Primary objective: To evaluate in patients with HER2 + breast cancer, treated only by trastuzumab, the impact of three months individualized physical activity intervention (55 minutes, 3 times per week) on left ventricular ejection fraction (LVEF).~Secondary objectives: To evaluate the impact of physical activity intervention on body composition, muscle function, metabolic, hormonal and inflammatory responses, pain, fatigue and quality of life.~This study examines patients aged 18 to 85 years, diagnosed with early breast cancer with HER2 overexpression confirmed histologically and eligible to receive treatment with trastuzumab (adjuvant).~This study includes 3 assessments phases: baseline (T0), 3 months (T3) and 6 months (T6) for both arms.~The programme is organised as follows: Arm A standard oncologic care coupled with physical activity intervention (3 times / week)  for 3 months ; Arm B (control group) standard oncologic care.~Between T3 and T6, volontary physical activity level will follow by actimetry.
Purpose of the study is to examine the effects of 3 months of physical activity intervention on myocardial function (Left ventricular ejection fraction) in patients with HER2+ breast cancer