Heart rate sensing during exercise by means of photoplethysmography

Monitoring heart rate (HR) during exercise can be used not only for early detection of a change in physical condition but also as an indication of the exercise training condition. Photoplethysmography (PPG) is a simple andnon-invasive methodfor HR sensing, but its use during exercise is difficult, since the PPG output is contaminatedwith motion artifact (MA). Especially for vigorous exercise, MA has the same frequency component as that of bloodvolume pulses (BVPs) in the PPG output, andno linear MA cancellation technique such as bandpass filtering (BPF) can work effectively. This paper proposes a PPG-basedMA canceling HR sensor. It is equipped with two sensors; one is a normal PPG sensor with a light emitting diode (LED)photo detector (PD) that contacts the skin to detect BVPs andMA, andthe other is an MA sensor with a LEDPD that does not contact the skin to detect only MA. Applying the outputs from the two sensors to an adaptive filter, the BVPs can be obtainedeasily. We evaluatedthe performance of the proposedHR sensor in ten subjects who performeda series of resting, walking, running andjumping. The results revealedthat the proposedHR sensor, together with an outlier rejection method, achieved root mean square errors (RMSEs) of 7.1 bpm, 6.4 bpm and6.1 bpm for walking, running andjumping, respectively, when using a Holter monitor as the reference. These results imply a reduction of RMSE by a maximum of 83％ comparedto no MA cancellation. © 2016, Japan Soc. of Med. Electronics and Biol. Engineering. All rights reserved.