Effect of the Different Hip Joint Angle Definition on the Estimation of Muscle-Tendon Complex Length of the Hip Extensor Muscles in Counter Movement Jump

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The purpose of this study was to consider the discrepancy between the trunk-based and the pelvis-based hip joint angles, and the difference between the MTC lengths estimated on the basis of the two types of hip joint angles during dynamic movement. Ten male athletes performed CMJ. Joint kinematics and the MTC length of long head of biceps femoris were compared between the trunk-based hip angle and the pelvis-based hip angle. In the CMJ, the angular displacements of the trunk-based were significantly higher than that of pelvis-based hip joints. There were no significant correlations between the two types of hip joint angles except at initial position. As for long head of biceps femoris, in contrast with the trunk-based-hip joint angle derived MTC length, which shows stretching in the descending phase and shortening in the ascending phase, the pelvis-based hip joint angle derived MTC length shows little changes. These results suggest that the trunk-based and pelvis-based hip joint angles have similar patterns of fluctuation; however, the angular displacement of each type is not equivalent. These results also suggest that we may misinterpret the type of muscle contraction because the definitions of hip joint angle are different even for the same muscle type.