אני נושם עמוק עוד פעם, ואחר כך מרגיש שהשריר בגב פחות כואב.

Yes. נושם and מרגיש are masculine singular present-tense forms.

So this sentence is being said by a male speaker:

• אני נושם = I breathe / I am breathing
• אני מרגיש = I feel

If the speaker were female, it would be:

• אני נושמת עמוק עוד פעם, ואחר כך מרגישה שהשריר בגב פחות כואב.

In Hebrew present tense, the verb often shows the speaker’s gender.

Hebrew present tense forms often look like what English speakers think of as participles, but they function as the normal present tense.

So אני נושם can mean:

• I breathe
• I am breathing

The exact English translation depends on context. Hebrew does not always make the same simple-vs.-continuous distinction that English does.

Here עמוק means deep / deeply, and in this kind of expression Hebrew commonly uses the masculine singular adjective form almost like an adverb.

So:

• לנשום עמוק = to breathe deeply / take a deep breath

Even though in English we would expect an adverb like deeply, Hebrew often uses an adjective form in this way.

This is a very common pattern in spoken and written Hebrew.

עוד פעם means again or one more time.

Literally:

• עוד = more / עוד
• פעם = time / פעם

So together:

• עוד פעם = another time, again

A very common synonym is שוב.

Examples:

• אני מנסה עוד פעם = I try again
• אני מנסה שוב = I try again

Both are natural, though עוד פעם often feels a bit more conversational.

ואחר כך means and then / and afterward / and after that.

Breakdown:

• ו = and
• אחר כך = afterward / then / after that

So the sentence structure is:

• אני נושם עמוק עוד פעם = I breathe deeply again
• ואחר כך = and then / afterward
• מרגיש... = feel...

Hebrew very often attaches ו directly to the next word.

Because the subject is still the same, Hebrew can leave it out.

So:

• אני נושם עמוק עוד פעם, ואחר כך מרגיש...

means:

• I breathe deeply again, and then [I] feel...

Repeating אני would also be possible:

• אני נושם עמוק עוד פעם, ואחר כך אני מרגיש...

But it is not necessary, and leaving it out sounds natural.

The ש is the Hebrew word that, attached directly to the following word.

So:

• ש + השריר = שהשריר
• literally: that the muscle...

In this sentence:

• מרגיש שהשריר בגב פחות כואב
• [I] feel that the muscle in the back hurts less

This attached ש is extremely common in Hebrew.

You will often see it in sentences like:

• אני חושב שזה נכון = I think that it’s correct
• אני יודע שהוא כאן = I know that he is here

השריר means the muscle, so it takes the definite article ה.

But בגב is built from:

• ב = in / at
• גב = back

When certain prepositions like ב, ל, and כ come before a definite noun, they often combine with ה. For example:

• ב + הבית becomes בבית = in the house

However, in this sentence בגב is being used more like in the back or in [my/the] back, where the context makes the reference clear. With body parts, Hebrew often relies on context instead of spelling everything out the way English does.

So השריר בגב is a natural way to say:

• the muscle in the back
• or in context, the muscle in my back

Hebrew often leaves possession understood from context, especially with body parts and personal experience.

So although English often says:

• the muscle in my back

Hebrew may simply say:

• השריר בגב

and the listener understands whose back is meant from the situation.

Hebrew can say בגב שלי if it wants to be more explicit:

• השריר בגב שלי = the muscle in my back

But in many contexts, that would sound less natural or unnecessarily explicit.

It behaves like a predicate adjective / participle-like form and is often translated into English with the verb hurt.

So:

• השריר כואב = the muscle hurts / the muscle is painful

In this sentence:

• השריר בגב פחות כואב = the muscle in the back hurts less / is less painful

For learners, it is often easiest to understand כואב here as hurts / painful.

You will see this word in expressions like:

• כואב לי הראש = my head hurts
• כואבת לי היד = my hand hurts

Notice that the form changes for gender:

• masculine singular: כואב
• feminine singular: כואבת

פחות כואב means less painful / hurts less.

This order is very natural because פחות directly modifies כואב.

• פחות כואב = less painful

You may also hear כואב פחות in speech, and it can be understandable and natural in some contexts. But פחות כואב is a very common and smooth way to express the idea.

So in this sentence:

• השריר בגב פחות כואב
• the muscle in my back hurts less

Because מרגיש here is followed by a whole clause, not a direct object noun.

Compare:

• אני מרגיש כאב = I feel pain
  Here כאב is a noun.

But:

• אני מרגיש שהשריר בגב פחות כואב = I feel that the muscle in my back hurts less
  Here everything after ש is a clause: that the muscle in the back hurts less.

You do not use את before an entire clause introduced by ש.

Yes, it is natural Hebrew.

It has a conversational flow:

• אני נושם עמוק עוד פעם = I breathe deeply / take another deep breath
• ואחר כך מרגיש = and then feel
• שהשריר בגב פחות כואב = that the muscle in my back hurts less

A speaker might also choose slightly different wording, for example:

• אני לוקח עוד נשימה עמוקה, ואחר כך מרגיש שהשריר בגב פחות כואב.
• אני נושם עמוק שוב, ואז מרגיש שהשריר בגב פחות כואב.

But the original sentence is perfectly normal.

You would change the present-tense verb forms to feminine singular:

אני נושמת עמוק עוד פעם, ואחר כך מרגישה שהשריר בגב פחות כואב.

Changes:

• נושם → נושמת
• מרגיש → מרגישה

Everything else stays the same.