Jefferson Curl, mostly felt in hamstrings?

I have been doing them with a 45lb barbell, which it looks like is used in the video (although that might not be too important)

 

I feel it a lot in my hamstrings, but not in my back really. Am I not curling properly, or is this normal since it is similar to a stretch motion?

The last time I did J-Curls (which was a while back) I felt a stretch throughout my whole posterior chain.  I followed a cue of lowering the bar vertebrae by vertebrae slowly and it seemed to work pretty well.

 

It really is a slow and very deliberate movement.

 

And your weight is spot on!

I have done this for a while, before the F1, for me it's a hamstring stretch.

I believe it is used as a hamstrings stretch since it is for manna.

Good deal. Thanks everyone

Primarily hamstring if you're doing it right.  Focus on rolling slowly vertebrae by vertebrae so as not to get too much leverage on the lower back.  It will stretch the whole posterior chain to an extent, but you really want to target the hamstrings.  I have a severe spondylolisthesis due to stress fractures in my lumbar spine, I have to be really careful doing Jefferson curls, but it just forces me to do it right!