Little nod to ACD canon today, because I couldn't not use this particular word!
A while after they first moved in together, John found himself wondering how exactly Sherlock kept up his toned, lithe physique. It wasn't that he was lazy, exactly, but it seemed like his entire fitness regimen involved abstaining from food and running around playing silly buggers. Neither of which would explain the toned forearms, the subtly defined pectorals, the thick but flexible neck John often found himself admiring in a detached sort of way.
Eventually, he started to notice a pattern. Once a week, unless a case had gotten in the way, Sherlock would disappear for a few hours and come back glistening with sweat and red in the face, and head directly for the shower. John resolved to follow him, to figure out what he was doing. He convinced himself it was strictly out of concern for Sherlock.
He ended up following Sherlock into a shady gym. He hid patiently in a corner, and found himself slightly breathless when Sherlock stepped out of the changing rooms, wearing nothing but a pair of worn silk trunks, his knuckles tightly wrapped with tape and a bit of padding. His torso was bare. John felt his heart pounding in his chest.
Before stepping into the ring, Sherlock smirked pointedly in John's direction.
"John, I had no idea you had any interest in boxing."