Most gyms are lit from directly overhead with cool fluorescents. That light fills every hollow, kills the shadow that shows a muscle's shape, and pushes skin slightly green. A physique that looks incredible in person photographs flat.
The kit
One hard source — a bare-bulb strobe or a small reflector — placed high and to the side, roughly 45 degrees off the athlete and above their eye line. That single light rebuilds the shadow that separates a shoulder from an arm, a quad from a hamstring.
Kill as much of the overhead as you can, or shoot in a corner where it falls off. Underexpose the room by a stop so your key does the work.
One hard light, high and to the side, beats four soft ones every time.
For competition-lean athletes we add a second hard light directly behind for a rim that traces the outline. For off-season or lifestyle work we drop it — the softer look reads as strength rather than stage.