This study will investigate the influence of systemic adjuvant/neoadjuvant therapy (SAT: chemotherapy +/- anti-HER2 antibodies (trastuzumab +/- pertuzumab) on heart function/rhythm and cardio-respiratory fitness in recently diagnosed breast cancer patients. In some patients, SAT damages the heart (so-called 'cardiotoxicity') and this can have a serious impact on the patient's quality of life and overall survival. It has also been suggested that anticancer therapies may lead to repolarization abnormalities, QT prolongation and autonomic dysfunction, clinically reflected by an increase in HR and a reduction in heart rate variability (HRV). There is a lack of information in the literature regarding the extent and time-course of changes in cardiac function, cardiac rhythm and cardio-respiratory performance ('fitness') in these patients. Moreover, the differential influences of specific treatment regimes (e.g. SAT or SAT plus radiotherapy) and different chemotherapy drugs on cardio-respiratory performance remain unclear. A better understanding of these issues is the primary aim of this study.