| Objective:This research was to test the repetitive lifting capacity from floor to knuckle height of the youth, at the same time, monitor the muscle sEMG of back and lower limbs, and the biomechanical mechanism variation of body, create the simulation model that can calculate the changed force on the joints, discuss the mechanism kinematics and dynamics of repetitive lifting, analysis the influence factors of repetitive lifting together with the individual differences, identify the advantages and disadvantages of different lifting posture, optimize the lifting technique.Methods:A total of 40 male university students was tested their body shape, the isometric peak torque, isotonic power and endurance of the right lower limb extension muscles in hip, knee, ankle joint. BTE system and sEMG instrument, motion analysis system and force platform were synchronized, the subjects repeated twice the test of repetitive lifting capacity, tried to continue until fatigue; The lifting frequency was 12 times/min, wooden box(35cm×25cm×25cm) was 13kg, undertaken squat and semi-squat by turns,30min interval of time; and recorded sEMG, measured the resting heart rate, twice immediate heart rate after the test of repetitive lifting capacity and the discomfort self-ratings, the resting heart rate after 28min. Before and after the test of repetitive lifting capacity, the subjects hands separated 35cm, handle was about the height of knee, two feet separated equal to the width of shoulder; repeated two sets the maximum torso pull, each set was 3 times, continues 6s,15min interval of time, and recorded sEMG.Results:The test of repetitive lifting capacity indicated the parameters of squat better than semi-squat in total work output, power, lifting times and duration; Total work output as the dependent variable, the multiple regression analysis showed that the strength and muscular endurance of knee extensor muscle and the maximum torso pull were the conjunct positive independent variable for two lifting posture. IMDF change showed that the local muscle fatigue accumulation appeared at the upper back, lower back and thigh under the squat, the lower back and leg under the semi-squat. MDF and MNF change showed that the local muscle fatigue accumulation appeared at the lower back and thigh.Two lifting posture, The peak resultant force and pressure of trunk and limbs from large to small is lumbosacral-lower limbs-upper limbs-C7; The ankle, knee and S1 were the most obvious position for the peak shear force; the lumbosacral under the semi-squat and the lower limbs under the squat were the most obvious position for the peak unbalanced force, followed by the upper limbs and C7. The peak resultant force and pressure of trunk and limbs (except the wrist) under the semi-squat were less than under the squat; The peak unbalanced and shear force of limbs (except the wrist) under the semi-squat were less than under the squat, the lumbosacral under the semi-squat were larger than under the squat. Two lifting posture, the peak resultant force and pressure of the lumbosacral increased by turns from top to bottom, the peak unbalanced force decreased to some extent; the peak shear force decreased under the squat and increased under the semi-squat; And relatively safe limits, the peak shear force increased under the semi-squat was larger than the peak pressure force increased under the squat.Conclusion:The strength and muscular endurance of knee extensor muscle and the maximum torso pull were conjunct positive influencing factor for the repetitive lifting capacity under two lifting posture. Two lifting posture, the force of trunk and limbs were larger than upper limbs; the resultant force and pressure of body under the semi-squat were less than under the squat; the unbalanced and shear force of limbs under the semi-squat were less than under the squat, the lumbosacral under the semi-squat were larger than under the squat. Two lifting posture, the resultant force and pressure of lumbosacral increased by turns from top to bottom, the unbalanced force decreased to some extent; the shear force decreased under the squat and increased under the semi-squat. Compared with the semi-squat, squat is a predominant manual lifting technique.
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