The standard biceps curl is hardest at the mid-range, where the lever arm is the longest. But for variety we can change that, making the top or peak contraction the toughest part of the movement.
Lean forward at a 45-degree angle.
Don't drop the elbows down. Keep the upper arm in line with the body.
Perform curls in this position, squeezing hard at the top.
There's no such thing as isolation in training or in life. Everything you do, have done, and will do, affects everything else. Success is a synergistic – not additive – phenomenon.