The estimated yearly cost of lost-time work injuries and illnesses is $140 billion. The average cost of musculoskeletal disorders (MSDs) exceeds all other claims. These injuries persist in spite of ergonomic interventions addressing known risk factors. Work/rest ratios have not received a significant amount of attention, particularly in low back disorders, and it is hypothesized that the lack of adequate rest within a work cycle may contribute to muscle fatigue and ultimately injury. The aim of the current study is to determine the duty cycle and cycle time combinations that contribute to muscle fatigue and injury.

Stimulating and recording electrodes were surgically implanted into the medial longissimus muscle of a total of 58 Male Sprague-Dawley rats (400- 450g). These rats were separated into 4 work/rest groups as well as a 1 day and 3 day experiment. Fatigue, based on decreased M-wave amplitude and area throughout the 1-day experiment was greatest in the work/rest group consisting of the highest duty cycle and shortest cycle time. This group was significantly different (p<0.05) than the group exposed to the lowest duty cycle and longest cycle time. Fatigue due to increased M-wave duration was observed in the group with the highest duty cycle and longest cycle time. Higher duty cycles had the largest effect on fatigue over the duration of the experiment, while longer cycle times were implicated in fatigue from the beginning to the end of each cycle. Fatigue in the 3 day experiment occurred from the beginning to the end of each day as well as from the beginning to the end of each cycle. Comparison between days was not reliable due to potential obstruction of the M-wave signal due to tissue healing around the electrodes.

The assessment of injury was performed through histological, histochemical, and immunohistochemical stains. Neither muscle injury nor regeneration was detected through NADH, ED1, desmin, and vimentin stains. The H&E reaction revealed a small percentage of increased inflammatory cell activity compared to the sham rats for the 1-day experiment. The low duty cycle with long cycle time group had a significantly higher white blood cell count compared to the high duty cycle and long cycle time. The 3-day experiment resulted in an overall elevated white blood cell count, including the sham group, compared to the 1-day experiment. There was little correlation between the M-wave fatigue data and the histology injury data. Overall, results from this experiment provide insight into muscle fatigue and injury due to various work/rest ratios.