Don't stretch beforehand, your muscles are cold and this could lead to injury. As for the straining feeling, not sure what you mean exactly. If you don't feel any strain you're not pushing yourself hard enough, but if it's an unusual feeling then you should get it checked out. Maybe your wrists aren't as strong as your biceps and the weight you're using is putting too much strain on them. It could also be that you're doing a lot of reps and you're feeling the burn from lactate buildup. Like I said if it feels out of the ordinary then don't do that exercise until you get the doc to look