you don't have to sit up perfectly but don't lean back too far because it becomes a cable row and not a lat pulldown. A little lean never hurt anybody. Also don't swing. I see people use the lean back just to get momentum to get more weight down.

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MONSTER: My Strength Endurance Journal, Part 2: The Strength Endurance Journal Returns <--NEW

BULKING: My 5X5 Journal

CUTTING: My CKD Max-OT Journal, My HST Cutting Journal