Browsing by Author "David B. Kaber, Committee Member"

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Biomechanical Analysis of Eccentric and Concentric Lifting Exertions
(2008-03-09) Shu, Yu; Simon M. Hsiang, Committee Member; Gregory D. Buckner, Committee Member; David B. Kaber, Committee Member; Gary A. Mirka, Committee Chair
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.
Effect of Wrist Splint Orthoses on Forearm Muscle Activity and Upper Extremity Kinematics
(2005-02-22) Shu, Yu; Gary A. Mirka, Committee Chair; David A. Dickey, Committee Member; David B. Kaber, Committee Member
Ergonomics is concerned with understanding the interactions between humans and other elements in a system. Physical ergonomics is concerned with the prevention of musculoskeletal disorders (MSDs), - a topic of particular importance to industry because of the high costs of MSDS in worker' lost time, health care costs, worker' compensation, etc. Awkward postures have been identified as a risk factor in the development of several MSDs. In particular, awkward postures of the wrist are often the focus of intervention efforts. Wrist splint orthoses (WSOs) are intended to protect the wrist by limiting wrist motions and there have been reports of relief of wrist disorders such as carpal tunnel syndrome and tendonitis when the orthoses are worn at night. However, their use in work environments should be carefully considered because of the complex interaction of required wrist postures and the work environment/task. The focus of the current study was to evaluate the impact of wearing a wrist splint orthosis while performing tasks requiring deviated wrist postures. Ten subjects performed two experimental tasks. In the first experiment, the subjects performed a series of simple, single-plane wrist exertions at varied wrist angles (flexion/extension and ulnar deviations) with and without a wrist splint orthosis. Electromyographic (EMG) activity of three forearm muscles (flexor carpi radialis, flexor carpi ulnaris and extensor carpi ulnaris) were recorded as they performed these exertions. In the second experiment, upper extremity kinematics of the wrist, elbow, shoulder and torso were recorded as the subjects performed a simulated computer jumper installation task at varied work surface angles with and without a wrist splint orthosis. The results of the EMG experiment revealed a strong interaction between wrist angle and wearing a wrist splint orthosis. For example, at the neutral wrist posture, wearing a WSO did not elevate the normalized EMG of the studied muscles. At 48° of wrist flexion, wearing WSO increased the normalized EMG of flexor carpi radialis by nearly 600% (32% of max vs. 5% of max, p<0.001). Similar trends were seen in deviated postures in other planes. In the study of the upper extremity kinematics during the jumper installation experiment, the results showed a strong effect (p<0.001) of the wrist splint on the shoulder abduction angle. Wearing WSO increased the shoulder abduction angle significantly (average 43.5° vs. 32.3°, p<0.01) during the jumper experiment indicating that the subjects adapted to the limited range of motion of the wrist by increasing shoulder movement. The results of this research provide important quantitative data relative to the recommendations of wrist splint utilization in the work environment to protect worker from occupational injuries and disorders. This study showed that wearing a wrist splint orthosis can increase the activity of the forearm muscles, thereby increasing exposure to another risk factor (force); and wearing the WSO can induce awkward postures in other parts of body, thereby increasing the risk of MSDs to other body regions. These results indicate that the practice of having a worker wear WSO during work activities should be carefully considered relative to task demands.
Viscoelastic Responses of the Lumbar spine during Prolonged Stooping
(2005-08-15) Shin, Gwanseob; Elizabeth G. Loboa, Committee Co-Chair; Gary A. Mirka, Committee Co-Chair; David B. Kaber, Committee Member; Peter L. Mente, Committee Member
There is considerable evidence that awkward postures of the low back are related to the incidence of low back disorders (LBDs). Specifically, the stooped or fully flexed posture maintained over a prolonged period of time has been known to lead to LBDs in many industrials tasks but the specific biomechanics/physiology of this link is not fully developed. This study combined empirical work with finite element analyses to explore this relationship. The empirical work focused on quantifying the time-dependent responses of the lumbar spine during a prolonged stooped posture by assessing the changes in the sagittal plane range of lumbar flexion and the electromyographic activity of the back extensor musculature in the isokinetic lifts during and after prolonged stooping. Ten healthy participants performed a regimen of a 10-minute stooping period followed by a 10-minute upright standing recovery period, with an isokinetic lift every 2.5 minutes. Results showed significant creep effects of the flexion angle and the increased activity of extensor muscles during stooping to compensate for the reduced extensor moment producing capability of the passive tissues. The 10-minute upright standing did not produce a full recovery of the lumbar spine tissues but a 30-second rest break in the middle of the stooping period moderated these viscoelastic responses. A three-dimensional finite element (FE) model of the lumbar spine was developed to predict the responses of the passive and active tissues of the low back during the prolonged stooping and recovery period. This model employed a nonlinear stress-strain relationship describing the viscoelastic material properties of individual components of the lumbar spine. The trunk flexion tasks that were performed in the in vivo empirical work were simulated in the FE model and the predicted results (range of motion, muscle activation levels, etc.) were compared with experimental results to validate the model. The predicted results by the FE model showed high correlation (R>0.9) with the in vivo experimental results, confirming the capability of the FE model as a potential tool for risk assessment of the prolonged stooping tasks. Results of the in vivo experiment suggested the importance of proper duty cycles in reducing LBD risks due to repetitive prolonged stooping in work-related tasks. The FE model of this study showed potential to simulate various prolonged stooped postures in occupational tasks and predict time-dependent stress/strain of individual spinal tissues. The data from these simulations can be used to design better work postures and duty cycles that can reduce the risks for LBDs, without sacrificing work productivity.