Strictly speaking, when playing basketball, what must a man wear in regards to covering his body? I had learned that a man has to cover from his breastbone to and including his upper thigh. I remember a rabbi showing this to me inside. I want to find the source, or to hear if I am mistaken.
If I'm remembering correctly, this would imply that a man can wear a sleeveless T-shirt ("muscle shirt") and shorts to play basketball. Am I right?
Anecdotal answer: when I was in Yeshiva in a Chareidi neighborhood of Jerusalem, I once went to go play basketball in a local open court, with a t-shirt (and I think shorts? I can't remember). At one point, I noticed 2 girls, one in her early teens and one pre-teen and the older girl was covering the eyes of the younger, and herself averting her gaze, as they walked past.
The next time I was in class, I asked my Rosh Yeshiva about this and he did basically state that men should cover themselves up to their elbows and knees, and even if not, one should respect the local attitudes.
See this answer on the halacha of how men should dress, in terms of tzniut.
When I asked a Rav about this he said the local custom is by far the strongest factor in determining this kind of modesty.
He said when people live on an Island and are accustomed to being shirtless or in a bathing suit or shorts all day, it may not be forbidden to play Volleyball or Basketball without a shirt.
But if the prevailing custom is to wear shirts, then one should do so. Modesty should prevent you from "standing out." If you really believe you "stood out" to those girls to the point that they had a hard time not looking at you, then you should consider dressing more modestly.
Note: I'm not going to source this answer anymore than what I've stated above. Feel free to downvote.
