Influence of quadriceps femoris fatigue on low back load during lifting of loads at different distances from the toes

[Purpose] In this study, the effects of muscle fatigue involving only the quadriceps femoris on low back load during lifting were dynamically and mechanically investigated in relation to the degree of muscle fatigue and distance from a heavy object. [Methods] We measured the joint angle, electromyography and ground reaction force of 18 healthy male Students during lifting at 3 fatigue levels of loads placed at 3 distances from the toes. [Results] At 25% fatigue of the quadriceps femoris, at the intermediate point of lifting, the knees were mostly extended and the body trunk was leaning forward, thus altering the mode of lifting. Myoelectrical activity of the quadriceps femoris was also decreased, and myoelectrical activity of the lumbar muscle decreased during the phase from the start of muscle activity to when the object was lifted off the floor. At 50% fatigue of the quadriceps femoris, no change in joint angles was seen during lifting, and as myoelectrical activity of the quadriceps femoris decreased, myoelectrical activity of the lumbar muscle increased during the phase from the start of muscle activity to when the object was lifted off the floor. In terms of ground reaction force, greater distance was associated with greater anterior load. [Conclusion] These results suggest that when the level of quadriceps femoris fatigue is relatively low, altering the mode of lifting somewhat lessens low back load, but when the level of quadriceps femoris fatigue is relatively high, compensatory changes cannot be made and an increase in low back load can increase the risk of lumbar injury.