Learn Finnish: Hän treenaa säännöllisesti, jotta selkä vahvistuu. - He trains regularly so that his back gets stronger.

Jotta specifically means 'in order that' or 'so that', showing the purpose behind an action. If you used että (that), it would just introduce a neutral fact rather than explaining why he trains.

Is the comma before jotta mandatory?

Yes. In Finnish, subordinate clauses are almost always separated from the main sentence by a comma. Since jotta introduces a subordinate clause, the comma is required.

Why does the sentence just say selkä instead of hänen selkänsä (his back)?

When talking about body parts, Finnish often leaves out the possessive words if the context makes it obvious. Since he is the one training, it is naturally understood that his own back is the one getting stronger.

How does the verb vahvistuu relate to the word for strong (vahva)?

The verb vahvistua (to get stronger) is built from the adjective vahva (strong). It is an automative verb, meaning the subject undergoes the change itself without needing a direct object. Compare it to vahvistaa, which means to actively strengthen something else.

How is the adverb säännöllisesti built?