shabbos is about menucha…rest…all that lovely jazz…lifting weights is not. now you may be right that technically its not assur, but then again neither is leaving your TV on for shabbos. so if you really want to lift weights youre gonna do it anyway so stop wasting time asking people on teh CR for justification.
the only possible reason i can think of is if you require some kind of physical therapy which includes a daily regiment of weight lifting. besides, why the heck can you not just wait until motzaei shabbos?