When training biceps don't think that you have to lift the bar or dumbbells all the way up to your chin. (I see this mistake all the time). By doing that you shift the elbows forward and the stress is taken off the biceps which defeats the purpose of the exercise. To keep the stress all the time on the biceps try to keep the elbows tucked in the side and without moving them rase the bar somewhere to the chest height. That's full rep of movement while maintaining constant stress on the biceps.