I did a search and can't really seem to find anything regarding this.
Today at the gym I was doing barbell preachers. There is something I have noticed when doing preachers that I think may be hindering the exercise. It seems as though that once your forearms break the vertical plane your biceps are no longer fighting the gravitational resistance and actually become relaxed. In a standard curl with a proper stance, even at full flexion your bi's are still fighting that gravitational pull.
I've never really thought about it too much, but does anyone else think that this hinders what they could actually be gaining from the exercise? Perhaps bring the bar up to the exact moment before the vertical plane would produce better results?