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Weight and height are used in computing body mass index, an indicator of risk for developing obesity-associated diseases. (Photo credit: Wikipedia)

An oft repeated response to the statement that running injuries continue to occur at very high levels despite “improved” quality of running shoes over the past several decades goes something like this: “Well, shoes have gotten better, but the runners have gotten worse.” In other words, the fact that contemporary runners tend to be heavier and less fit washes out any injury-reducing benefit provided by modern running shoes.

Now, this post actually has nothing to do with running shoes, but rather looks at the question of whether heavier, slower runners are in fact more likely to get injured. Let’s ignore the obvious fact that anyone who hangs around runners would likely observe that injuries hit people of all shapes and sizes – I know plenty of thin, eminently fit people who have gotten hurt. But, although anecdotal observations like this can be helpful, they are far from scientific.

Fortunately, several scientific studies have addressed this question, and one was just published in a 2012 issue of the Journal of Physical Activity and Health. The study, titled “Impact of a High Body Mass Index on Lower Extremity Injury in Marathon/Half-Marathon Participants,” was written by a team from the Mayo Clinic led by Tyler Vadeboncoeur.

The design of this study was fairly simple. The researchers recruited 194 individuals who were participating in a half-marathon/marathon, recorded their BMI just prior to the start of the race, and administered two surveys. The first survey was filled out prior to the race and asked participants if they had suffered a lower extremity injury that had affected their training within the 30 days prior to the race (this survey also collected demographic data and included a personal health questionnaire). The second survey was administered after the race and asked whether participants had suffered an injury during the race.

Male (n = 55) and female (n = 139) subjects were each divided up into three groups based on their BMI (note, the authors openly discuss the limitations of BMI as a measure of body composition, but for practical reasons it was the easiest method to use just prior to the start of a race). You can consider these groups to be Low BMI, Moderate BMI, and High BMI – BMI ranges for each group are provided in the table below:

Tertile Females Males Low BMI 18.1–21.8 20.3–24.9 Moderate BMI 21.9–25.9 25.0–26.8 High BMI 26.1–42.2 26.9–34.6

They then looked at how a variety of factors (e.g., age, general health indicators, pre-race peak training volume, race finish time, injury outcomes) differed between the groups. Interestingly, they found the following with respect to injuries:

Females – Pre-Race Injuries (no significant differences among groups)

Low BMI – 24%

Mod BMI – 18%

High BMI – 9%

Females – Injuries During Race

Low BMI – 35% ( significantly different from other BMI groups)

Mod BMI – 10%

High BMI – 12%

Males – Pre-Race Injuries (no significant differences; only 7 injuries total)

Low BMI – 5%

Mod BMI – 12%

High BMI – 22%

Males – Injuries During Race (no significant differences; only 5 injuries total)

Low BMI – 15%

Mod BMI – 6%

High BMI – 6%

I’m going to focus on the female results here because the male sample was small (this was a race for breast cancer, so gender participation was heavily skewed), and there were so few injuries observed among them that it’s pointless to delve into the data for the guys.

What these results show for females is that there was no increase in injury risk among individuals in the High BMI grouping. In fact, the authors report that “ With every 1-unit increase in BMI a female was 13% less likely to suffer a race-related injury .” That’s right, High BMI females were on average less likely to suffer an injury during the actual race than lower BMI females, and there was also a trend for them to be less likely to suffer and injury in training. Why might this be?

One possible explanation is that the authors found that “lower BMI females had

longer peak weekly training miles (P = .007) and ran faster in both the full and half-marathons (P < .001 for both distances).” Running higher mileage and running to compete have both been among the only factors consistently linked to increased running injury risk (I discuss this in depth in Chapter 2 of my book), and it’s thus possible that High BMI runners are actually protected by running a slower pace and fewer miles than their Low BMI counterparts. It follows, then, that this study cannot say whether higher BMI runners would be at greater risk if they maintained the same training volume as those in the lower BMI groups, or if they were running the same speed.

The authors conclude the paper with the following paragraph:

“Many people with a high BMI participated in our open-entry marathon. While training for and participating in our marathon/half-marathon was associated with a risk of lower extremity injury, a high BMI was not an independent risk factor. As such, based on our findings, interested high BMI runners should not be discouraged from participating in endurance running based on their BMI alone. In females, a higher BMI may even be protective. These findings should be confirmed in a larger study allowing for more detailed control of confounders.”

I should point out that this is not the only study to show that high BMI is either not correlated with increased injury risk (see Macera et al., 1989), or was actually found to be protective (see Taunton et al., 2003).

The take home message here is that if you’re a Clydesdale or an Athena, you might be a bit slower than some of the other runners lining up at the start of a race, but you may actually be less likely to get hurt than some of your more fleet-footed counterparts. Not a bad tradeoff if you ask me, and certainly calls into question the argument that the reason injury rates have remained high is because because contemporary runners aren’t all built like Frank Shorter or Bill Rogers.