A wearable medical device for detecting and treating life-threatening arrhythmias includes a wearable vest portion, sensing electrodes, therapy pads, at least one motion sensor, and a cardiac monitor. The cardiac monitor is configured to detect whether the patient is experiencing a treatable arrhythmia using an ECG signal and in response, determine a confidence level for the detected treatable arrhythmia using outputs from the at least one motion sensor. Determining the confidence level includes detecting patient motion prior to the detection of the treatable arrhythmia. Determining the confidence level further includes increasing the confidence level in response to determining that the patient is motionless coinciding with the detection of the treatable arrhythmia, or decreasing the confidence level in response to determining the patient motion after the detection of the treatable arrhythmia. The cardiac monitor is configured to determine whether to deliver a treatment shock based on the confidence level.