When it comes to the bicep curl it's one of the most mis-performed exercises I see. But you may be thinking isn't it one of the standard exercises? Yes.
So here are the two biggest problems I typically see with the bicep curl.
1. Rocking of the body. I get it, you want to look like you are crushing it in the gym, but when your whole body starts rocking with the movement it's time to check any ego you may have and lower the weight. The reason is because when you start rocking your biceps are no longer doing the majority of the work. Instead the momentum of the movement is carrying you.
2. The second most incorrect action on the bicep curl is not lowering the bar (or dumbbells) all the way down before curling. Full contraction of the muscle is going to work it far more than half curls.