Influence of weight and frequency on thigh and lower-trunk motion during repetitive lifting employing stoop and squat techniques

Changes in kinematics as a function of lifting weight and frequency was investigated in sagittal symmetric repetitive lifting. For every lift cycle (lowering and lifting) the motion range between the upright position (0 °) and the maximum angular displacement of the thigh and lower-trunk body segments was recorded. Ten subjects performed five repetitive lifting bouts with different weight/frequency combinations, using both stoop and squat lifting techniques. In total, 6384 lifts were analysed. The lifting weight or frequency did not influence the motion ranges in stoop lifting. In squat lifting the weight lifted did not appear to have any influence on the motion ranges, while the thigh motion range was significantly smaller at lifting frequency of 20 lifts min−1 than at a frequency of 10. A significant gradual decrease in the thigh motion range and corresponding increase in the lower-trunk motion range were seen for a majority of the subjects during squat lifting at frequency 20. These changes suggest that quadriceps muscle strength is the limiting factor in repetitive squat lifting. Also the variation in motion ranges was greater in squat lifting than in stoop lifting.