Is it a metaphor when a writer describes a character as having arms contorted into knots?

Solution 1:

Per the OED, knot is a very old word, attested from the year 1000 and coming to us from the Old English cnotta meaning a knot in the sense of the tied and intertwined piece of rope. 860 years later the word was applied to thickened nodules of tissue in both plants and animals, so we can talk about knots in trees and knotted muscles. At the time of this first use, you could say that the usage was metaphoric. The OED marks it "transferred," which I think is close to your choice, "figurative". So arm muscles, lips, and maybe the skin around the eyes can be "contorted into knots," and those knots are literal in the transferred meanings. This is different from the clearly metaphorical, for instance saying that "her eyebrows contorted into crawling caterpillars."

Solution 2:

First, let's consider the meaning of the word metaphor: a figure of speech in which a word or phrase is applied to an object or action to which it is not literally applicable.

Considering that parts of the human body cannot literally contort into knots, the phrase can be considered to be a metaphor.

In the case of your example, which I believe comes from Inside Out and Back Again, by Thanhha Lai, the term 'eyes contorted into knots' is used to describe the character's displeasure at having to welcome unwanted guests.