How to Know When Muscle Size Differences Are a Problem | Dr. Andy Galpin
In a Nutshell
Muscle size differences are often not real problems—they can result from posture, injury protection, or normal anatomical variation rather than actual morphological asymmetry. When true asymmetry exists, research shows that exceeding ~3.5% in lower body muscle size (or ~1% in women) significantly raises injury risk, while upper body tolerance is higher (shoulder strength differences of 15-20% are normal, but elbow/wrist should stay near 5%). Morphology measurements are prioritized over functional tests because they’re easier to standardize, and fat infiltration plus connective tissue quality become critical predictors of long-term mobility outcomes as we age.
These notes were generated by AI and may contain inaccuracies.
The first question you should ask yourself, if you notice a real difference in muscle size, is: Is this difference real at all? The classic example is the trapezius muscles, those muscles located at the back of your neck, between the neck and the shoulder blade. We see this very commonly. If you look at someone, you might see things like: "Wow, the right trapezius muscle is much smaller than the left one." So, if you go to Instagram, TikTok, or YouTube, you'll see tips saying: "Do more trapezius exercises or shrug your shoulders to the right side." You can do that until you feel extremely tired, and it probably won't change anything. Because in this case it is not actually bigger. It looks different because something else is injured. In other words, if you look at someone on camera, let's assume their shoulders are perfectly straight and parallel: if they had a neck injury, for example, and they raised their left shoulder slightly to protect that injury, that would make their left trapezius muscle look noticeably larger. But it is not. They are just shrugging. So, what you might see as differences in muscle mass is really just variations in posture or protection against injury. So, before you start an 8-week training program in which you increase the exercise volume by 20% on one side of your body, make sure there is a real morphological difference.
Another thing we should pay attention to, and which you will notice when people's bodies become very thin, is the presence of large asymmetry, especially in the back muscles (lats) and abdominal muscles. The abdominal muscles are the most visible. Almost no one has perfectly symmetrical six-pack abs. This is normal. Remember that your bones are not perfectly symmetrical. Therefore, the way your muscles start and end on your body will not be at the same distance. It will not have the same shape. They will look different simply because of structural and anatomical factors over which you have no control. Therefore, our first stop on this journey, to explain whether we are facing a real problem or not, is to first make sure that we are not seeing just differences in position, and secondly to make sure that it is not just an anatomical issue.
Other things we learned mostly came from general research on aging in terms of morphology, variations, and when these things become important. These are usually people who either lead sedentary lives or, at best, engage in minimal physical activity. We will see things like asymmetry in the muscle mass of the lower limb. When this asymmetry exceeds 3.5%, the injury rate really starts to rise. Specifically, one of the research papers found that this increases your risk of getting sick by up to 35%. This actually seems very logical and obvious. If you have a muscle that is larger than its counterpart, it will almost certainly lead to a movement disorder. Because the movement depends on skill, and this is not necessarily a big problem as much as the muscle itself being larger. Because skill can be corrected very quickly. But if one muscle is significantly larger or smaller than another, it means that something important may be happening.
Another similar story is here. One paper found that a 1% increase in asymmetry increases the risk of developing the condition by up to 83% in women. So you can see the point here, and I'm not going to get too bogged down in percentages like an 80% or 35% risk of infection. The point here is that these small imbalances of 1% or 3% make a substantial difference, what we call a difference of long-term clinical significance. Do you remember when you said 10% at the beginning? Well, here's some compelling evidence that when it comes to muscle size, unless you have a reason for it, and even then, we might argue that a 3.5% difference in size between symmetrical muscles, especially in the lower body, might be something you want to consider correcting rather than leaving it as it is.
From the aging research, what caught my attention most when reading many of those papers was the size, fat infiltration, and quality of connective tissue. These things can predict what are called career outcomes. In other words, I can look at how much fat is in your muscles if you are over fifty, and I can predict your physique. I can predict how fast you will walk. I can predict your flexibility and strength. For the third, fourth, or perhaps fifth time now, these fields are receiving considerable scientific attention right now because they are trying to help us understand what is really happening to muscles as we age. The fact that this component can directly explain and predict how you walk and how you move in the world is very compelling to me to pay attention to. We must also acknowledge that this formation, and that quality, accumulate in a way that is completely different from the function. As I said, I really would like to emphasize this point, since I can make a convincing argument, at least in my mind, that because aspects of function are so context-dependent, I think morphology might be a better area to focus on.
Remember what I said earlier about how the calculations differ, and so does the standardization, and whether the performance is fast, slow, or strong, and what test you are running, this makes standardization extremely difficult, and almost impractical. So, quite frankly, what we did, and here's our behind-the-scenes secret: we really did turn more towards the study of morphology. Because I know it doesn't tell us how the muscle works, but because it's much easier to standardize, bad data is worse than no data at all. Therefore, we have at least turned to obtaining high-quality data, albeit limited, by using techniques for imaging muscle size and quality. Because of this, when I see asymmetry in the shape, symmetry, or quality of the muscle, my threshold for taking action drops significantly.
I mentioned earlier that there is some evidence that 3.5% is a low percentage. If you see a 3.5% difference in your strength, I don't care at all. But from the perspective of muscle size, that may be important.
There is also a great study that has been published that I wanted to share with you. It was just released, I think in February of last year. It was a comprehensive review and meta-analysis looking at upper body asymmetry, and it caught my attention because most of the research done here, as I mentioned, focuses on the lower body. But it looked at nearly 90 studies. What distinguishes it is, and I will publish this research paper. I believe it is available to everyone, so you can download this chart and see for yourselves. But what is really interesting is that they looked at a different set of tests by joint and movement, and produced amazing graphs about the level of asymmetry in different parts of the body.
What caught my attention is that, in general, in places like the shoulder, it is very common to have a difference of 15 to 20% in strength between your right shoulder and your left shoulder, for example. Anatomy enthusiasts and motion buffs will recognize this term, but it encompassed everything from internal to external rotation. I looked at adduction, abduction, flexion, extension and many other different movement patterns in the shoulders, and all the studies combined concluded that there are common variations ranging from 15 to 20%. But when looking at other parts such as the elbow and wrist, those percentages dropped significantly, usually to around 5%. So, if you're really going to delve into this area, whether you're dealing with a shoulder or elbow problem or working on strengthening these areas, I think it's wise to pay attention and say, "Well, the limit for action in the shoulder is probably higher." Perhaps if the percentage is 10% or less, I can ignore it. But if the percentage is 10% or less in the attachment, I cannot ignore that. Because this is more significant, it is natural that there should be an almost perfect match between your right elbow and your left elbow. Your wrist, forearm, and fingers should be very symmetrical, but it may simply be that our shoulders are not designed to be that symmetrical. Therefore, I might call that more "normal."
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