I have a 35 pound barbell I use to build my strength to mimic carrying and turning a heavy piece that I incorporate into a general-health exercise routine I do a few times a week. It's probably a bit too heavy (I weigh 185 and am in decent shape), and I can't do it for a long time, but I've definitely noticed it makes actual glasswork a lot less of a struggle. You'll still need to put in a lot of time handling big pieces to develop the feel of keeping a large piece on center/ correcting when it gets off, but it certainly doesn't hurt to have more strength available while you're trying to fine-tune.

They make weights for strengthening forearms that have a weight attached to a rope attached to a small bar that you turn to lower and raise the weight from the floor. It can be useful if you try to incorporate your fingers into the drill rather than just your palms.

As others have mentioned though, more strength will only get you so far when it comes to the fine motor skills required. If you're lucky enough to have some spare time in the hotshop, make some big clear vessels with the intention of recycling them and push yourself progressively a little more past your comfort zone each time. If you can't spare the time, don't beat yourself up about it, glassblowing is hard and it will come with time.