The goal of this project is to quantify the acute and chronic effects of exercise on hemodynamic conditions[unreadable] in the infrarenal aorta of human subjects with small AAA (AAA diameter equal to or > 3, equal to or < 5 cm). The subjects will be a subset of the patients in the standard therapy arm and exercise intervention arm described in Specific Aim 2 of Project IV: Evaluation of Exercise Therapy for Small AAA. We will test the hypotheses that: (i) Shape matters: Differences in shear and dynamic tensile forces acting on the vessel wall, resulting from differences in[unreadable] aneurysm shape, are predictive of AAA growth rate, (ii) Size matters: As AAA enlarge, adverse hemodynamic[unreadable] conditions (including regions of low mean wall shear stress and high particle residence time) are exacerbated[unreadable] under resting conditions, (iii) Structure and motion matter: Differences in wall thickness, tissue composition,[unreadable] cyclic wall motion, and fluid-solid interactions affect AAA enlargement, (iv) Exercise matters: Increased[unreadable] infrarenal blood flow resulting from acute lower limb exercise, eliminates regions of adverse hemodynamic[unreadable] conditions, dramatically increasing wall shear stress and reducing particle residence time in all subjects[unreadable] regardless of AAA shape or size, (v) Persistence matters: Regular exercise slows AAA progression affecting[unreadable] size and shape, and results in more favorable hemodynamics and vessel wall motion. We will test these[unreadable] hypotheses by quantifying hemodynamics and wall tensile stresses under resting and exercise conditions in[unreadable] the abdominal aorta of patients with small AAA randomized to chronic exercise therapy or standard therapy.[unreadable] Our specific aims are: (1) Quantify time-varying abdominal aortic anatomy in AAA patients, (2) Quantify[unreadable] abdominal aortic blood flow at rest and during dynamic exercise using a custom MR-compatible bike in a 0.5T[unreadable] open MRI, and (3) Develop and validate computational methods to model blood flow, pressure, and wall[unreadable] motion in "patient-specific" computational models of the abdominal aorta of patients with small AAA[unreadable] randomized to chronic exercise therapy or standard therapy.