gymism

"Gymism" (noun): The involuntary urge or ritual of flexing one's muscles in front of any mirror, often accompanied by a moment of self-admiration or seeking visual confirmation of progress. Gymism is a common phenomenon among fitness enthusiasts who can't resist showcasing their hard-earned gains whenever faced with reflective surfaces.

Strolling through town, Sam Sulek spotted his reflection in a shop window and couldn't resist a spontaneous 'gymism,' flexing his muscles with a grin.