Runners should stop leaning forward so much to avoid overuse injuries

DENVER, Colo. — Avid runners are constantly chasing a new personal best time or distance, but oftentimes the pursuit of excellence can result in a litany of injuries. Now, researchers from the University of Colorado, Denver find that an unlikely remedy may help runners avoid such injuries. They say athletes just need to stop leaning forward so much.

Let’s explain. The trunk flexion refers to the angle at which a runner bends forward from the hip. This angle fluctuates greatly from person to person. Most runners self-report an angle ranging anywhere from -2 degrees to 25 degrees. This latest research reports that the greater the trunk flexion angle, the bigger the impact on stride length, joint movements, and ground reaction forces. In simpler terms, how far one leans forward as they run may have a big impact on subsequent knee pain, back pain, and medial tibial stress syndrome.

“This was a pet peeve turned into a study,” says Anna Warrener, PhD, lead author and assistant professor of anthropology at CU Denver, in a media release. “When (co-researcher) Daniel Lieberman was out preparing for his marathons, he noticed other people leaning too far forward as they ran, which had so many implications for their lower limbs. Our study was built to find out what they were.”

Daniel Liberman, PhD, from the Department of Human Evolutionary Biology at Harvard University collaborated on the early stages of this project with Dr. Warrener.

Leaning forward shortens your stride

The human body’s head, arms, and trunk make up just about 68 percent of total body mass. So, even minor fluctuations in one’s trunk flexion may substantially alter lower-limb kinematics and ground reaction forces (GRF) while out for a run or jog.

To research this topic, study authors recruited 23 injury-free, recreational runners (ages 18-23). The team recorded each participant as they ran for 15 seconds at their self-selected trunk position. In addition to participants’ trunk position of course, researchers also asked each subject to try running at a 10, 20, and 30-degree angle of flexion.

“We had to create a way in which we could reasonably force someone into a forward lean that didn’t make them so uncomfortable that they changed everything about their gait,” Dr. Warrener explains.

Researchers accomplished this by hanging a lightweight, plastic dowel from the ceiling just above the runners’ heads. They then moved the dowel up or down, depending on the necessary angle.

Contradicting the research team’s original hypothesis, the experiments showed that average stride length decreased by 0.39 inches and stride frequency increased from 86.3 strides per minute to 92.8 strides/min. Also, overstride in relation to the hip increased by 28 percent.

“The relationship between strike frequency and stride length surprised us,” Dr. Warrener adds. “We thought that the more you lean forward, your leg would need to extend further to keep your body mass from falling outside the support area. As a result, overstride and stride frequency would go up. The inverse was true. Stride length got shorter and stride rate increased.”

Running is a ‘whole body experience’

Researchers theorize that this may be caused by a drop in the “aerial phase.” In other words, less leg air time means shorter steps.

“The act of swinging your leg is really expensive as you’re running,” Dr. Warrener comments. “Swinging it faster as you lean forward may mean a higher locomotor cost.”

In comparison to subjects’ trunk flexion of choice, increased angles produced a more flexed hip and bent knee joint. More leaning also changed both foot and lower limb position, causing a bigger impact on body ground reaction forces. Rate of loading jumped by 29 percent and vertical ground reaction force impact transients increased by 20 percent.

All in all, these findings suggest that excessive trunk flexion may be a leading cause of running overuse injuries.

“The big picture takeaway is that running is not all about what is happening from the trunk down–it’s a whole-body experience,” Dr. Warrener concludes. “Researchers should think about the downstream effects of trunk flexion when studying running biomechanics.”

The study is published in Human Movement Science. 

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