Athletic performance is an important factor to consider when designing athletic footwear. Improvements in agility drill performance can lead to improved sports performance. Changing footwear characteristics can be one way to help improve performance. The design and materialization of the upper of a shoe can impact biomechanical metrics and needs to be further explored during agility movements. Therefore, the first purpose of this study was to investigate the impact that stiffening the upper panel of an athletic shoe could have on agility performance using biomechanical outcome measures that correlate with athletic performance. Stiffening the upper panel significantly altered contact time during the lateral skater jump with the stiffest shoe having the shortest contact time compared to the other two shoe stiffness conditions (p=0.020-0.046). Shoe upper panel stiffness significantly changed peak concentric power in the anterior-posterior drill where the least stiff shoe produced the highest power compared to the stiffest shoe (p=0.006). The same dataset was used to address the second goal of this study which was to model the foot and ankle complex as a system of gears to understand force attenuation and propulsion during agility tasks. Although no differences were found between footwear conditions in gearing, it was shown that modeling the foot and ankle system through gearing can advance our understanding of the impact that footwear changes can have on performance and may be an important outcome measure in future footwear studies. / Master of Science / Athletic performance is an important factor to consider when designing athletic footwear. Through changes in shoe design, it is possible to improve agility drill performance. The design and materials used to construct the upper (top portion of the shoe) of the shoe can change the way an athlete completes a task and therefore, these measures need to be explored during several agility movements. The first goal of this study was to determine the impact of stiffening the upper portion of an athletic shoe on agility performance using outcome measures that are associated with improved athletic performance. Stiffening the upper portion of the shoe altered the contact time during the lateral skater jump with the stiffest shoe having the shortest contact time compared to the other two shoe conditions. Shoe upper panel stiffness also impacted peak concentric power in the anterior-posterior drill where the least stiff shoe produced the highest power compared to the stiffest shoe. The same dataset was used to model the foot and ankle as a system of gears in order to understand how an individual absorbs force and then utilizes force to move the body forward or to the side when completing agility tasks. Although no differences were found between footwear conditions when assessing gearing, it was shown that modeling the foot and ankle system in this way could aid in the understanding of how footwear changes can alter performance.