It can depend on your throwing technique. I play hooker and I find I use my core muscles, my lats as well as triceps and biceps. It's all about keeping a tight core. If you want to add length and power to your throw I strongly recommend 'leg ups'. I don't know what they are actually called but basically you lie flat on your back and lift your legs together a few inches off the ground and hold them in the air. If you are new to this try it for a minute on minute off. also leg raises, bicycles and side-to-sides are useful. Crunches and sit ups are not very useful I feel. As for the rest of the muscles you need to build explosive power so when you do any standard exersize (e.g. bench press, shoulder press, dips etc.) you should lower the weight you usually would lift and sort of 'explode' when you do the exercise. i.e. lift them with speed/power.
