Punctuation of this: Their movements, quick and effortless; mine, lumbering and heavy
Solution 1:
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.:
Their movements, quick and effortless—mine, lumbering and heavy
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!