Quote Originally Posted by JamesC View Post
Feels like a strain on the spinal erector.

I'm right handed. Mixed grip. Right hand over, left hand under.
A few things:
When warming up you should either switch grips every set OR keep the "palms facing you" grip until you hit the heavy sets.
The strain MAY have been caused by the weight traveling too far away from you, which MAY happen on the side with the "over hand" when using an "over hand+underhand" grip.
I would suggest, and this is simply my opinion, to use the underhand grip on your dominate side BUT I have heard for some the opposite works, so...