Structural balance in the human body, if perfectly developed, would allow all of your muscles to be proportionately developed. You’d have the perfect amount of flexibility. Your joints would move smoothly through a full range of motion.
Too bad we don’t live in a perfect world. The truth is that the stresses of daily life, our jobs, and sports cause imbalances in almost everyone. You’re a little tight here or a little weak there. You slouch in your chair at work or in the car. Maybe your hands hurt from typing all day, or maybe it’s your shoulder from doing bench press.
The good news is that you can train to fix those problems.