Abstract Background Heart failure (HF) and atrial fibrillation (AF) are often concomitant and act in a vicious cycle. Atrial fibrillation is associated with greater functional limitations and increased morbidity and mortality in patients with HF. Moreover, AF associated with HF increases patients' physical inactivity, worsening their clinical condition, and prognosis. Exercise training is safe and has clear benefits in HF. However, these benefits have not been demonstrated when AF is associated with HF. Case summary We present the case of a 57-year-old man with permanent AF and HF with reduced ejection fraction, who underwent 12 weeks of exercise training that included cardiopulmonary exercise testing, neuromuscular sympathetic activity (NMSA), and muscle blood flow (MBF) before and after training. Discussion Exercise training was shown to have a potential benefit in reducing the activity of the sympathetic nerve and increasing muscle blood flow, as well as increasing VO2peak and decreasing the VE/VCO2 slope in a patient with AF associated with HF with reduced ejection fraction. These results may indicate favourable clinical implications in this group of patients.