I would hazard a guess and say that a combination of bicep AND tricep exercises are good for arm wrestling power and strength. Our gym used to have a tricep press machine which you sat in and pressed the pads away from you, similar to a preacher curl machine but one that worked in the opposite way - hard to explain really. Nevertheless, it was good for triceps and also the foreams got a great workuot too depending on how you angled your arms. I would also say Hammer curls, EZ barbell curls, and any good old-fashioned heavy rowing exercises for back would also be productive.