A portable electronic device include one or more electrodes for calculating distances and rotational angles between the device and the user is disclosed. Based on the calculated distances and rotational angles, a physical activity of the user can be determined. Additionally, the calculated distances and rotational angles can be used for compensation of optical artifacts in one or more signals detected by the device. User movement or physical activity can introduce optical artifacts, which can lead to erroneous determination of the one or more characteristics. The calculated distances and rotational angles can be used to reduce or remove the optical artifacts, leading to a more accurate determination of the one or more characteristics of the user.