A comprehensive study of static and dynamic stresses for symmetric and asymmetric lifting activities

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)


Industrial Engineering

First Committee Member

Shihab S. Asfour, Committee Chair


The main objectives of this study were: (1) to develop a generalized computerized three dimensional dynamic biomechanical model for manual materials handling activities. The model was used to determine the mechanical stresses imposed on the various joints of the human body, (2) to investigate the importance of including the dynamic stresses in the biomechanical analysis of lifting tasks by computing and comparing both the static and dynamic forces and moments at the shoulder, L5/S1 and knee joints, and (3) to investigate the effects of some task variables on the peak forces and moments at the shoulder, L5/S1 and knee joints for floor to table lifting tasks. The task variables investigated were: (i) weight of load (7, 14 and 21 Kg), (ii) angle of twist 0$\sp\circ$-symmetric and 90$\sp\circ$-asymmetric), (iii) lifting speed (fast and normal) and (iv) lifting technique (squat and stoop).A generalized computerized three dimensional dynamic biomechanical model was developed to calculate separately, the static and dynamic forces and moments acting at each major joint of the human body as well as the L5/S1. The L5/S1 is a disc in the spinal column between the fifth lumbar vertebra and the first sacral vertebra. Two different methods for solving the indeterminate structure at the feet were employed in the model. In the first method, a pre-defined ratio was used to distribute the loads at the hips. In the second method, a force platform was used to measure the reaction forces and moments at the right foot. Then using the equilibrium equations, the reactions at the left foot were computed.The results of this study indicate that the dynamic effect in lifting tasks is high and cannot be neglected. Dynamic forces (moments) were found to account for at least 42 percent of the total forces (moments) and in some instances were as high as three times the static forces (moments). Therefore, analyzing a dynamic task such as lifting, using static models is inappropriate. If static models have to be used, then the correction factors reported in this study could be used to account for the dynamic effects.The total stresses at the shoulder, L5/S1 and knee joint were significantly affected by the amount of load lifted. It has been observed that as the load increases, the stresses at the three joints also increase. The total stresses at the L5/S1 joint were also affected by the lifting technique utilized. The stoop technique resulted in higher values than the squat technique. No significant effect was found in this study from the two levels of the angle of twist or the lifting speed. The normal lifting speed for each subject was close to his fast lifting speed. This explains why lifting speed was not found to be significant in this study. (Abstract shortened by UMI.)


Engineering, Biomedical; Engineering, Industrial; Biophysics, General

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