Turtleflexing

The act of flexing one's muscles so hard that their head appears to retract inside of their body.

"Look at that bodybuilder showing off across the gym; you can barely see his face past all that muscle!"
"Yeah, it's a bit much. He spends half his time here turtleflexing in the mirror."

Turtleflexing

When someone flexes their bulging muscles and it causes their neck to sink inside of their torso.

He was turtleflexing so hard that his chin disappeared!