Methods and apparatus for monitoring a subject are described. A monitoring device configured to be attached to a body of a subject includes a sensor that is configured to detect and/or measure physiological information from the subject and at least one motion sensor configured to detect and/or measure subject motion information. The physiological sensor and motion sensor are in communication with a processor that is configured to receive and analyze signals produced by the physiological sensor and motion sensor. The processor is configured to process motion sensor signals to identify an activity characteristic of the subject. Once an activity characteristic is identified, the processor is configured to select a noise reference in response to identification of the activity characteristic of the subject, and then process physiological sensor signals using the noise reference to generate an output signal having reduced noise relative to the physiological sensor signal, to produce physiological information about the subject.
