Na macie do ćwiczeń rozciągam mięsień i wzmacniam staw.

Because na can require different cases depending on meaning.

Here it means on in the sense of location, so na takes the locative case:

• mata = mat
• na macie = on the mat

If na expressed movement onto something, you would usually use the accusative instead:

• kładę się na matę = I lie down onto the mat

So in your sentence, na macie shows where the action is happening.

Literally, it is:

• na = on
• macie = mat (locative form of mata)
• do ćwiczeń = for exercises / for exercise

So the whole phrase is literally on an exercise mat or on the mat for exercises.

In natural English, on an exercise mat is the best translation.

Do usually takes the genitive case, so ćwiczenia changes to ćwiczeń.

Breakdown:

• ćwiczenia = exercises
• do ćwiczeń = for exercises / for working out

This phrase works like a noun modifier:

• mata do ćwiczeń = exercise mat

This is a very common Polish pattern:

• kubek do kawy = cup for coffee
• buty do biegania = shoes for running
• sala do tańca = room for dancing

So do + genitive often means for or used for.

Because both verbs are in the 1st person singular present tense: I stretch, I strengthen.

• rozciągać = to stretch

• rozciągam = I stretch / I am stretching

• wzmacniać = to strengthen
• wzmacniam = I strengthen / I am strengthening

This -am ending is common for many verbs in the ja form, though not all verbs follow exactly the same pattern.

They can mean both.

Polish does not usually distinguish between:

• I stretch
• I am stretching

with different present-tense forms the way English does.

So:

• rozciągam mięsień can mean I stretch a muscle or I am stretching a muscle
• wzmacniam staw can mean I strengthen a joint or I am strengthening a joint

Context tells you whether it is:

• a general habit
• something happening right now

They are in the accusative case, because they are the direct objects of the verbs:

• rozciągam mięsień = I stretch a muscle
• wzmacniam staw = I strengthen a joint

However, both nouns happen to look the same as the nominative here.

That is because they are masculine inanimate nouns, and in the singular, masculine inanimate nouns usually have:

• nominative = accusative

So:

• mięsień = nominative / accusative singular
• staw = nominative / accusative singular

Because Polish has no articles.

There is no direct equivalent of English a/an or the in ordinary sentences.

So:

• mięsień can mean a muscle or the muscle
• staw can mean a joint or the joint

Which one is intended depends on context.

That is very normal in Polish, and learners need to get used to understanding definiteness from the situation rather than from articles.

Yes, they could be plural, but the singular is perfectly natural here.

The singular can mean:

• one specific muscle and one specific joint
• a general statement about this kind of exercise
• one body part at a time

If you wanted plurals, you could say something like:

• rozciągam mięśnie i wzmacniam stawy = I stretch muscles and strengthen joints

But the singular sounds very natural if the speaker is talking about a specific exercise or a typical action.

Yes. Polish word order is more flexible than English word order.

The basic version here is:

• Na macie do ćwiczeń rozciągam mięsień i wzmacniam staw.

This puts the location phrase first, giving it some emphasis or making it the setting.

You could also say:

• Rozciągam mięsień i wzmacniam staw na macie do ćwiczeń.

That is also grammatical, but the emphasis changes slightly. It sounds a bit more like the main point is the action, and the location is added afterward.

So the original sentence is natural, and fronting Na macie do ćwiczeń is a common way to structure the sentence.

They are imperfective verbs.

That matters because imperfective verbs are used for:

• ongoing actions
• repeated actions
• general statements
• present tense

So in this sentence, the imperfective aspect is exactly what you would expect.

If you wanted a perfective verb, it would usually describe a completed action, but perfective verbs do not normally have a true present tense meaning in Polish. Their present forms usually refer to the future.

So here:

• rozciągam
• wzmacniam

are the natural forms for I stretch / I am stretching.

The base form is mięsień.

It is a masculine noun, and its forms can feel a bit tricky because the stem changes in some cases. For example:

• mięsień = nominative singular
• mięśnia = genitive singular
• mięśniowi = dative singular
• mięśniem = instrumental singular
• mięśniu = locative singular
• mięśnie = nominative plural

So yes, it is one of those nouns where you should learn more than just the base form.

In your sentence, though, you only need to recognize that mięsień is the direct object and that its accusative singular looks the same as the nominative.

A rough English-friendly guide:

• Na macie do ćwiczeń ≈ nah MAH-chye doh CHVEE-cheny
• rozciągam ≈ ross-CHON-gam
• mięsień ≈ MYEN-shen
• wzmacniam ≈ vz-MATS-nyam
• staw ≈ stav

A few useful pronunciation notes:

• ci before a vowel often sounds like a soft ch sound
• ę is a nasal vowel; in real speech it often changes depending on the following sound
• wz at the start of wzmacniam can be hard for English speakers; the w sounds like English v
• ł is not in this sentence, but if you see it elsewhere, it sounds like English w

If you want a more exact pronunciation, the standard IPA for the whole sentence is approximately:

/na ˈmat͡ɕɛ dɔ t͡ɕˈfʲit͡ʂɛɲ rɔsˈt͡ɕɔŋɡam ˈmʲɛɕɛɲ i vzˈmat͡ɕɲam staf/

Yes, here it clearly means joint.

The word staw can mean:

• joint in anatomy
• pond in another context

But because the sentence talks about stretching and strengthening on an exercise mat, the anatomical meaning is the obvious one.

This is a good example of how context usually removes ambiguity in Polish.