In gay adult video, "muscle" points to body type. It covers men with visibly developed, trained physiques, from lean and defined to heavier bodybuilder builds. The word describes appearance only, so it says nothing about what someone does on camera or how they relate to a partner. It often sits alongside terms of a different kind, such as hairy, which is also about looks, or bareback, which refers to sex without a condom rather than to a physical trait. Keeping those two sorts of terms apart makes this page easier to read.