The Development and Evaluation of a Cumulative Exposure Integration Method Based on Fatigue Failure Theory

Abstract

Cumulative exposure to physical risk factors has been identified as one of the major contributors to the development of workplace musculoskeletal disorders (MSDs). Linear integration methods have been widely used to quantify cumulative spinal loading in the workplace. There is a fundamental assumption behind these methods: the impacts of force and time on the cumulative exposure measure are treated equally. However, evidence from cadaver studies on tendons, ligaments, cartilage, and spinal motion segments suggests a fatigue failure process in the development of MSDs. Under fatigue failure theory, the impact of force is exponentially higher than exposure time in the cumulative damage measure. The aim of this dissertation is to evaluate the linear integration assumption in estimating cumulative damage and to develop and evaluate a novel fatigue failure theory based estimation model for the low back using continuous exposure history.

Thirty male college students were recruited to perform eccentric exercises with the elbow flexors of the non-dominant arm under one of three loading conditions (high, medium, and low force) which all have the same area under the loading curve. Create kinase (CK), resting elbow angle and maximum isometric voluntary contraction (MIVC) were measured before, immediately after the exercise, and in three follow up days (2, 4, and 8 days after the exercise). The relaxed elbow angle and MIVC were significantly reduced in the high force group compared with the other groups. Create kinase results did not show statistical differences among the groups. The results of this experiment suggest that the linear integration method may underestimate the impact of high force loading in estimating cumulative muscle damage.

A fatigue failure based model for estimating low back cumulative damage was developed. In this model, damage for each repetition was estimated using an S-N curve, which was developed based on cadaver study data, and adjusted stress amplitude, which was calculated using rainflow counting and Goodman/Gerber methods. Cumulative damage was estimated by summing the damage from each repetition. Two epidemiology databases that contain exposure data and corresponding health outcomes for the low back were used to evaluate the proposed model. Comparisons with linear and squared integration methods were made. The fatigue failure based model performed best in both databases with the highest odds ratios and the highest number of significant results from the logistic regression tests when compared with the other two methods.

These results suggest that MSDs are potentially the result of cumulative trauma. The proposed fatigue failure theory based model's estimate of cumulative damage was highly associated with negative health outcomes.