The spine erectors have the longest recovery time of all muscle groups. The deadlift and squats strain the low to a significant degree. And if you do something like front squats, stiff-legged deadlifts or walks by a farmer or … well, then you have the point. There is literally no reason to tackle the direct work in the lower back, such as hyperextensions and the like.
In fact, performing such work could be the real problem with the deadlift plateau you are experiencing. Therefore I suggest to squat and lift on the same training day during the week. This gives the lower back sufficient recovery time for the rest of the week.
Build your upper back, your lats and ham. Show me a guy with a big deadlift and I'll show you a guy who can chin and row. Sure, there are going to be some really fat guys who can pull on big and do no chins, but the average guy needs to build up strength throughout the upper back if he wants to lift big weights.
Which lifts should you use for your body? Lats and back? Choose those that allow a high level of progressive overload and that you really enjoy doing. It is up to you to find out these things through experience.
As for the thigh muscles, I went with the stiff leg lift (out of a big deficit) and the good morning during my powerlifting career.
Stiff Leg Deadlift from Deficit
I trained them with completely different modalities. I found that the stiff leg crews were something I could push the load on, but I kept the good mornings light and concentrated on the track.
No other lift built my rear chain like stiff leg crews from a 4-inch diameter deficit. From the upper back to erectors to the thigh muscles, this was my goal for the total chain destruction of the back chain. I ended up working on over 600 pounds for iterations.
On a good morning, I kept the load in the range of 185 to 225 pounds, even though my deadlift was consistently over 700. That's the difference in approaching the load if you decide to include these two lifts in your crosshull program.
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