Progressive Fatigue Effects on Manual Lifting Factors

The purpose of this study was to evaluate the effect of progressive fatigue on factors that previously have been associated with increased risk of low back pain in various occupational settings, during a repetitive lifting task where a freestyle lifting technique was used. A laboratory experiment was conducted to evaluate electromyography amplitude, kinematic, and kinetic parameters of repetitive freestyle lifting during a 2-hour lifting period. 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, trunk flexion angle at initiation of the lift, and knee angle at initiation of the lift 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, indicating that dynamic strength may change over time depending on lifting posture selected. (C) 2009 Wiley Periodicals, Inc.