Almost every major muscle in your body has a twin.
Left pec, right pec; left quad, right quad; left triceps, right triceps; left lat, right lat; and so on. Thus, a muscle imbalance is a size and/or strength discrepancy between two matching muscle groups. For example, it’s common for people to have one arm or pec that’s larger than the other. Bodybuilders refer to this as 'asymmetry.'
Sometimes you can see these imbalances in the mirror, and sometimes you can’t, but you often notice them in your training (one limb is stronger than the other). For example, if one side of the bar tends to ascend faster than the other on your bench press, it may be due to one or more muscle imbalances on the trailing side.
HOW TO FIX IT?
THE POWER OF UNILATERAL EXERCISES
Start to correct imbalances by adding more unilateral exercises to your workout. Unilateral exercises allow you to isolate one side of your body from the other. Kinesis Strength machines, resistance bands, dumbbells, and single side cables are all pieces of equipment that will help you focus your attention on your weaker side.
The reason we want to work unilateral exercises into the plan, and avoid movements that allow both sides to work at once, is simple: the strong side will automatically take over. If you are doing bicep curls for example, and your left side is stronger than your right, then you will likely lift with greater assist from the left, instead of equally from both sides.