Work-Related Musculoskeletal Disorders in the Rat
ART Literature Review:
by Everett Johnson, DC, ART
Work-Related Musculoskeletal Disorders in the Rat.
A 2003 study published in Journal of Neurotrauma developed a rat model to look at trauma to the median nerve due to work-related musculoskeletal disorders. The model involves the rats being trained to perform repetitive reaching and grasping tasks that mimic work place demands. This model allowed the authors to examine a task performed and determine if there is related behavioral changes and tissue injury in the rat median nerve.
39 experimental rats were trained for 12 days to reach for food pellets through bars for 5 minutes per day. When they could perform this task they were transferred to another training station until they could reach in the feeding tube and grab a pellet for 20 minutes a day. When this was accomplished the project was started and the rats began their tasks of reaching for food pellets for 2 hours a day, 3 days a week for 12 weeks. The work day was broken up into .5 hour blocks that were separated by 1.5 hours of rest. The rats used their preferred limbs and some rats used both right and left limbs interchangeably.
The results of the study are quite remarkable. Behavioral and physiological changes were noted in the rats. Motor performance of the rats had declined after the first 5 weeks of the study, increased back to baseline by week 8 and then fell again for weeks 9 through 11. Duration of the tasks also declined, rose and declined again in a similar pattern over the 12 week period. The authors suggest the changes could be due to the number of animals that were sacrificed during the study to perform analysis at different stages of the study. They also noticed the animals developed different “scooping” and “raking” motions to get the pellets. From weeks 1 to 7 there was an increase from 18 to 100 percent of rats that performed these alternative motions to accomplish the task.
The authors also noted increase in inflammatory chemicals and demyelination of the median nerve during the length of the study compared to rats in the control group. Fibrosis was observed under microscope on occasion during the course of the study. The nerve conduction velocity of one of the 10 week animals could not be performed due to the development of a large connective tissue nodule around the median nerve in the carpal tunnel. Difference in nerve conduction velocity for the reaching limbs was significant. Control limbs measured average 52.4 m/s where the study
group reach limbs measured 47.7 m/s. The authors found a correlation between the nerve conduction velocity and the reach rate for the animals.
The results of this study demonstrates that significant changes are made in the median nerve in rats when trained to perform repetitive motions over a 12 week period of time. These findings suggest that high repetition, low force tasks may induce fibrotic and inflammatory changes in the tissues. Workers experiencing symptoms of a musculoskeletal disorder and continue to aggravate the condition without seeking care may exacerbate the changes in the nerves that have already occurred.
Clark B, Et Al. Median nerve trauma in a rat model of work-related musculoskeletal disorder. J Neurotrauma 2003; 20(7): 681-695