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.
Endurance training may make you leaner, but it'll eat up muscle, damage your cardiovascular system, elevate cortisol, put you in an acidic state, and might just kill you.