The psoas major muscle is essential for bipedal walking – An analysis using a novel upright bipedal-walking android model

Bipedalism is a unique function in humans. Various investigations in bipedal walking have assessed the kinetic chain from the pelvis to the lower limbs. However, few studies have investigated the functions of the upper body including the psoas major muscles. In the present study, a bipedal-walking human full-body skeletal model, "the bipedal android model", was generated by attaching air cylinder devices to simulate the contraction and relaxation of various muscles required for bipedal walking, including the psoas major muscles. The bipedal-walking principle was discussed using the model.Every part of a human full-body skeletal model was connected by wires or cables to retain the mobility of each joint. Then the psoas major (PM), gluteus minimus (GM), long head of biceps femoris (BF), quadriceps femoris (QF), and semimembranosus (SM) muscles were simulated in the skeletal model using air cylinders. The gait pattern was observed by synchronizing the contraction of PM, GM, QF and SM, and relaxation of BF of the ipsilateral side together with the reversed patterns in the contralateral side, and then switching the phase by every step. The gait pattern in dysfunction of PM or GM muscles was also observed by disconnecting the corresponding air cylinders.The synchronized contraction of PM, GM, QF and SM generates the force to tilt the upper body to ipsilateral side, followed by elevation of the lower limb together with the forward rotation of the pelvis in the contralateral side to swing the leg forward. The next step was generated by reversing the contraction phase at the landing of the swung leg. The dysfunction of PM muscle disabled effective gait in the model, while GM did not.The bipedal android model indicated that the psoas major muscles play a crucial role in bipedal walking in human.