Free weights. Brave "the cave" and enjoy. Have a trainer show you if you haven't used them before.
Free weights make your body balance and shift and this uses and strengthens more of your muscles than just the one you think you're focusing on. Weight machines, on the other hand, isolate just one set of muscles. Your whole body isn't involved, so it doesn't get stronger.
Lift heavy. It's amazing how you'll feel.
~~ Synger ~~