Single Arm Cable Rear Delt Fly

The single arm cable rear delt fly trains the posterior deltoid against a cable rather than a dumbbell, and the difference is in the resistance curve. A dumbbell loads the rear delt only near the top of the arc, because gravity acts straight down and offers almost nothing when the arm hangs; a cable pulls along its own line, so the muscle works from the first degree of the movement to the last. Working one arm at a time lets the shoulder blade travel through its full range without the other side limiting it. The rear delt is small and easily overpowered, so the correct weight here is lighter than intuition suggests. If you need to swing the torso or bend the elbow to complete a repetition, larger muscles have taken over and the exercise has stopped doing its job.