In the opening song of the anime 「ダンベル何キロ持てる？」, the phrase 「キレてるよ！」 can be heard e.g. at 00:12 in the linked video. According to this website, it gets translated as "You're doing it!". However, according to jisho.org, the meaning should rather be "I'm angry" (or something like that). How do I make sense of the 「キレてるよ！」 in the song?
It's bodybuilders' slang. Japanese bodybuilding fans somehow use unique ways of praising someone's muscles, and キレてるよ is one of the most common. This キレてる describes a well-developed and "separated" muscle with little subcutaneous fat, such as six-pack abs. I think this is from カット, another term that refers to beautiful separation of muscles after losing subcutaneous fat.