Runners: Leaning Forward Increases Force on Hip Extensors Interview with:
Anna Warrener Ph.D.
Department of Anthropology
University of Colorado Denver
Denver, CO

Dr. Warrener  What is the background for this study?

Response: We became interested in looking at trunk position during running because one of us (Daniel Lieberman) had anecdotally observed people running with a variety of trunk positions, and when we went looking for academic literature on the topic, we found it was fairly scarce. We suspected that trunk position could have a major impact on the forces experienced by the lower limbs during running and even affect aspects of gait (stride length and time). So we developed a model predicting how these forces and movements might change as trunk flexion/forward leaning increased.

Our primary predictions were that more forward lean would increase “overstride” which is the distance in side view between the hip and the heel as it contacts the ground (a measure of how far your are extending your leg when you step). This in turn would increase the impact forces experienced by the lower limb at initial contact which have previously been shown to increase the risk of repetitive stress injuries. We also predicted that stride would get longer and take more time because extending the leading leg out farther forward (overstriding) would be necessary to keep the body center of mass within a base of support above the limbs. This more extended limb, we predicted, would change the angles and forces about the ankle, knee and hip joints. What are the main findings?

Response: What we found was largely in line with our predictions with a few interesting exceptions. Greater forward lean did increase “overstride” and the impact forces experienced by the leading limb at foot-ground contact. The moments of force at the hip were substantially increased, meaning that the hip extensors had to work harder as people leaned farther forward, while knee and ankle moments of force actually decreased.

One surprising finding was that stride length actually decreased slightly with greater lean and stride frequency increased, which means that even though people stretched their leading limb out farther (overstride) their steps were shorter and faster which seems counterintuitive. We think that means they were spending less time during the gait cycle in the air (aerial phase) but we didn’t collect the data needed to directly test that.

The other interesting finding was just how variable people are in their running form. While we saw very similar responses in all the variables we measured as we had people run with greater forward lean, their “baseline” for each variable was really different. For example, some people started out with very high impact forces and they got higher the more they leaned. The same trend was true for others, but their baseline impact force was really low, so even at the highest lean conditions, they might not have an impact as great as someone else without lean. I think these idiosyncratic running behaviors are really interesting and in my mind help explain why it can be so difficult to pin down specific aspects of form to various injuries. What should readers take away from your report?

Response: I think the big takeaway is that the position of your trunk is pretty influential on the biomechanics of your lower limb while you are running. Our researched tried to provide a comprehensive model for how all these variables may be linked, but this is a jumping off point for more research since we didn’t set up a study to determine the “proper” running form (if there is such a thing) or the direct impact on injury.

I think this study is exciting because it provides a lot of testable hypotheses for the future. I personally am also really fascinated by the differences we observed between runners even as they responded similarly to increasing trunk flexion/lean. I have been wondering a lot about where these differences arise. What recommendations do you have for future research as a result of this work?

Response: One of the things we would like to look at in the future is the interesting results of decreasing stride length and time and whether it is being caused by a reduction in time spent in the air during a stride. This seems really important to us because swinging your leg during running is actually very metabolically costly. If air time is reduced, that probably means that a runner is having to swing that leg faster and is using more oxygen to run with that kind of form. Also, it would be interesting to extend our findings to actual occurrence of injuries to see if people who habitually lean forward more during running are more prone to developing certain types of musculoskeletal problems. Is there anything else you would like to add?

Response: We thank Lee Saxby in our acknowledgments because it was his conversations with Daniel Lieberman that started us working on this project.


Anna Warrener, Robert Tamai, Daniel E. Lieberman,
The effect of trunk flexion angle on lower limb mechanics during running,
Human Movement Science,
Volume 78, 2021, 102817, ISSN 0167-9457,

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