The purpose of the current study was to investigate the mechanism of the maintenance of sagittal trunk posture in maximal sprint running. Twelve male sprinters performed 50-m sprint running with maximal effort. kinematic and ground reaction force data were collected at approximately 40 m from the starting point. an inverse dynamics approach was applied to calculate joint forces and torques at the hip and lumbosacral joints. The pelvic anterior-posterior tilt components from hip extension torque and from the moment of hip joint force were calculated. The lumbosacral extension torque was greater than lumbosacral lateral flexion and torsional rotation torque during stance phase. The first and second peak values of the lumbosacral extension torque were 6.58 ± 1.58 Nm/kg and 3.11 ± 0.76 Nm/kg, respectively. The mechanical load to flex the lumbar region was due to hip extensor, and the posture was not affected by the impact force via the supporting leg. The large lumbosacral extension torque was exerted to maintain the posture while resisting this mechanical load by hip extensor. These findings suggested that lumbar extensor were instantaneously required to exert maximal torque, and sprinters needed to strengthen the lumbar extensor to decrease the load on tissues around the spine.