Izmjenjujte uspravno i opušteno držanje tijela.

Here izmjenjujte means alternate, switch between, or keep changing from one to the other.

It comes from the verb izmjenjivati, which is an imperfective verb. In this kind of instruction, it suggests a repeated or ongoing action, not just a one-time change.

So the sentence is telling someone to keep alternating between two kinds of posture.

This is a question about aspect, which is very important in Croatian.

• izmjenjujte = imperfective imperative
  
  • suggests repetition, continuation, or habit
  • keep alternating
• izmijenite = perfective imperative
  
  • suggests a completed action, often one change
  • change/switch once

Since this sentence is giving advice about posture, Croatian uses the imperfective form because the idea is repeat this action over time.

Grammatically, izmjenjujte is the 2nd person plural imperative.

That means it can be used:

• when speaking to more than one person
• or as a formal/polite singular command to one person

So it can mean:

• you all, alternate...
• or please alternate... when addressing one person formally

The informal singular would be izmjenjuj.

Because they agree with držanje, which is a neuter singular noun.

Here they are functioning as adjectives, not adverbs:

• uspravno držanje = upright posture
• opušteno držanje = relaxed posture

Since držanje is neuter singular, the adjectives must also be in the neuter singular form:

• uspravno
• opušteno

They are adjectives here.

A learner might think they are adverbs because forms in -o often look adverbial. But in this sentence they describe the noun držanje, so they are adjectival.

Compare:

• Sjedite uspravno.
  Here uspravno is an adverb: Sit upright.
• uspravno držanje tijela
  Here uspravno is an adjective: upright body posture

So the surrounding structure tells you what the word is doing.

Držanje is in the accusative singular because it is the direct object of izmjenjujte.

However, for many neuter nouns in Croatian, the nominative and accusative singular look the same. So:

• nominative: držanje
• accusative: držanje

That is why the form does not change.

Tijela is genitive singular of tijelo (body).

The phrase držanje tijela literally means holding of the body, and more naturally body posture or simply posture.

This is a very common Croatian structure:

• noun + genitive noun

So:

• držanje tijela = posture of the body / body posture

Croatian often avoids repeating a noun when it is already clear.

So:

• uspravno i opušteno držanje tijela

is a shortened version of something like:

• uspravno držanje tijela i opušteno držanje tijela

In English, we often say alternate between upright and relaxed posture. Croatian can compress this idea more neatly and leave the noun phrase only once.

Because Croatian does not need one in this structure.

English often says:

• alternate between A and B

Croatian can simply use:

• izmjenjujte A i B

So i literally means and, but with a verb like izmjenjivati, the whole sentence naturally gives the meaning alternate between the two things.

Yes, it is a common expression.

Literally:

• držanje = holding, bearing
• tijela = of the body

Together:

• držanje tijela = body posture
• often just posture

It is a normal phrase in contexts like:

• health
• exercise
• ergonomics
• physiotherapy

Yes, Croatian word order is fairly flexible, but this version is the most natural for an instruction.

• Izmjenjujte uspravno i opušteno držanje tijela.

This sounds neutral and standard.

You could change the order for emphasis, but in ordinary advice or instructions, the original sentence is the best choice:

• verb first
• then the object phrase

The base dictionary forms would be:

• uspravno držanje tijela = upright body posture
• opušteno držanje tijela = relaxed body posture

If you strip it down to the head noun:

• držanje = posture / bearing / holding

And the full noun phrase is built by adding:

• an adjective before it
• a genitive noun after it

So the pattern is:

• [adjective] + držanje + [genitive noun]

For example:

• pravilno držanje tijela = correct posture
• loše držanje tijela = bad posture