To strengthen your back—the most commonly injured part of the body—it's important to condition both the muscles in it and the ones that support it, notably the abs.
For the bird dog, start on your hands and knees. Tighten your abs and simultaneously lift your right arm and your left leg until they're in line with your back. Keep them straight as you hold for 15 seconds, then gently return to the start position. Do 10 reps, then switch sides and repeat.
Source: Medical Xpress