Whether we’re aware of it or not, most of us have a slight difference in the length of our legs. In the majority of cases, this is relatively small – the average difference is about half a centimeter or so (and, in case you were wondering, the right leg is more often shorter than the left). Plenty of us go through life completely unaware of it, but for some people, the difference may be more noticeable.

Often, someone has no idea that their legs are different lengths until they visit a therapist with an injury (often in the back), and it is pointed out to them. Sometimes even very subtle differences can get the blame for a whole host of musculoskeletal aches and pains.

There are two kinds of leg length discrepancies that doctors or therapists may talk about. One is the anatomical leg length discrepancy, which is where the bones of the leg have simply grown differently on each side. It can include cases where there are deformities of the bones at the ankle or pelvis, or where someone has suffered an injury to the growth plate as a child, resulting in a lack of normal development.
The other type is known as a functional leg length discrepancy. This is when the bones are the same length when they are measured, but other factors – such as asymmetries in the pelvis or a scoliosis of the spine – create the appearance of a leg length discrepancy.

Will a difference in leg lengths give you back pain?

You may have heard that having legs of different lengths can cause pain or long term problems by “putting your back out of alignment.” Is this true, or is it a convenient story to sell people treatments that they might not need? Let’s take a look at what the evidence says.  

First of all, it turns out that identifying a leg length discrepancy isn’t always straightforward. For larger differences, measuring with a tape measure is pretty accurate, but below about 1 centimeter or so, it’s hard to be sure without X-ray or CT scan confirmation. If your therapist doesn’t have access to these scans, then it might be that the tiny leg length discrepancy that you (allegedly) need expensive orthotics to correct doesn’t even exist at all.

Let’s say, though, that you really do have a leg length discrepancy. How worried should you be? As so often is the case, the answer is that it depends. Correcting a leg length discrepancy by putting a heel lift in your shoe won’t necessarily give you a benefit, and could, in fact, make things worse rather than better. Conversely, researchers have found that giving someone an artificial leg length discrepancy by just sticking stuff to their feet actually has surprisingly little effect on how their lower back moves when they walk around. So it appears that you shouldn’t jump to blame your leg length discrepancy for your back pain without asking a few other questions first.

The first question is, how big is the difference? Various studies have looked at whether leg length discrepancies are associated with back pain. There is no hard and fast rule as to how much difference is significant, but most research has found that a difference of less than 2 centimeters doesn’t cause back pain or other clinically relevant symptoms in most people. The human body is extremely good at adapting to small asymmetries, especially when the difference has been there since childhood.
Size is not the only factor, though. How you use your body might determine whether a leg length discrepancy will affect you or not. One study appears to confirm the common-sense notion that it’s likely to be more relevant to people who spend a lot of time on their feet than those who don’t. For people who perform heavy and repeated loading, even smaller differences might become important.
It’s worth considering the history of the back pain, and what caused the leg length discrepancy in the first place. If your leg length discrepancy has been with you since childhood, but your back pain started last Saturday after you got drunk and fell down the stairs, your leg length is pretty unlikely to be a major factor. If, on the other hand, you had surgery on a badly broken leg that suddenly left you with one leg several inches shorter than the other, and you spend a lot of time at work carrying heavy boxes, then it’s a far more likely culprit.

Finally, back pain is not the only thing to think about. There have also been suggestions that leg length discrepancies may put people at greater risk of lower limb problems; for example, some studies have found a link between a leg length discrepancy and the development of knee or hip osteoarthritis. Interestingly, though, there’s no clear connection between which side had the shorter leg, and the side with the osteoarthritis. If there’s a biomechanical link here, it’s a complex one. Clearly there is much more research to be done. The jury is still out on that one.

If your leg length inequality is functional rather than structural, the evidence is even more thin on the ground. An apparent leg length discrepancy can be caused by a number of things, including pelvic torsion or hypertonicity of the back muscles. As a result, a functional leg length discrepancy may accompany back pain, but it's difficult to know whether it's a cause or a symptom of the underlying problem, and whether correcting it with orthotics will help. In some cases, it appears to, but the mechanisms behind this are still uncertain. Treatment targeted at the back muscles, on the other hand, may help to reduce both the back pain and the functional leg length discrepancy.  

The verdict? Partially true

A small (< 1cm) leg length discrepancy is unlikely to have much of an effect on your back. Larger differences may be related to back pain in some people, but this is less common than you might imagine. If it seems to be the case for you, then your therapist may suggest trying a heel lift (either temporarily, or longer term) to see if it eases your symptoms. There’s no foolproof way to tell for sure whether a leg length discrepancy is the cause of your problem, though, so it’s always important to review the treatment after a few weeks to see whether or not it is helping!