The body both absorbs RF, meaning there has to be a safe absorption rate (SAR) and creates impedance with it. It also creates radio shadows. What’s more, larger individuals have more of this effect. At Fitbit there was a guy who I’ll refer to only by his first name — Tim — who was our first port of call for whether or not our prototypes were getting the job done, RF-wise. He was a very large human. (proportionally speaking — he was also very fit!)