Aquest múscul està cansat.

Because múscul is a masculine singular noun, and aquest has to agree with it.

Catalan demonstratives change for gender and number:

• aquest = masculine singular
• aquesta = feminine singular
• aquests = masculine plural
• aquestes = feminine plural

So:

• aquest múscul
• but aquesta cama

Catalan has two main verbs for to be: ser and estar.

Here, estar is used because cansat describes a state or condition: the muscle is tired, not permanently defined that way.

A very useful rule for learners is:

• ser = identity, classification, essential characteristics
• estar = condition, state, location, many temporary situations

So està cansat is the normal way to say is tired.

Because adjectives in Catalan usually agree with the noun they describe.

Since múscul is masculine singular, the adjective also has to be masculine singular:

• múscul cansat
• cama cansada
• músculs cansats
• cames cansades

In this sentence, cansat matches múscul.

There is no missing article.

Aquest already works as the determiner, so you simply say:

• aquest múscul = this muscle

If you say:

• el múscul = the muscle

That changes the meaning.

And el aquest múscul is not standard Catalan.

They mainly show which syllable is stressed.

• múscul: stress is on the first syllable → MÚS-cul
• està: stress is on the last syllable → es-TÀ

Accent marks are very important in Catalan because they help with pronunciation and sometimes distinguish forms that would otherwise look similar.

A rough pronunciation guide is:

• Aquest → uh-KEST
• múscul → MOOS-kool
• està → es-TA
• cansat → kun-SAT or kan-SAT, depending on accent/variety

So the whole sentence is roughly:

uh-KEST MOOS-kool es-TA kun-SAT

That is only an approximation for English speakers, but it gives the basic stress pattern.

Yes. Catalan can use cansat/cansada for body parts that feel tired or fatigued.

For example:

• Tinc les cames cansades = My legs are tired
• Té els ulls cansats = He/She has tired eyes

So Aquest múscul està cansat sounds natural.

You would say:

Aquests músculs estan cansats.

Notice that several words change:

• aquest → aquests
• múscul → músculs
• està → estan
• cansat → cansats

Catalan shows agreement very clearly across the sentence.

You would normally use aquell:

Aquell múscul està cansat.

So the basic contrast is:

• aquest múscul = this muscle
• aquell múscul = that muscle

This is a useful pair to learn together.