The heart rate sensor 305 is {HEAD ~ a robust blood volume pulse sensor} positioned about {TAIL ~ the center} of the forehead and a rechargeable or replaceable battery module 307 is located over one of the ears 
The heart rate sensor 305 is { HEAD ~ a robust blood volume pulse sensor } positioned about { TAIL ~ the center } of the forehead and a rechargeable or replaceable battery module 307 is located over one of the ears
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH