Building a Broader Back
The latissimus dorsi (aka The Lats) is responsible for extension, transverse extension, flexion, adduction and internal rotation actions of the shoulder joint.
The action of the lats draws the upper arm downward and backward and rotates it inward (as illustrated in the downstroke in swimming the 'Australian crawl'). In climbing it joins with the pectoral and abdominal muscles to pull the body upward. The two latissimus dorsi muscles also assist in respiration by raising the lower ribs.
The latissimus dorsi is the widest and most powerful muscle of the back. Properly trained its what gives bodybuilders the sought after V taper.
Kinesiology & Physiology
Generally speaking, all types of chins and pulldowns promote width, while most bent over rowing exercises give you thickness in the middle of your back.
One Arm Dumbbell Row
Place your left knee and left arm on a flat bench for support while you grasp a dumbbell in your right hand. Begin with your torso parallel to the ground and extend the dumbbell down and slightly forward; upon achieving full extension, start pulling the dumbbell up toward your hip. Make sure that your elbow is held close to your body as you lift the weight; your torso should remain in the parallel-to-the-ground posture throughout the movement. Do the desired number of reps, then switch sides.
One Arm Dumbbell Row Demonstration
Bent-Over Barbell Row
AKA Barbell Bent Rows
To work your rhomboids, posterior deltoids and trapezius, bend over and keep your back straight, holding barbell with a medium-to-wide grip. Slowly pull the weight up until it lightly touches your chest, then return to the starting position.
When you use large-diameter plates on the bar, you'll have to stand on a thick block of wood or a flat exercise bench during the exercise to keep the plates free of the floor in the bottom position of the movement. You can do variations of this from from narrow (hands touching) to as wide as the bar permits.
Bent-Over Barbell Row Demonstration
Seated Cable Row
Take hold of the handles, and sit with your feet braced against a crossbar for support knees slightly bent. Extend your arms, stretching completely forward from the waist and pull the handles to just below the rib cage. Be sure not to bring your torso beyond perpendicular to the ground. Keep the weight under control and slowly stretch forward once again.
Seated Cable Row Demonstration
To strengthen the latissimus dorsi muscles, take a wide grip on the lot pull-down bar and, while kneeling or sitting, draw it down until it touches the tops of your shoulders, either in front of your neck or behind. It's a great widener for the outer lats.