Seems simple enough: flexing your biceps to bring a weight as high as possible. That generally defines the arm-curling movements that you do on biceps day. But, it’s not that simple. If you try it yourself, you’ll find that your elbows pull forward from your sides, and that’s problematic.
First, just thinking that you need to take your bicep curls as high as possible to ensure a full range of motion is mistaken. Remember, curls are single-joint movements, and once your elbows pull forward from a position by your sides, you’ll suddenly start recruiting the shoulder joint (front delts, specifically), in addition to the elbow. You’ll also notice that in this endpoint position, your hands are now stacked directly above your elbows, meaning that there’s reduced tension – a nice little resting spot, if you will. So much for your peak contraction.
The fix is easy: simply lock your elbows by your sides for the duration of the set. Stop the upward motion of the positive rep when you can no longer maintain this arm position, which for most people is when the bar is just above shoulder level. Alternatively, shift the movement to cables. Even if your hands are stacked above your elbows, the line of pull is coming from the side (rather than directly downward), so you still can’t rest at the top.