Therefore, the purpose of this study is to evaluate, in patients with HER2 positive breast cancer and treated exclusively by trastuzumab, the impact on cardiac function (as assessed by LVEF) of an individualized, intermittent aerobic exercise training regimen (55 min, 3 times a week), comprising both moderate and high intensity exercise, for a period of 3 months. This evidence concerns the gene ERBB2 and breast cancer.