Il aime faire du sport pour renforcer ses muscles.

In French, many activities like sports are introduced with the verb faire, not jouer, except when you refer to specific ball games (e.g. jouer au football, jouer au tennis).

• faire du sport treats “sport” as a general, uncountable activity (“to do some sport/exercise”).
• You never say faire sport without an article, and jouer un sport is not idiomatic.

Here du = de + le is the partitive article, used to talk about an unspecified quantity of an uncountable or mass noun.

• sport is singular because it’s treated as an abstract, mass concept (“some sport/exercise”), not as individual countable items.
• You wouldn’t say faire des sports in everyday French; faire du sport covers the general idea of exercising.

When the subject of both verbs is the same, French uses pour + infinitive to express purpose (“in order to…”).
Example breakdown:

• Il aime faire du sport → main action
• pour renforcer ses muscles → why he does it

You cannot insert à or another preposition here; pour directly before the infinitive is the standard way to show intention.

Yes, you can. afin de + infinitive is more formal but has the same meaning (“in order to strengthen his muscles”).

• pour + infinitive = everyday, neutral
• afin de + infinitive = slightly more formal or written style

renforcer means “to strengthen” or “to make stronger.” Other verbs are possible but carry subtle differences:

• fortifier also means “to fortify,” often used for habits or defenses
• augmenter means “to increase,” more general and could apply to quantity rather than strength

Here, renforcer ses muscles is the most idiomatic way to say “to build/strengthen one’s muscles.”

No. In modern French, aimer is followed directly by an infinitive without à.

• Correct: Il aime faire…
• Incorrect/archaic: Il aime à faire…

If you want to use à, you need a different verb (e.g. commencer à faire, apprendre à faire).