Brystkassen beveger seg under innblåsningen.

Because Norwegian usually forms compound nouns as a single word.
Brystkassen is made from:

• bryst = chest / breast
• kasse = box

So brystkasse literally means something like chest box, but in normal English it means rib cage or chest depending on context.

This one-word spelling is very normal in Norwegian. English often uses separate words where Norwegian prefers compounds.

The ending -en makes the noun definite, like the in English.

• en brystkasse = a chest / a rib cage
• brystkassen = the chest / the rib cage

Norwegian often puts the definite article at the end of the noun instead of using a separate word like English the.

Because å bevege seg is a very common reflexive verb pattern in Norwegian, meaning to move.

• bevege can mean to move something
• bevege seg means to move oneself / to move

So in this sentence, the chest is moving, not moving something else:

• Brystkassen beveger seg = the chest moves

The word seg is the reflexive pronoun, similar to itself in English, although in natural English we usually just say moves, not moves itself.

Seg is a reflexive pronoun used for himself, herself, itself, themselves depending on the subject.

Examples:

• Han vasker seg. = He washes himself.
• Hun setter seg. = She sits down.
• Brystkassen beveger seg. = The chest moves.

So seg does not have one fixed English translation. Its job is grammatical: it shows that the action refers back to the subject.

Beveger is present tense.

The infinitive is å bevege seg = to move.
The present tense is beveger seg = moves / is moving.

In Norwegian, the present tense can often translate to either:

• moves
• is moving

depending on context.

Innblåsningen is a definite singular noun.

It comes from innblåsning, which means insufflation, blowing air in, or in a first-aid/CPR context, the rescue breath / the inflation depending on the exact context.

Breakdown:

• innblåsning = an insufflation / a breath blown in
• innblåsningen = the insufflation / the breath being delivered

Again, -en is the definite ending.

Because Norwegian usually marks definiteness by adding an ending to the noun.

So instead of saying a separate word like English the insufflation, Norwegian says:

• innblåsningen

The same thing happens with brystkassen.

Under is a preposition meaning during, under, or while depending on context.

In this sentence, under innblåsningen means during the insufflation or during the breath being given.

So it tells you when the chest moves.

Yes. This is standard Norwegian main-clause word order:

• Brystkassen = subject
• beveger = verb
• seg = reflexive pronoun
• under innblåsningen = prepositional phrase

So the pattern is basically:

Subject + verb + reflexive pronoun + time phrase

If you move the time phrase to the front, Norwegian usually requires inversion:

• Under innblåsningen beveger brystkassen seg.

Notice that the verb beveger comes before the subject brystkassen after the fronted phrase.

Yes, but the meaning is slightly less specific.

• brystet = the chest / the breast area
• brystkassen = the rib cage / chest cavity area

In a medical or first-aid context, brystkassen is often more precise, because it refers to the chest rising and falling physically.

So Brystet beveger seg is understandable, but Brystkassen beveger seg sounds more technical or medically appropriate.

A rough guide:

• beveger ≈ beh-VEH-ger
• seg in standard Bokmål speech is often pronounced close to sai or sei, though pronunciation can vary by dialect and speaking style

Norwegian pronunciation varies a lot by region, so learners will hear some differences. The most important thing is recognizing seg as the reflexive pronoun, even if it sounds a little different from speaker to speaker.

Yes.

Indefinite:

• en brystkasse = a chest / a rib cage
• en innblåsning = an insufflation / a rescue breath

Definite:

• brystkassen = the chest / the rib cage
• innblåsningen = the insufflation / the rescue breath

In your sentence, the definite forms are used because it refers to a specific chest and a specific breath/inflation in the situation being described.