.INTRODUCTIONSmartphones are being extensively used for activity recog-nition in recent studies, because they are carried by almosteveryone and are equipped with various onboard sensors, suchas an accelerometer and a gyroscope [1], [2]. Recently, wrist-worn devices, such as smartwatches, are coming into themarket with such on-board sensors [3], [4], which can be usedfor human activity recognition as well [5]. The combinationof these two devices provides us with richer information thatcan be used to detect various human activities