The proposed study seeks to leverage acute exercise physiology as a tool to better understand multiple clinical and mechanistic issues in exercise oncology. An acute exercise bout causes functional changes such as increases in heart rate and stroke volume leading to increased cardiac output and increased systemic blood flow. These systemic effects also increase tumor blood flow (perfusion). Additionally, an increase in mean arterial pressure increases oxygen diffusion distance within the tumor. Further, acute exercise mobilizes NK cells, cytokine signaling, and a myriad of other molecular and cellular transducers of exercise. Our study of acute exercise physiology in breast cancer patients will address several significant knowledge gaps both clinically and mechanistically.