Corrugator
On the first picture, a Triangle person is showing straining, with a frown on their face.
Next, they are shown with neutral face.
Then, they are straining again.
After that - neutral face. A voice comes from off-stagge: “What are you doing?”
Finally - they are straining again, pointing to the spot between their eyebrows, saying: “I’m exercising my corrugator!”
I was looking at an anatomy atlas, and I thought the corrugator is such a perfect name for a muscle! Now, when I see someone frown I think: “Ah, they are corrugating!”