I'm not sure how to punctuate this:
Their movements, quick and effortless; mine, lumbering and heavy.
Maybe I got it right. Any thoughts?
This is correct semantically (and quite nicely written if I do say so myself). If you are looking for an alternative to the semicolon, you might consider an em dash, e.g.:
Em dashes are technically less correct than a semicolon in this situation, but I feel they add a different flavor to the text. Good luck with your writing!
Is not a sentence. I think it context might have been something like this:
Either way, it's a fragment.