Sentence:  He rubbed his hands together. "Believe it or not, there was a time when I considered giving acting a go. What do you think, Miss Fox?" He <WSD>flexed</WSD> impressive biceps. "Would I have had a chance against the Schwarzeneggers and the Chuck Norris types?" 
Meaning of the word marked with <WSD> tags:  To tighten the muscles for display of size or strength.