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.
The bar placement in the Zercher squat makes it a lot easier to activate the core and control the low back and pelvis. A straight bar without chains will work too.