The effects of quadriceps fatigue on the technique of lifting.

A biomechanical analysis was performed of lifting before and after fatiguing the quadriceps muscles. The hypothesis tested was that when the quadriceps muscles were fatigued the lifter would change lifting technique from more of a squat (leg) lift to more of a stoop (back) lift to decrease the demand on the quadriceps muscles. The hypothesis was broadly supported, and three variables changed significantly with fatigue: trunk angular velocity, which increased, and knee moment integral and hip angles, which both decreased. These changes are all consistent with a change from more of a squat toward a stoop technique. The fact that the technique of lifting changes with quadriceps muscle fatigue underlines the importance of the physiologic condition of these muscles and suggests that rehabilitation of low-back-injured workers should include the quadriceps muscles. The amount of work performed should be controlled to avoid the development of local muscle fatigue and subsequent changes in performance.