It's probably a sign of the historical importance of walking, running, and horse riding in Italy/Rome. A man who can walk well, a man "in legs" is a capable man.
I've always understood "in gamba" to correspond to our English expression, "with it," which can have many applications. WordReference also translates it as "on the ball" or "on top of things," and gives "In gamba!" as a way of saying "Take care!" "Capable" seems a rather boring way to translate it.
I also vote for WordReference to help get at the nuances of these expressions.
Maybe this idiom has something to do with the Hebrew connection between the word רגיל <rageel> (=used to something/capable of something), derived from the root ר.ג.ל (RGL), and the word רגל <Regel> (=leg) of the same root. So far i've encountered many similar phenomenons, where the Italian association of words was identical to the Hebrew equivalent...
And i agree with you; it looks like no one is interested on doing comments in this page. I think it's a sad situation for us, languages learners, not expressing our feelings. Maybe, we are afraid of making mistakes, not knowing that it is another way of learning. Greeetings!
If "He is a capable man" is what they wanted as an answer, they should have put "È un uomo capace." There are several different ways to translate this idiom, and then you get marked wrong if you use one of the other translations. That's way too rigid a method for teaching a language; I've noticed this a lot about DuoLingo. It's not a good way to learn idioms.
Words for body parts can be both remarkably stable, as can be seen in the words for 'nose' and 'heart', for example, and changeable. Slangish metaphors become, for example, the "regular" terms. Classical Latin crus (crura) 'leg' was replaced in Late Latin by gamba, borrowed from Greek kampe 'curvature'...In Japanese, gambaru means 'hang in there, do one's best'. But I remember gamba from the French form jambe, cf. il a des jambes.
This is definitely not what the Italian sentence means.
A male whose primary sexual interest centers on women's legs, sometimes in distinction to one who is interested in tits or asses.
He's a leg man; he prefers legs to tits.
Enough of the idioms. I am trying to learn to speak the language. Before I learn the idioms I just want to be able to understand the words I am speaking. At the end of the course, there should be a whole section on idioms, but not while we are still trying to learn the definitions of words and sentence structures.
If you are really trying to have conversations with people who actually speak the language, idioms are a vital part of the language. REALLY listen to your conversations some time - idioms such as these make up a pretty large portion of your speech. Some of them have become so fossilized you don't even realize that they started AS idioms (metaphors, largely).
I would avoid this kind of "made" sentences, as for Spaniards (and probably for other non native english speakers) are just impossible as we have no way to guess them.
Must be also said that after more than 350 lessons is like the second or third time that I come across one of those ;)