One or more sensors sense patient heart rate and patient activity and provide related signals to a processor. The processor is responsive to sensor signals and obtains an activity threshold based on historical patient heart-rate signals and historical patient activity signals. The processor periodically derives a correlation value from current patient heart-rate signals and current patient activity signals and compares the correlation values to the activity threshold. A start of activity is noted when a correlation value exceeds the activity threshold; while an end of activity is noted when a correlation value fails to exceed the activity threshold. Activity is identified as exercise when correlation values continue to exceed the activity threshold for a predetermine amount of time, while an end of exercise is noted when correlation values fall below the activity threshold for a predetermined amount of time.