Whoop tracking devices were originally developed for professional athletes, and can be worn on wrists, upper arms, ankles or, apparently, the latest model can be worn against your body in your underwear. Surgeons often like them because they comply with bare-below-the-elbow policies. They also have good app support.

Approx cost = £27pcm for 24 months, device included.
Further bands and body options available to buy separately.