Deadlifting and lower back pain

Ive developed an issue where when I pull heavy, I sometimes will strain one of the spinal erectors on my lower right side.

I think the reason is that I have terrible mobility in my hips and hammies and find it difficult to keep my lower back in extension at the bottom of the lift. My right hammie is not as flexible as my left.

Given lack of flexibility, when lift gets heavy, I round my back and pull the muscle.

Ive taken opportunity of the COVID downtime to work on flexibility and have been working my way back up since Ive been able to get back in the gym. So far so good.

Any other ideas on what could be going on here?

Are you pulling with a mixed grip? If so, would you mind posting a video of your deadlift?

Yes, mixed grip. Here is a video from today’s training

YouTube

Banged out a 5 rep pr today at 280. Felt a tiny twinge on the last two such that I didn’t attempt a 6th rep. But left gym feeling fine. Think it was because i aggravated it pressing on Sunday.

Just my non-professional 2 cents: you're dropping your hips, allowing the bar to roll so it's most likely ahead of midfoot, and better extension could be had by lowering the lumbars while raising the thoracic.