Electrical energy is conveyed via an implanted tissue stimulation system into tissue of the patient over a period of time. Electrical parameter data (e.g., impedance data and/or field potential data) is measured based on the electrical energy conveyed into the tissue of the patient, whereby the electrical parameter data is modulated in response to the physical activity of the patient to generate a time-varying signal (e.g., an oscillating signal). The time-varying signal is analyzed, and the physical activity of the patient (e.g., the physical activity level of the patient or the physical events performed by the patient) is tracking during the time period based on the analyzed time-varying signal.