I was watching Star Trek: The Motion Picture yesterday and saw their uniforms (below): you will note there is a box of some description they wear around their waists. I was just wondering what is this box's purpose?
According to the Memory Alpha page on this particular uniform, that little black box they wear is:
a life support monitor belt buckle (non-canonically named the perscan device)
Out of canon, we learn more about the Perscan device
The perscan (or perscan device) was a medical device used by Starfleet in the 23rd century. It was a tiny scanner-transceiver which monitored the body's vital signs and transmitted the results to sickbay at all times, providing them with continuous updates on their physical status. In the uniforms introduced in the 2270s, the perscan was part of the belt ornament. (TOS novelization: Star Trek: The Motion Picture)
This fits in with McCoy's remark early in the film about the sickbay looking more like a computer lab than a medical facility.