Traditional lat pulldowns are a great exercise for building muscle mass, but like all bilateral (two-armed) shoulder extension exercises, they overlook a basic truth of human movement: Our bodies are designed for reciprocal motion.
This is obvious when you think of lower body movements like walking and running. We're upright, bipedal animals that use reciprocal patterns of gait and extensible, elastic tissues to propel ourselves through space.
Our lats didn't evolve for us to do pull-ups or machine rows. We're built for action, specifically, climbing and running, both of which occur in an alternating pattern. Think climbing a ladder or sprinting from a hungry lion—with integrated contributions from the abs for side bending and twisting.
And here's a variation that uses weights on both sides, alternating sides for each rep.
These variations will not only help you build a muscular back, they'll help you move better, too.