In my blogging backlog, two underwear images from Daily Jocks, from the Echt Apparel company (of Australia):
The Lowe [‘Lion’] Stringer
The Equip Stringer
The linguistic path here starts with tank top, for a type of sleeveless t-shirt. From that, by truncation, tank ‘tank top’.
Then a particular type of tank, one with thin (sometimes string-like) shoulder straps (and, usually, a deeply scooped front): the stringer tank. From that, by truncation, stringer ‘stringer tank, stringer tank top’.
So: the stringers in #1 and #2.
Stringers are especially associated with bodybuilders. The two guys above are well bulked up, but the next two are out in the further regions of muscular development:
At the other end of the scale, we have a nicely muscled but slender young man in slim stringers (from the New Arrival company):