Collapse and reformation of the medial longitudinal arch during gait is controlled passively and actively. If either tissue group fatigues over the duration of a run, the change in arch mechanics may increase risk of running injuries. However, a 3-dimensional kinematic analysis of the medial longitudinal arch after a prolonged run has not been performed. Additionally, rarely has arch collapse been quantified for walking and running in the same study. PURPOSE: To compare arch mechanics before and after a 45 minute run and to compare walking and running arch deformation. METHODS: Thirty runners performed barefoot walking and running trials before and after a 45 minute treadmill run. Reflective markers were placed on the foot and lower limb. Arch lengthening, navicular displacement, and arch height index quantified arch motion. Arch rigidity index and dynamic arch stiffness, a new measurement, quantified resistance to collapse. RESULTS: There was a significant gender y time interaction for arch rigidity index, decreasing after the run for men and increasing for women. There was no main effect for either time or gender for any other dependent variable. Walking and running, however, were significantly different for all relevant variables. Arch collapse was significantly greater for running than walking. CONCLUSION: The structures of the medial longitudinal arch of the foot may have adapted to the cyclical loading of the run by recruiting other muscles, or the arch may be resilient to change after a non-exhausting run. Greater arch deformation during running was likely a function of increased plantarflexion moment and ground reaction forces compared to walking.