A method or apparatus for monitoring human activity using an inertial sensor is described. The method includes monitoring accelerations, and identifying a current user activity from a plurality of user activities based on the accelerations. In one embodiment, each of the plurality of user activities is associated with one of a plurality of types of periodic human motions that are detectable by the portable electronic device, and wherein the identification of the current user activity is made based on detecting two or more instances of the periodic human motion associated with the user activity. The method further includes counting periodic human motions appropriate to the current user activity.