Sensor location on the body
The location of the OHRM on the body presents unique challenges that vary significantly by location. Most form factors today with OHRMs are in one of three places:
Ear – in audio earbuds
Arm – on armbands for either the upper or lower arm
Wrist – in smartwatches or activity trackers
It turns out that the wrist is one of the worst places for accurate PPG monitoring of heart rate because of the much higher optical noise created in that region (muscle, tendon, bone, etc.) and because of the high degree of variability in vascular structure and perfusion across the human race. The forearm is considerably better because of the higher density of blood vessels near the surface of the skin. However, the ear is by far the best location on the body for OHRM because it is essentially just cartilage and capillaries, which don’t move much even when the body is in vigorous motion, thereby drastically reducing the optical noise that must be filtered. In particular, a dense collection of arterioles exists between the anti-tragus and concha of the ear, enabling a higher PPG signal-to-noise in the ear versus other body locations. Altogether, both of these properties enable biometric earbuds to offer greater accuracy and reliability, especially when comparing form-factors for aggressive exercises, such as Cross-Fit exercises, as shown in the