* present atrial fibrillation * diagnosed with acute myocarditis * health conditions that prevent participating in the exercise intervention * patients who cannot undergo mr scans (e.g. kidney disease or metallic implants) * treatment with il-6 receptor antagonists (tocilizumab, kevzara) within the last month due to drug interference with the cardiopulmonary exercise adaptations.

* present atrial fibrillation * diagnosed with acute myocarditis * health conditions that prevent participating in the exercise intervention * patients who cannot undergo mr scans (e.g. kidney disease or metallic implants) * treatment with il-6 receptor antagonists (tocilizumab, kevzara) within the last month due to drug interference with the cardiopulmonary exercise adaptations.

- present atrial fibrillation - diagnosed with acute myocarditis - health conditions that prevent participating in the exercise intervention - patients who cannot undergo mr scans (e.g. kidney disease or metallic implants) - treatment with il-6 receptor antagonists (tocilizumab, kevzara) within the last month due to drug interference with the cardiopulmonary exercise adaptations.

- present atrial fibrillation - diagnosed with acute myocarditis - health conditions that prevent participating in the exercise intervention - patients who cannot undergo mr scans (e.g. kidney disease or metallic implants) - treatment with il-6 receptor antagonists (tocilizumab, kevzara) within the last month due to drug interference with the cardiopulmonary exercise adaptations.