Doctor of Philosophy (PhD)


Engineering Science (Interdepartmental Program)

Document Type



Musculoskeletal disorders account for 32 percent of work related injuries and illnesses. Extensive studies have been conducted to understand and explore the factors that affect the development of musculoskeletal disorders to provide effective control strategies. One of the most important factors that allow effective control strategies is the biomechanical factors. Among the biomechanical risk factors, exposures to repetitive static and vibratory activities are known to result in musculoskeletal disorders. A ligamento–muscular reflex activity exists in humans and animals to stabilize the knee, shoulder, elbow, ankle joints, and spine. This reflex activity decreases over time when static loading is applied, which leads to joint instability and decreased safety of the spine. However, the recovery process of this reflexive muscle activity to its original state is a complicated phenomenon and the research efforts in this area are limited to a few animal based experiments. Although the recovery process is modeled and known for animals, the process for human muscle recovery is still unknown. This study concentrates on the behavior and recovery of the human lumbar muscle when subjected to static loading. Ten males participated in this study where the electrical activity of their erector-spinae muscle for two different lumbar levels was recorded for twenty minutes of loading and three hours of recovery period to observe and model the muscle behavior. The behavior of recovery was modeled using a bi-exponential structure previously developed for animal experiments. However, the analysis of the EMG showed that the sitting task during rest periods created additional static loading on the lower back. The effects of this task were introduced to the model by adding a third exponential component referred to as the “daily activity” factor. Overall, the results support the previous findings about the behavior of the lower back muscles that were developed by animal experiments. However, the mathematical description of the phenomenon is modified by taking the daily activity factor into account. Understanding the time periods and phases for the recovery is essential, since a better understanding of the phenomenon can lead to optimal design of work/rest periods in occupational as well as sports activities.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Fereydoun Aghazadeh