Is it ok to arch one's back when performing lat pulldowns?

Question

I am aware that one should avoid performing lat pulldowns with the bar behind the neck. (mirror)

Is it ok to arch one's back when performing lat pulldowns?

Example where the back is arched:

Example where the back is not arched:

Answer 1

Not only is it OK, it's something you SHOULD do.

Pull your shoulders down and back. It pushes your chest out, and engages your lats, which is the entire point of the exercise.

Answer 2

In my personal opinion, I think you get a better contraction when you arch your back and activate more back muscles. To add to this post, another big mistake is to start swinging the bar down when you go up in weight.