Form questions on Jefferson curls

Hello, a couple form questions on the Jefferson curls:

1) Should you tense your quads or just keep the leg straight without forcing extra quad tension?

2) Is there a specific breathing pattern or can you just breathe freely?

Thanks a lot!

Hi Giulio, 

1)knees locked ensures a complete stretch of the posterior chain and makes it easier.

2)breath as much natural as possible. do not hold your breath during forward flexion, never.