If you want to engage as much of your back musculature as possible during your seated rows (or horizontal pulls in general), it's imperative you add some movement from the hip joint. I call this "toprock."

By applying a little sway with a neutral spine, you'll remain safe while keeping the emphasis away from the biceps and away from unwanted shoulder glide once the weight begins to get a little heavy.

Think about it: The "intro to training" textbooks says the form police will arrest you if you keep anything other than a completely rigid torso when doing rows, but applying a bit of momentum is perfectly acceptable and isn't "cheating." This is me doing what I consider to be a standard set of heavier seated rows:

As you can see, I'm maintaining good tension throughout the rep, and never is there any rounding of the lumbar (or thoracic) spine. And, as the video below shows, I believe this same principle should apply to all row variations, within reason:

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