A physiotherapist from the mid-20th century, Dr. Vladimir Janda discovered both the upper and lower-crossed syndromes. The upper-crossed syndrome is a muscle imbalance in which the upper back is weak and the chest is too tight, pulling the shoulders and head forwards. The lower-crossed syndrome occurs because the lower back muscles are too tight and the abdominals are too weak.
To correct the imbalance between the lower back muscles and the abdominals you should focus on a couple exercises that isolate and strengthen the abs.
Upper Back Weakness
Your upper back muscles tend to get long and loose and need to be specifically targeted by rowing motions, where you pull weight towards you. The key in these movements is to pull your shoulder blades back and together, sticking your chest out. That small movement makes the upper back muscles work the hardest.
Left vs. Right Imbalance
Many people favor one side during day-to-day activities which can make one side of your back stronger than the other. To balance things out, try doing these exercises that let you work one side of your back at a time.