Yoshiko Mikawa is the character from Assemble Insert. In the Hero Audition Finals are held to the public, but it's nothing more than 28 people randomly chosen off the street. The first contestant is Yoshiko Mikawa, a beautiful young woman who can bend iron bars with her bare hands. However, when she flexes, her face reddens and her muscles bulge out, so Hattori decides she's not fit to be an idol. After Yoshiko come dozens of lame contestant, and the audience slowly leaves.
Yoshiko gaining muscle mass when strenuously trying to bend a lead pipe. When the deed is done, she immediately returns to a more slender shape.