The biomechanics of occupational shoulder injuries.
Karduna AR; Gracely E
Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, R03-OH-003869, 2004 Mar; :1-17
The first aim of this proposal was to determine if low load repetitive arm motion would lead to altered scapular motion patterns, thus providing a biomechanical basis for NIOSH's recent report. Since it would have be impractical to implement this type of protocol in an actual work environment to assess muscle impairments, the second aim of this proposal was to identify an index of muscle fatigue that will explain altered motion patterns with repetitive motion. It was believed that this would provide a basis for the development of a screening tool to assess impairment, analogous to that currently being investigated for the low back. Although NIOSH has identified a clear epidemiological link between repetitive arm motion and shoulder disorders in the workplace, there are few scientific data available regarding the biomechanics of this connection. This is surprising considering that occupational shoulder disorders have a direct medical cost of $4 billion dollars annually in the United States. There is evidence that repetitive motion is associated with muscle fatigue and abnormal shoulder motion, which in turn may lead to damage of the rotator cuff musculature. Subjects without a history of shoulder injury were recruited for this study. The first step was to collect baseline measurement of electromyographical activity (EMG) and kinematics during arm elevation. Subjects then went through a low force, high repetition fatigue protocol. Immediately afterwards, EMG and kinematic data collection was repeated. EMG data demonstrated that our protocol did result in muscle fatigue. Significant changes in scapular rotations' were noted following the fatigue protocol for all three rotations. For posterior tilting, there was a general linear trend found, with a significant decrease in motion at the beginning of elevation and a significant increase at the end of elevation. There was a significant increase in upward rotation at all elevation angles, with the maximal response noted in the mid range of motion. There was a significant increase in external rotation at 50 degrees of elevation and above, with a linear increase in the response as elevation angles increased. No statistically significant relationships were found between an EMG fatigue index and changes in scapular kinematics. When examining the relationship between the time until fatigue and these same kinematic changes, a significant relationship was found for upward rotation, but not for external rotation or posterior tilting. Although previous work had demonstrated that fatigue due to maximal force exertion resulting in alterations in scapular kinematics, this is the first study to demonstrate that low load activities can lead to similar results. These results may have important applications for workers, since the activity level in the present study was selected to correspond to actual work exposure levels. Since alterations in scapular kinematics have been shown to change the loading environment of the subacromial space, these results may further our understanding of the development of injuries to the rotator cuff in the work environment.
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