can I substitute sitting tricep dumbell extentions for lying barbell extentions? I've hit a plateu with the lying barbell extentions and want to mix it up, just wondering if sitting dumbell extentions would be a good way to go, or if theres another more effective excercise. I'm also doing pushdowns and I'm using all the weight on the machine, is there an excercise that would be equivalent to pushdowns using free weights?