What specific exercise will increase my back strength?

There are numerous exercises that can improve back strength. Traditionally, exercises such as pull downs, rows and pull-ups can help in improving back strength. But it is more than just the exercise that will improve back strength. If you're not overloading the back muscles, there is no reason for them to get stronger. So you must gradually challenge the back muscles to they can adapt to the new stresses placed upon them and thus become stronger. So no matter what back exercise you choose, you can start by performing 1 set of 10 repetitions using a weight where the last couple of repetitions are challenging. As that becomes easier, you can increase the weight you are using and/or increase the volume from 1 set, to 2 sets, to 3 sets while gradually increasing the weight. This type of progression will continuously overload the back giving it something new to have to adapt to and thus, increase strength.

Here are 2 great exercises you can do to strengthen your back.  You can do these at home or in the gym.  If you are at home and you do not have dumbbells, you can easily use 2 soup cans to get you started. Start out doing a set of 10-12 and work your way up to doing 3 sets of 10-12. 

Standing Dumbbell Cobra: 2-Arm

Start

1.  Stand with your feet pointed straight ahead, draw in your navel and tuck your chin. 

2.  Bend at your waist until your chest is at a 45-degree angle to the ground. 

3.  Extend both of your arms and let them hang in front of your body with a dumbbell in each hand. 

Movement

4.  Keeping your elbows straight, bring your arms to the sides of your body in a circular pattern by pinching your shoulder blades together and down.  Hold and then return to the starting position. 

Bent-over Dumbbell Row: 2-Arm

Start

1.  Stand with your feet pointed straight ahead and draw in your navel. 

2.  Bend at your waist until your chest is at a 45 degree angle to the ground. 

3.  Extend your arms and let them hang in front of your body with a dumbbell in each hand. 

Movement

4.  Row the dumbbells, bringing your thumbs up toward your armpits. 

5.  Contract your shoulder blades together and be careful not to arch your back.  Return the dumbbells to the starting position and repeat.