How Different Knee Flexion Angles Influence the Hip Extensor in the Prone Position

[Purpose] The present study examined the effects of knee flexion angle on hip extensor muscle activity. [Subjects and Methods] Twenty healthy subjects maintained knee flexion angles of 0 degrees, 30 degrees, 60 degrees, 90 degrees and 110 degrees in the prone position and performed maximal voluntary contraction in hip extension. Maximum torque in hip extension at the different angles was measured, and surface electromyogram activities of the gluteus maximus (GM), biceps femoris (BF) and semitendinosus (ST) were recorded and normalized by the maximum voluntary isometric contraction (MVIC). [Results] The maximum torque of the hip extensor showed significant decreases between 0 degrees and 60 degrees, 90 degrees and 110 degrees of knee flexion. The muscle activity of BF was significantly high at 0 degrees, and GM showed a significantly higher activity than both BF and ST at 60 degrees, 90 degrees and 110 degrees of knee flexion. [Conclusion] The maximum torque in hip extension and muscle activities of BF and ST were significantly high at 0 degrees but they decreased at knee flexion angles of more than 60 degrees. Therefore, we consider that more than 60 degrees of knee joint flexion is required to increase GM activity, and to reduce the muscle activities of BF and ST.