Trabecular Bone Project
A layman's recap of our study on trabecular bone in the calcaneus of runners
(The full paper is published here: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0188200 )
Because bone is a living tissue, adapting during life to use, certain bones may reflect habitual running. Researchers already use some bone measures--especially in the "cortical" bone that makes up the long bones of the body-- to infer how active an individual was during life. Trabecular bone- the porous bone found in joints and in the bones of the feet- is also gaining favor as a diagnostic tool. It too adapts during life to the strains it experiences.
Below: Human calcaneus with trabecular bone showing through the eroded cortical bone.
In 2014 we set out with the following research questions: Does the calcaneus (the heel bone of the foot) reflect bone adaptation resultant from endurance running? And if so, can we identify these adaptive properties? Eventually, such understanding may let us examine hominin fossils, providing a way of more directly assessing whether our ancestors engaged in habitual running.
Knowing that trabecular thickness, bone density and alignment change due to loading, we expected these variables to be higher in the bones of runners, and the calcaneus of forefoot runners to have the most robust trabecular measures of all. (The Achilles tendon, which attaches to the calcaneus, exerts huge forces on the bone, and these forces are greater in forefoot runners.)
A recruitment campaign netted us 19 test subjects: 13 runners, of whom 6 were forefoot strikers and 6 rearfoot strikers (see right) with one guy who switched up footstrike depending on pace. Six were nonrunners-- our control group. Runners came to the UMass Biomechanics Lab where we fitted them with tracking markers and recorded their running stride with high-speed motion capture to be sure of their footstrike method (forefoot or rearfoot).
Above right: At the Biomechanics Lab, you are seen as nothing but a disembodied form of tracking markers.
Next, runners and nonrunners alike travelled to Worcester Polytechnic Institue for a dose of radiation in Dr. Karen Troy's high resolution peripheral quantitative computed tomography machine. (It scans and builds an image of the internal structure of the bone, measuring all sorts of trabecular properties along the way.) ...We pointed the scanner to focus on one region of the calcaneus:
...and used another computer program to reconstruct the images for further analysis:
The computer in the WPI lab has onboard software the computes the trabecular variables we wanted to measure. Bone orientation measures required using another program and significantly more user input. At that point, all that was left to do was feed the data through a statistical program to analyze differences between testgroups.
Interestingly, the thickness and mineral density of trabecular bone in our nonrunners were highly correlated with body weight. This makes sense: more body weight means more stress on this bone, which you're standing and walking on all day. And in the absence of a repetitive, high-strain activity like running, these forces should be the most important factor influencing bone adaptation.
In runners, trabecular thickness and bone mineral density were not correlated with body weight, and were instead highly correlated with variables related to running: weekly running mileage, years they've been running, and the age at which they started running.
There were no statistically significant differences in the bone of nonrunners vs. runners. However, our nonrunners weighed a lot more, and because we know (and our results suggest) that greater body mass leads to greater bone adaptation, we adjusted our nonrunners' bone measures for body mass. (Essentially, this is a statistical way of taking body weight out of the equation and seeing what the results would be if everyone had the same body mass.) After this adjustment, we found that forefoot runners had significantly greater trabecular thickness (and nearly-significantly) greater bone mineral density than nonrunners.
We know that trabecular bone adapts in response to strain. Our results suggest that endurance running causes increased trabecular thickness and probably mineral density in the calcaneus, near where the Achilles tendon attaches. Further, bone gets thicker and denser as runners run more volume, have been running for more years, and start early in life (when bone is more responsive to loading).
Why did our results show that only the forefoot runners had thicker and denser trabecular bone? Why not the rearfoot runners too? Probably because the forefoot runners in our study run more mileage and have been running for longer than the rearfoot runners. Thus, our rearfoot runner subgroup was also a "these guys run more and have been running for longer" subgroup. This lead us to conclude that bone in the calcaneus becomes reliably and measureably different in runners vs. nonrunners only with sufficient years and volume of running. (A greater sample size might show that it only takes a little bit of running to create these differences, but we had a small sample size, making it harder to detect differences.) This also means that our finding of greater bone adaptation in forefoot vs. rearfoot runners may not be due to footstrike, but rather volume and years of running. Footstrike may have an effect on trabecular bone in the calcaneus but our study, with the runners we got, wasn't capable of detecting such a difference.
Finally, our results suggest that body weight has a measurable effect on trabecular bone in the calcaneus, in individuals who do not subject this bone to repeated high loads (i.e., running). This may seem like a simple and obvious observation but it hasn't been reported elsewhere and several reviewers initially questioned it.
What we left out of the published paper
My intent was to write a paleoanthropology paper. The purpose in looking for bone adaptation resultant from running is to someday be able to examine fossils to assess whether human ancestors were running. This would be a huge development, but it would require years of further research, which I won't be doing anytime soon as I'm on to a new project. Even then, the differences in bone between a human ancestor who ran a lot and one who didn't might be hard to detect in a fossil, because 1) fossils don't preserve perfectly, and 2) we'd need many, many fossil feet to get an adequate sense of whether a hominin population was running, and we don't have enough of these fossils. For these reasons- and because I ultimately published in a general science journal- the reviewers and editor insisted that we abandon this discussion in the paper. Hence, it's a simple bone adaptation paper.
We also completely left out all data that we collected on a second bone, the first metatarsal. I ultimately decided that the bone scans that we collected did not sample the same region in every participant's bone and therefore were misleading.
THANK YOU to the 19 men who volunteered their time and feet!