Your back hurts because you are hyperextending at the top of each rep.
Screenshot_20250116-231736.jpg
Once your shoulders are in place, just stop. No extra credit on the deadlift.
Not sure if that's directed at me specifically, but in case it is - I am not an SSC or a paid coach, just a guy with thoughts, opinion, and experience who helps out this community, but there are some SSCs that do online work. You can look here: Coaching | Starting Strength .
I know that Maj. Broggi's company, The Strength Co. does online coaching. From his Twitter feed, I think he's getting ready for a military deployment, but he's not the only coach they have, so they would be worth asking: Coaching
– The Strength Co. .
I have had the privilege of visiting and training once with Phil Meggers at his gym, Testify Strength and Conditioning. They also list online coaching as a service they provide: Testify Strength & Conditioning - Train With Us — Testify Strength & Conditioning .
I have never walked away from an opportunity to get coaching from an SSC and not been very happy I did so.