Po terapii manualnej napięcie mięśniowe jest mniejsze.

Because po normally requires the locative case when it means after in a time sequence.

So:

• basic form: terapia manualna
• after po: po terapii manualnej

Both words change because:

• terapia is a feminine noun, so in the locative singular it becomes terapii
• manualna must agree with terapia, so it becomes manualnej

So po terapii manualnej literally means after manual therapy.

It is locative singular.

Here is the breakdown:

• terapia → terapii
• manualna → manualnej

The locative is used after certain prepositions, and po is one of the most important ones.

Not always. Po can have several meanings depending on context, but one very common use is after, especially with events or actions.

In this sentence:

• po terapii manualnej = after manual therapy

In other contexts, po can also mean things like:

• along / on in some expressions
• by in expressions like po polsku = in Polish

So you should learn po together with its common patterns rather than as just one English word.

This is a noun + adjective combination:

• napięcie = tension
• mięśniowe = muscular / muscle-related

Together, napięcie mięśniowe means muscle tension.

Grammatically:

• napięcie is neuter singular
• mięśniowe must agree with it, so it is also neuter singular

That is why you see -e at the end of both mięśniowe and later mniejsze.

Polish often uses an adjective where English uses a noun as a modifier.

Compare:

• English: muscle tension
• Polish: napięcie mięśniowe = literally muscular tension

This is very common in Polish. Instead of stacking nouns the way English often does, Polish frequently uses adjectives derived from nouns.

Because mniejsze has to agree with the subject napięcie mięśniowe, which is neuter singular.

• mniejszy = masculine singular
• mniejsza = feminine singular
• mniejsze = neuter singular

Since napięcie is neuter, the correct form is:

• napięcie ... jest mniejsze

It is the comparative form of mały (small), but in this kind of sentence it often translates more naturally as lower or less rather than smaller.

So although mniejsze literally comes from smaller, in context:

• napięcie mięśniowe jest mniejsze = muscle tension is lower / less

Polish often uses mniejszy / mniejsze in places where English might prefer less or lower.

Because Polish does not have articles like English a / an / the.

So:

• napięcie mięśniowe can mean muscle tension, the muscle tension, or sometimes a muscle tension depending on context

You understand which meaning is intended from the situation, not from a separate article word.

Yes, in a normal full sentence like this, jest is the standard and natural choice.

• napięcie mięśniowe jest mniejsze = muscle tension is lower

In some styles, especially notes, headlines, or very informal speech, Polish may omit forms of to be, but in an ordinary neutral sentence, jest should be there.

Yes. Polish word order is more flexible than English because case endings show grammatical relationships.

The given version:

• Po terapii manualnej napięcie mięśniowe jest mniejsze.

is neutral and natural.

You could also say:

• Napięcie mięśniowe jest mniejsze po terapii manualnej.

This puts the main statement first and the time/context phrase later.

Both are correct, but the emphasis changes slightly:

• starting with Po terapii manualnej highlights the time/context
• starting with Napięcie mięśniowe highlights the subject first

Because in this sentence the subject is in the nominative case, which is the dictionary/basic form for many nouns.

Here:

• napięcie is nominative singular
• mięśniowe agrees with it in nominative singular neuter

Since Polish uses case endings, learners often expect every case to look very different, but sometimes the nominative form simply looks like the base word.

A useful clue is the ending -ęcie. Many Polish nouns ending in -e are neuter, and napięcie is one of them.

You can also see its gender from agreement:

• mięśniowe
• mniejsze

Both are neuter singular forms, so they confirm that napięcie is neuter.

By itself, it can be understood either way depending on context.

• po terapii manualnej could mean after manual therapy in a general sense
• it could also mean after the manual therapy / after a manual therapy session, if the situation makes that clear

Because Polish has no articles, context does that job. The sentence itself does not force only one interpretation.