Unfortunately, I have a very large muscle imbalance right now. In this case, it’s the result of my hamstring rupture and subsequent surgery. This is something I will be aiming to target as a primary part of my rehab.

Pretty logical — loss of muscle and strength in one leg, train it more to regain it.

This case fits very much in the “clinical” category. In the personal training & fitness world, most of the discussion surrounding “muscle imbalances” tends to be related to subclinical in nature.

I say subclinical, as there has not been any major injury or loss of function associated with the imbalance, and the aim of addressing it is generally based on a hypothesis.

Those hypotheses relate to the assumption that the human body should be symmetrical, and that deviation from this is going to lead to insult or injury. Unfortunately, or fortunately, most of these imbalances have little or no evidence to support their being a therapeutic target. 

You see, imbalances and asymmetry are normal. Many of us have variance in muscle strength, limb length, handedness, and more. I am right handed, and wouldn’t expect my left side to be as strong or well coordinated. Even Usain Bolt, arguably a decent athlete, has a large leg length discrepancy that results in variance between legs in force output. He seems to have performed well despite this.

In most cases, I don’t worry about clients having muscle imbalances, and don’t assume them to be part of their injury presentation. Where I am more concerned would be in the following cases:

• Large Deficit — a difference of 10% or so between sides (e.g. quad or hamstring strength) is relatively normal, but larger differences may present elevated risk of injury, particularly in sports where those muscles are already at risk.
• Functional Loss — where function is impaired by an imbalance, as can be the case in extremes of scoliosis or post-injury/operative muscle weakness. 
• Bodybuilding — in physique sports, symmetry is part of the grading criteria, and thus we need to seek this as a training outcome.

Most people do not fit into these categories. Most of us have small differences in strength, muscle insertions, appearance, and so on. This is the normal human form, and deviance from a non-existent ideal probably isn’t something you need to be too worried about correcting.