DL Form Check
Reply With QuoteIt doesn't look like you're squeezing your back and setting it at all. Look at the round line your back makes before you pull the 5th rep. This should flatten out before you pull.
It looks like you might be trying to set your back but instead your hips are falling. You may need to practice the lower back control drills to make sure you can contract those muscles. (drop your dick down between your legs)
Also, tall chest trying to point your nipples forwards to the wall will get your upper back tight which will help set your lower back from the top down.
