One of the safest and most effective ways to strengthen your back is to perform exercises on the floor. With most of your weight supported on the ground, you can target the muscles in your upper and middle back without putting any pressure on your lower back.
Lie face down on a padded, level surface or mat. Straighten your legs behind you with your ankles parallel to each other. Place your fingertips next to your rib cage and tuck your elbows in toward your body. Pin your shoulders back as you inhale and raise your chest and head away from the floor. Keep your elbows pointing back and your toes on the floor. Pause and hold this back arch for one to two breaths, then lower your chest.