis displacement, and 0.89~0.94 for deep aponeurosis displacement, and the reliability was high. On the other hand, that of muscle thickness, 0.38~0.87, and pennation angle, 0.10~0.56, were low. Furthermore, deep aponeurosis displacement increased significantly (p<0.01) with increasing muscle strength, whereas the relationships of the other indices were not significant.