A method monitors cardiac arrest during physical exercise and consequently activates a rescue request. The method continuously measures a user's heart rate to determine a heart rate in a sequence of predetermined time intervals, by a heart rate measurement device worn by the user; then, generating a signal representative of the heart rate detected in each interval. A motion sensor detects a user's motion condition and generates a motion signal representative of the motion detected. A portable processor receives the heart rate signal and communicates with the heart rate measurement device and with the motion sensor; then processes the signal representative of heart rate, to recognize a possible cardiac arrest condition. The processor processes the motion signal to detect a possible stillness condition. The processor activates a rescue request if both a cardiac arrest condition and a stillness condition have been recognized. A system executes the method.