Doctor of Philosophy (PhD)


Engineering Science (Interdepartmental Program)

Document Type



Much evidence suggests that the cause of lower back pain (LBP) and injury is frequently related to the posture of lifting, the load, muscle fatigue, and other factors. The purpose of this study was to evaluate the effect of progressive fatigue on factors that have previously been associated with increased risk of LBP in various occupational settings, during a repetitive lifting task where freestyle lifting technique was utilized. A laboratory experiment was conducted to evaluate several fatigue analysis, electromyography amplitude, kinematic, and kinetic parameters of repetitive freestyle lifting during a 2-hour lifting period. Each of ten (10) young adult male participants lifted a load from floor height to a lowering platform of 76cm height at a rate of 4 lifts/minute. The mass lifted was determined utilizing the psychophysical approach. The task consisted of 8 consecutive 15-minute periods of lifting, before, between, and after which subjective fatigue rating and strength measurements were taken, and during which kinematic variables were recorded. Effect of time, at α=0.05 level, was observed on subjective fatigue rating (p<0.0001) and on static strength (p=0.0184). Subjective fatigue rating increased over time, indicating that the participant "felt" increasingly fatigued as the experiment progressed. Static composite strength decreased an average of 20% from the beginning to the end of the experiment. Effect of lifting posture (semi-squat, semi-stoop, and stoop) was observed on peak trunk flexion angle (p=0.0122), trunk flexion angle at initiation of the lift (p=0.009), and knee angle at initiation of the lift (p=0.0007), indicating that in freestyle lifting, participants assume quantitatively different lifting techniques. A significant effect of the time-posture interaction was observed on the dynamic leg lift floor to knuckle height strength (0.0237), indicating that dynamic strength may change depending upon lifting posture selected. No generalizable effect of the independent variables was observed on the remaining parameters for all participants. Indicators of general physical fatigue, particularly dynamic floor to knuckle height leglift strength and subjective fatigue rating, were observed to possess some significant predictive capability in variation of a number of kinematic and force parameters.



Document Availability at the Time of Submission

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Committee Chair

Fereydoun Aghazadeh