I've been doing skullcrushers with dumbbells and I have been able to progress. I started with just 10lb to get used to it, then 15lb and now 20lb, but every time I do them my triceps feel really tight and like they are under a lot of pressure. I do not have this problem, when I do overhead dumb bell extensions. I am able to progress though, it just feels like there is a lot of pressure. Is there a warmup I should do?