Date of Award

Summer 2014

Document Type


Degree Name

Doctor of Philosophy (PhD)


Mechanical & Aerospace Engineering

Committee Director

Stacie I. Ringleb

Committee Director

Sebastian Y. Bawab

Committee Member

Joshua T. Weinhandl

Committee Member

Gene Hou


The anterior cruciate ligament (ACL) provides stability to the knee joint while performing activities such as a side step cut. Neuromuscular fatigue, a reduction in muscle force producing capabilities, alters lower extremity mechanics while performing a side step cut and may increase the risk of ACL injury, particularly in females. Musculoskeletal modeling allows for the measurement of muscle forces, which are difficult to measure in-vivo. Therefore, musculoskeletal modeling, may improve our understanding of the effects of neuromuscular fatigue on muscle force production and loading of the ACL. Therefore, the purpose of this study was to develop a musculoskeletal model which incorporated two analytical fatigue models by Tang et al. (2005) and Xia et al. (2008). These fatigue models were used to determine the effects of neuromuscular fatigue on muscle force production and ACL loading at various levels of fatigue (i.e. 10%, 25%, 50%, 75% and 90%) and were validated by comparing these results with experimental data. Six recreationally active females performed five anticipated side step cuts both before and after an isolated hamstrings fatigue protocol using the right lower extremity. Root mean square (RMS) differences were calculated between both fatigue models and the experimental hamstrings muscle force 1.91 N·kg-1 and 1.88 N·kg-1, for RMSTang and RMSXia, respectively. Despite similar RMS differences, the Xia et al. (2008) model was selected for analysis of fatigue as this model utilized general input parameters. The total quadriceps and hamstrings muscle forces demonstrated significant decreases (p0.05) due to fatigue. The limited number of participants in this study suggested an underpowered study and may help explain the lack of significance in various dependent variables including peak ACL loading. Using the model developed in this study can aid researchers in understanding the effects of fatigue on risk of ACL injury in order to develop better training programs in order to reduce the risk of injury.