Biomechanical Analysis of Eccentric and Concentric Lifting Exertions

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Title: Biomechanical Analysis of Eccentric and Concentric Lifting Exertions
Author: Shu, Yu
Advisors: Simon M. Hsiang, Committee Member
Gregory D. Buckner, Committee Member
David B. Kaber, Committee Member
Gary A. Mirka, Committee Chair
Abstract: There is considerable evidence that low back injuries and disorders are related to heavy loads applied to the human spine. Biomechanical models have been invented to examine spinal load patterns under various conditions and to estimate the risk of low back injuries. One group of these models are called electromyographic (EMG) -assisted biomechanical models which use the EMG activities of the trunk muscles to predict muscle force and the spine reaction forces. Previous EMG-assisted biomechanical models have been used to study the spinal loads under various conditions: trunk flexion⁄extension, lateral bending and twisting exertions. One of the challenges facing these models is that they rely heavily on the active muscle force component. In certain kinds of exertions (eccentric exertions and exertions at or near the full flexion trunk postures) the passive components of the extensor mechanism play a significant role in the net extensor moment, and these are not captured in the traditional EMG-assisted modeling technique. This study introduces a new EMG assisted biomechanical model that includes passive components. Empirical experiments were conducted to evaluate the improvements in model predictions when these passive tissue components were considered. Eighteen subjects participated in two groups of experiments. In experiment one, subjects performed repetitive, eccentric and concentric lifting motions in a controlled dynamometer task environment. In experiment two, subjects performed a repetitive, free dynamic lifting and lowering exertions. In both experiments, the subjects were asked to reach their full trunk flexion posture during the lifting motion. As they performed these tasks, the EMG activity of the major trunk muscles was collected. Based on the degree of trunk flexion experienced by the subjects, the passive tissue forces were estimated through the use of a finite element model of the lumbar region. Estimates of the net internal extensor moment were derived from two different EMG-assisted biomechanical models — one that included these passive tissue forces and one that did not. These predicted internal extensor moments were then compared with the net external moment calculated from the combination of the static and dynamic moments involved in lifting the load and moving the mass of the torso and this comparison provided insight into the utility of the inclusion of these passive tissue forces. The results indicated the necessity of involving passive components in the EMG- assisted biomechanical model when studying the trunk flexion/extension exertions at full trunk flexion postures. The mean absolute error between the measured load and model predicted load was significantly smaller for the model with passive components as compared to the model without passive components (19.6 vs. 25.5 Nm in experiment one, and 19.4 vs. 54.9 Nm in experiment two, respectively). The R squared value of the measured and predicted load demonstrated great improvements by involving passive components (37% to 66% in experiment one, 12% to 75% in experiment two, respectively). In a second phase of this research, this new EMG-assisted model was used to study the differences in the biomechanical response between lowering (eccentric) and lifting (concentric) exertions. Eccentric exertions induced significantly (p<0.05) higher mean maximum spine compression forces in both experiments as compared to concentric exertions (3114 vs. 3680N in experiment one, and 1870 vs. 2516N in experiment two, respectively). The variability of the spinal load in these two types of exertions was also compared. These results showed that there was significantly (p<0.05) greater variability in the eccentric exertions than in concentric exertions. These differences were shown to be affected by the lifting⁄lowering velocity, knee posture and load levels. The measure chosen to characterize this variability was the average absolute deviation from the median (AADM) of the compression values (where the median refers to the median values of the multiple repetitions of the same task). This AADM of the maximum compression force was 281N for concentric versus 472N for eccentric exertions in experiment one, and 134N versus 207N in experiment two. These results indicate both the mean and variability of the spinal compression loads are greater in eccentric exertions than in concentric exertions. This result has significant meaning when considering the relative risk of lifting and lowering exertions in the workplace. The EMG-assisted model introduced in this study demonstrated an innovative method to quantitatively include the effects of the passive components of the spine into the model and showed the importance of involving these passive components in the estimation of the spinal load at the full flexed posture and eccentric exertions. The results of this study have also provided some insight into the relative risk of eccentric vs. concentric exertions by understanding the trade-offs between the active and passive tissues of the spine during eccentric exertions.
Date: 2008-03-09
Degree: PhD
Discipline: Industrial Engineering

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