heart muscle is subject to two kinds of stress: physiologic stress, i.e. exercise; and pathologic stress, i.e. disease related. likewise, the heart has two potential responses to either stress: cardiac hypertrophy, which is a normal, physiologic, adaptive growth; or cardiac remodeling, which is an abnormal, pathologic, maladaptive growth. upon being subjected to either stress, the heart "chooses" to turn on one of the responses and turn off the other. if it has chosen the abnormal path, i.e. remodeling, exercise can reverse this choice by turning off remodeling and turning on hypertrophy. the mechanism for reversing this choice is the microrna mir-222 in cardiac muscle cells, which exercise up-regulates via unknown myokines. mir-222 represses genes involved in fibrosis and cell-cycle control.