I. Introduction
Monitoring and assistive technologies for elderly people are becoming important since the world's population is aging. Continuous monitoring of the daily activity level can provide important clues to estimate the health status[2]. Smartphone, as wearable sensors connected to wide area network, is highly suitable device for the continuous telemonitoring of outdoor activities of elderly people. Recently, there are several studies to estimate the energy expenditure of physical activities using sensors embedded in smartphone [6], [7]. Most of them utilize machine learning approach to recognize human activity from motion sensors. However, it is still difficult to recognize complex human activity accurately from only the motion data of smartphone even in the recent studies[11]. One way of improving the accuracy of human activity recognition is to reduce the number of activities.