A cardiac monitoring system includes cardiac sensing electrodes for external placement proximate to the patient to sense ECG signals of the patient; one or more accelerometers configured to generate patient activity data based on signals corresponding to changes in the patient's body position and movement; and a monitoring computer configured to correlate the ECG signals of the patient with the patient activity data generated from the one or more accelerometers to determine ECG signal contamination, analyze the ECG signals of the patient to extract at least heart rate data of the patient and detect a cardiac arrhythmia based at least in part on the heart rate data, and change a confidence in the detected cardiac arrhythmia based on the determined ECG signal contamination; wherein at least the patient activity data and the heart rate data are analyzed to determine a change in a physiological condition of the patient over time.