In this passage from Swimming in the Dark, what does "arched back defined" mean?

Question

I would like to know what "Your arched back defined" means in the following sentences:

Afterwards you got up and switched on the radio, sitting on your haunches, turning the tune button. Your arched back defined, your ass resting on your heels. Your tan had faded, I realised, and so had mine. Finally you found a station, a piano concerto, maybe Mozart. You lit a cigarette and came back to bed, the smoke gently floating, caressing the air. I felt weightless, again like one of the leaflets I had released into the air. I closed my eyes.

• Tomasz Jedrowski, Swimming in the Dark, Chapter 5

In this novel which is set in the 1980's in Poland under the socialist regime, where homosexuality was socially unacceptable, the protagonist Ludwik (a university graduate) left Poland in 1981 to live in the United States of America. And he remembers what it was like back then in Poland, where he went to the flat of his lover Janusz. After lovemaking, Janusz got up and switched on the radio, with his "arched back defined."

In this part, I wonder what this boldfaced expression means. Does it mean that his muscles at the back are defined, because his back was arched...?

I am an English learner from South Korea, so thank you for your patience in advance as I may not know obvious things. I would very much appreciate your help. :)

Answer 1

One of the meanings of the verb define is

To demark sharply the outlines or limits of an area or concept.

In this context, Janusz's arched back (arched because he is sitting on his haunches) may be clearly outlined against the background.

In bodybuilding, definition can mean

The degree to which individual muscles are distinct on the body.

However, the muscles need to be thoroughly warmed up for good definition to be visible in the back. One muscle group that may be visible without warming up is the spinal erectors on both sides of the spine.