This is a disappointing article to write as I been going on about it for more than 15 yrs, have written about it before in several other places and so have a lot of other people, but the myth just won’t go away. The myth is that when there is a leg length difference, the foot (or more specifically the subtalar joint) will pronate more on the longer leg side to compensate for the difference in leg length (and by implication, allegedly increase the risk for overuse injury due to that overpronation).
The myth did not start with the good work of Bill Sanner, but this work was used to reinforce the myth. What Sanner et al showed was that pronation at the subtalar joint can shorten a limb by up to almost 1 centimeter. This implies that the body could pronate the foot more to even up the leg length difference. Foot pronation as a compensation for a leg length difference is widely taught in Podiatry schools, at podiatry meetings and in podiatry textbooks. It does not often seem to come up in orthopedic, physical therapy or chiropractic text books – I have long been intrigued and wonder why this is the case? (I have my theories on this, but will keep them to myself for now ☺)
I am not talking here about those who have asymmetrical foot pronation for other reasons. This can lead to a functional leg length difference. I am talking about a structural problem in one or more bones of the lower limb, so one limb is shorter than the other. The propaganda is that the foot will pronate more on the longer side to shorten the limb, which can make sense as Sanner’s work showed that this can occur for up to 1 cm. However, I never bought into that myth as my own clinical experience was that I used to see the foot on the shorter limb pronate more just as often as the longer limb. I used to have students come up to me and ask me why the patient they were assessing was pronating more on the shorter side when they were taught the opposite. They were confused and could not understand. I used to ask them why would it? What evidence is there for it? … there never was any!
Try this: Stand up; try and actively pronate one foot; note how much energy that requires. Now just stand with a slightly flexed knee; note how little energy that requires compared to actively pronating the foot. If you were the body and had a structural leg length difference, how would you want to compensate? Pronate the foot which requires energy or flex the knee more which require much less energy?
What does the evidence say? A number of studies have looked at this: Bloedel & Hauger who used digitized video during treadmill running of 12 subjects with a leg length difference and found no statistical differences between the short and long leg for the amounts of calcaneal inversion and eversion. They concluded that subtalar joint kinematics are not affected by leg length differences. Other similar studies reported the same. We even did one and compared the calcaneal angle, navicular drift and navicular drop during static stance of a dozen people with a structural leg length difference and found no differences between the short and long leg:
How does the body compensate for a structural leg length difference? According to the Bloedel & Hauger study, they flex the knee more on the long side (see the ‘Try this’ above!).
I do have another issue with compensations for a leg length difference that I will get into another day, but in the meantime, ponder this: When you are running, only one leg is on the ground at a time. If there is a structural leg length difference, how does the body even know the leg is short if only one leg in on the ground?
As always, I go where the evidence takes me until convinced otherwise. and the evidence says that foot pronation is NOT a compensation for a structural leg length difference.
Bloedel PK, & Hauger B (1995). The effects of limb length discrepancy on subtalar joint kinematics during running. The Journal of orthopaedic and sports physical therapy, 22 (2), 60-4 PMID: 7581432