Seated cable row with palms-up grip
Emphasis: middle back, particularly the rhomboids
Sit on a rowing machine and lean forward to grasp the bar with an underhand grip that's slightly wider than shoulder-width.

 

Pull the handle toward your upper abs, bending your knees slightly to minimize the stress on your lower back. Pull your elbows back and bring your shoulder blades together, then slowly return to the starting position and repeat. Do not lean back beyond a 90-degree angle.