A wearable defibrillator consists of a vest (or belt) which is worn by the patient. The device monitors the patient's ECG with sensing electrodes and can monitor other patient conditions and in appropriate cases can treat certain conditions. An accelerometer(s) in the wearable defibrillator can allow for the device to determine the position, movements, forces applied to the patient, and/or the device. The device can use a least one patient motion detector generating a signal indicative of patient activity. Analysis of the signal can be indicative of patient activity appropriate for treatment or indication of device condition.