Have you been told that you have a leg length discrepancy?

Many runners have, as it’s a fairly common phenomenon, affecting anywhere from 40-70% of the population, and is often blamed for causing running injuries.

A leg length discrepancy seems fairly easy to fix: just put a heel lift in on the shorter side. But is there any substance to this? And what’s more, is there any evidence that treating a leg length discrepancy with a heel lift actually results in a better outcome?

Today’s article will look at the scientific  research on leg length discrepancy , and as usual, things aren’t as simple as they may seem.

Scientific research on leg length discrepancy in runners

Measuring leg length discrepancy

The first issue at hand when discussing whether or not a particular runner has a leg length discrepancy is whether or not the leg lengths are being accurately measured. Sounds easy, but in practice, it’s anything but.

Most doctors and physical therapists use a measuring tape and bony prominences on the pelvis and the ankle to determine the absolute lengths of your legs. Then, subtracting the two, it can be determined whether or not there is a difference.

But research has consistently shown that this is an inaccurate and unreliable way to measure. In a 1991 review article by S.T. McCaw and B.T. Bates, the authors describe how tape-measured leg lengths can be off by up to half an inch! Another study cited showed that another manual measurement method was only within a quarter of an inch from the true value 25% of the time.

Leg length discrepancies, especially small ones, can only be accurately diagnosed with medical imaging. Both x-rays and CT scans have been demonstrated to be highly accurate.

So, the first lesson to take away from this research is that you can’t be sure of a modest leg length discrepancy if it was measured by hand.

Will your body compensate for having a leg length discrepancy?

But let’s say you do have a true leg length discrepancy—from a purely mechanical perspective, it would seem obvious that you’d be off-balance, just like a car with one wheel larger than the others. But the body is a reactive machine: it can adapt to changes in running surface, footwear, and muscle strength. What’s to say it can’t change in response to a leg length discrepancy?

Several studies have been conducted to examine the extent to which the body can effectively compensate for a leg length discrepancy.

In a 2002 review article by Burke Gurney of the University of New Mexico, data is cited that unsurprisingly show that there are some demonstrable asymmetries in walking and running gait in subjects with a true leg length discrepancy or an artificially induced one (like walking with one shoe on). But interestingly, when these leg length discrepancies are corrected with a heel lift, there does not seem to be a consistent benefit.

Gurney cites a few sets of (unfortunately unpublished) data from other researchers that appear to show that compensatory heel lifts in the shoes of runners do not seem to improve efficiency, at least as measured by oxygen consumption.

And in an older study of walking gait, impact forces actually increased after the subjects, who had leg length discrepancies ranging from about 5-20mm, were given a heel lift to correct their discrepancy. What’s more, this increase in impact forces persisted even after a three-week “break in period,” illustrating that the lifts may have been throwing the body off balance instead of stabilizing it.

Leg length discrepancies and running injuries

However, leg length discrepancies have been linked to a few injuries—possibly because of the aforementioned asymmetries in gait. Low back pain has been a heavily researched topic when it comes to leg length discrepancies, as it’s hypothesized that one of the ways the body compensates for a shortened limb is twisting or curving the spine.

But despite Gurney citing over a dozen studies on low back pain and leg length discrepancies, there’s still no clear answer: several have found a higher incidence of leg length differences in people with low back pain, while others, including a well-designed prospective study of 257 college athletes, have found no connection between the two.

Hip pain, however, is better connected with the presence of a leg length discrepancy.

One doctor showed that, of the 254 patients with a leg length difference at his clinic for hip pain, 226 had pain on the side with the longer leg.

Stress fractures have also been connected with leg length differences. In a study by Kim Bennell and coworkers at the University of Melbourne, a leg length discrepancy was twice as common in women with a stress fracture vs. those without. Interestingly, however, the location of the stress fracture was not related to which leg was longer. Other work has also connected leg length differences to stress fractures in military recruits.

Final notes on leg length discrepancy in runners

In all, I suspect that leg length differences are overdiagnosed and overcorrected—very few doctors or physical therapists use x-rays or CT scans to measure leg length discrepancies, which probably leads to people being given heel lifts they don’t really need.

• If your doctor or physical therapist suggests that you have a leg length discrepancy, see if they’re willing to confirm it with an imaging study. Burke Gurney’s review recommends that 20mm (just over 3/4 of an inch) is a good approximate breaking point for who’d benefit from a heel lift, but notes that this is only a guide.

• Additionally, like any orthotic intervention, runners will react differently to a heel lift. Right now, we don’t have any reliable method of determining whether a heel lift will throw you off balance or put you on an even keel.

• Do keep in mind the two conditions (hip pain and stress fractures) that have been reliably linked with leg length differences, however. For now, your best bet is to ensure that your leg length discrepancy truly exists, and if it does, give a heel lift a trial run. If it doesn’t fix your problems in a few weeks or so, you’ll likely have to look elsewhere for answers.

References

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