Systems and methods related to the field of cardiac resuscitation, and in particular to devices for assisting rescuers in performing cardio-pulmonary resuscitation (CPR) are described herein. In some aspects, an external defibrillator includes a light sensor, an accelerometer, and a computing unit connected to memory that stores computer instructions for determining a compression rate for the chest compressions and determining, based on the information from the light sensor and information about the compression rate, whether a rescuer is releasing the chest of a person during manual CPR chest compressions.