Ohio State University, Columbus, Ohio, 2003 Dec; :1-238
Recent reviews of the epidemiology literature have concluded that a number of work-related physical factors show consistent and positive associations with development of low back disorders. Among the workplace factors associated with back disorders are jobs involving manual materials handling activities, jobs involving frequent or prolonged bending of the torso, and jobs requiring frequent lifting of loads. A common theme underlying these workplace factors is that all involve situations where the tissues of the lumbosacral spine will be repeatedly subjected to elevated levels of compression and shear forces. It is known that repeated application of sizable loads to biological materials can and will result in the development of fatigue failure. However, relatively few studies have examined this failure mechanism using lumbosacral motion segments. None have attempted to simulate repeated loading of lumbosacral motion segments associated with lifting moderate weights in various angles of torso flexion. Accordingly, the purpose of this study was to carefully simulate spinal postures and loads associated with lifting a 9-kg weight in three torso flexion angles (0-degree, 22.5-degree, and 45-degree torso flexion). Twelve human cadaver spines (average age 81 + 8 years) each were dissected into three motion segments (L1-L2, L3-L4, and L5-S1), which were then randomly assigned to a loading condition representative of each of the three torso flexion angles. Care was taken to reproduce the posture of the motion segments to those observed in vivo via use of multiple radiographs during the fixation period. Tests were done in a humidified environmental chamber at a temperature of 37 degrees C. Creep loading was performed for 15 minutes to condition the specimen, and then cyclic loads at 0.33 Hz were imposed until failure or until 10,020 cycles were completed. Failure was taken as a displacement of 10 mm after the termination of the creep loading period. Simulated torso flexion angles had a dramatic impact on the number of cycles to failure of lumbosacral motion segments. Lifting the load in a simulated neutral trunk posture resulted in failure after an average of 8,253 cycles (+2,895), while the 22.5-degree torso flexion simulation resulted in an average of 3,257 cycles (+4,443), and motion segments experiencing the 45-degree condition averaged 263 (+646) cycles to failure. Torso flexion accounted for slightly over 50% of the total variance in fatigue life. Results suggest repetitive lifting in flexed torso postures may greatly increase the rapidity of fatigue failure in lumbosacral tissues.
NIOSH Pittsburgh Research Laboratory, P.O. Box 18070, Pittsburgh, PA 15236