I need help to lossen up ER Optimus Prime legs, before it breaks!

Took him of the shelf and wanted to transform him but for whatever reason his both legs are supper stiff and barely move. And since I do not want to break him(I saw few posts about his legs snapping off) and neither he doesn't have screw or ball joint I'm not sure how to do it. If someone already did it or has an idea what could work I would really appreciate it.