Study finds reduced vertical loading in stroller running

For many parents with young children, getting back into a running routine means dragging out the jogging stroller. While it's no surprise that pushing a bulky three-wheeler can feel harder and change the way you run, a new study led by researchers from Penn State Berks found that runners experience less impact per step, reducing their overall risk for injury.

The study, which was published in the journal PLOS One, also revealed a potential trade-off. While the impact per step is lower when running with a stroller, the torsion or twisting forces of the foot pushing off the ground increased. This motion, however, is not as consistently associated with injuries as step impact, the researchers said.

"While there are a lot of data on running economy and effort, this is the first to look at how loading forces change with stroller running," said Allison Altman Singles, associate professor of kinesiology and of mechanical engineering and senior author on the study.

Understanding this 'biomechanical trade-off' could inform stroller design, coaching strategies, as well as injury prevention and rehabilitation protocols for those who run with strollers."

Allison Altman Singles, Associate Professor, Kinesiology, The Pennsylvania State University

Up to 79% of runners experience injuries each year, according to the research team. The researchers said they set out to evaluate if and how key forces on the body associated with common overuse injuries change when running with a stroller.

They examined metrics related to vertical loading and torsional loading - the amount the foot twists on the floor when a runner pushes off the ground and moves forward. High vertical loading rates - or how fast force is transferred into the body - are associated with an increased risk of overuse injuries like knee pain, stress fractures and plantar fasciitis. An increase in torsional loading is also linked to stress-related injuries in the lower leg.

For this study, the researchers recruited thirty-eight healthy runners - men and women who were injury-free and ran at least five miles per week. Each participant ran with a stroller and without a stroller over a force plate, which measured the impact of each step.

The researchers found that when participants ran with a stroller, there was a significant reduction across all vertical loading metrics by at least 8% and up to 17%. In other words, there was less impact per step, suggesting a potential decrease in injury risk.

"When we're running with a stroller, we unweight ourselves vertically because we put our hands on the handlebars and lean into it a little. That allows some of our weight to shift and go through the stroller instead of our legs," Singles said.

On the other hand, torsional loading metrics increased significantly with stroller running, with some measures rising by more than four-fold. The arms and torso naturally rotate while running in order to counterbalance the natural movement of the legs. Holding the handlebars could be limiting the rotation in the upper body, the researchers explained, which in turn increasing twisting forces under the feet to compensate. Stroller running may also require more twisting forces to propel the stroller forward and to keep it moving in a straight line.

While the increase in torsional loading could be a concern, Singles said that it's much less established as an injury risk factor compared to vertical loading metrics and could potentially be countered with improved stroller design or running cues. However, more research is needed.

"The number one takeaway is that stroller running is not dangerous. This study shows that, in many cases, you have a reduced risk of overuse injury because of the stroller itself," Singles said.

The research team is continuing to study stroller running including further examining these biomechanical changes and the types of injuries that stroller runners are prone to.

Other authors on the paper include first author Joseph Mahoney, who was previously associate professor of mechanical engineering and of kinesiology at Penn State Berks and is currently associate professor of mechanical engineering at Alvernia University, and Benjamin Infantolino, associate professor of kinesiology at Penn State Berks. Amy Lista and Diego Carbajal, who both earned an undergraduate degree from Penn State Berks, also contributed to the paper.

Funding from Penn State and Alvernia University supported this work.