1. A system for determining an activity level recommendation for a patient, the system comprising: at least one computer system configured to: receive data describing a geometry of at least a portion of a patient's heart; identify a cardiac event and a blood flow characteristic threshold value associated with the cardiac event; determine a three-dimensional model representing the geometry of at least the portion of the patient's heart based on the received data; determine a physics-based model of blood flow through the three-dimensional model, the physics-based model including a resistance of blood flow at the patient's myocardium simulated at a selected level of physical activity of the patient; model the flow of blood through the three-dimensional model using the physics-based model of blood flow through the three-dimensional model simulated at the selected level of physical activity; calculate, from the modeled flow of blood through the three-dimensional model simulated at the selected level of physical activity, a value of a blood flow characteristic within the patient's heart at the selected level of physical activity; update the physics-based model of blood flow by iteratively reducing the resistance to blood flow at the patient's myocardium and recalculating the value of the blood flow characteristic through each updated physics-based model, until the recalculated value exceeds the blood flow characteristic threshold value associated with the cardiac event; determine, based on one or more stored patient activities classified according to intensity level, a classification of patient activity intensity level corresponding to the physics-based model at which the recalculated value exceeded the blood flow characteristic threshold value associated with the cardiac event; and generate an activity level recommendation based on the determined classification of the patient activity intensity level.