Thanks!

This isn't a strict definition of reflexive verbs, but: when the verb is applied to the subject (so the action is done to the same object as has already been stated (stated here from the verb form later in the sentence)) a reflexive pronoun is required. (The reason this isn't a strict definition is because this isn't always true, but it's a useful definition and I often prefer useful to strict.)

Here si is the reflexive pronoun. In English, the equivalent is himself/herself/itself etc..

In English, he trains does not leave ambiguous who is being trained. We know it is the he who is getting trained. So in English, it isn't really reflexive. However, whether he is training himself, or whether he is being trained by someone else is ambiguous.

Anyway, adding himself/herself/itself doesn't actually break the English meaning. I think there is a strong case for including it in an alternative answer.

The only reason not to include it as an alternative answer, in my opinion, would be if the Italian Si allena leaves ambiguous who is doing the training. Not being a native speaker of Italian, I am unsure whether that is left ambiguous, so although I might think there is a strong case, I could be wrong. I very much think it's not ambiguous, but I don't know for sure.

Any native speakers want to comment?

I agree (and realise how stupid the question sounds) but from a translation perspective I just want to know if I am on the right track. From a pragmatic perspective it may not even matter as I too can only think of very obscure situations where it would train itself.

Believe it or not, but a year ago I took up Spanish on Duolingo entirely on my own, and later, after some practice with the native speakers, I passed a real Spanish exam and now I study Spanish in the university as one of the main subjects, and it seems pretty easy!

Thank you Duolingo!!!

I agree with Gregory. Duolingo's grammatical jargon is frequently different from the Italian textbooks I am using. It's often difficult to find the right reference, especially when it comes to all these crazy clitics(?) and other pronouns and two-letter articles and whatever. My textbooks have pages of them, all with overlapping semantic roles. Duolingo's German is much, much better.

Well, except that it's not. Supposedly, Duolingo teaches grammar (the are units devoted entirely to grammar!), and its model of teaching vocabulary is kind of broken (which is probably clear to anyone who used any reasonable spaced repetition system). Assume that there are 16 questions/words/sentences in some lesson, and I remember 8 of them very well, 4 of them not so well, and 4 of them I keep forgetting. Instead of concentrating on the 4 (or maybe 8) troublesome ones, Duolingo insists on me repeating things I know well, sometimes a few times. This is a gross inefficiency, not only because I'm wasting time - also because repeating things on the verge of forgetting and not too early leaves a stronger trace in memory. I'm continuing using it anyway, partly as an experiment, partly because I'm a completionist;) (and I didn't finish the built-in Italian lessons yet), but I have to say that I'm rather disappointed at this design flaw. So I cannot agree that it's the "best vocab trainer" - it's far from that. Its few advantages are the price (there are free SR systems out there, but the time overhead of typing in lots of vocabulary is substantial - OTOH, one might argue that this helps remembering, too, so it can be treated as a part of the learning process) and the gamey feeling (which appeals to game geeks like me).