Learn Hardcore French: Avant, ses muscles étaient plus faibles.

Here Avant is an adverb of time meaning “previously” or “before (in time).” It’s not the preposition avant + de; it simply sets the scene in the past.

Why is it ses and not son or sa?

Ses is the third-person plural possessive adjective (“his” or “her”). Because muscles is plural, you always use ses regardless of the gender of the owner.

Why is muscles plural here?

In French, muscle usually refers to any individual muscle or the concept of muscle. When speaking of someone’s overall strength or physique, you pluralize it: ses muscles (their muscles).

Why is the verb étaient in the imparfait and not in the passé composé (ont été) or passé simple (furent)?

The imparfait describes an ongoing or habitual state in the past. Here we’re talking about how their muscles used to be weaker over a period of time, not a one-off event, so imparfait (ils étaient) is the natural choice.

Why is there no que after plus faibles for the comparative?

Normally a comparison goes plus… que…, but when the second element is contextually clear (here, “than they are now”), French often drops que. It’s elliptical: Avant, ses muscles étaient plus faibles (Before, their muscles were weaker [than now]).

Could you say moins forts instead of plus faibles?

Yes, moins forts (“less strong”) is a valid alternative. Plus faibles and moins forts are semantically equivalent here, though plus faibles is slightly more direct since faible is the negative form of fort.

Where does the adjective faibles go in relation to the noun?

Most descriptive adjectives follow the noun in French. Faibles is one of those that generally comes after: muscles faibles. It’d sound odd to say faibles muscles unless you want poetic or emphatic effect.

How do you pronounce faibles and muscles?

muscles: /mysl/ (the final -es is silent; the -cl gives a /kl/ sound)
faibles: /fɛbl/ (the -es is silent; the ai is /ɛ/)