Running with Science: Army biomechanics expert opens new research area to help runners run right

April 18, 2011

Story Highlights

  • ARL researchers helped develop a training program for runners who are at risk for stress fractures.
  • Findings from the study could help reduce risk of stress fractures.
  • Research could also save the U.S. military nearly $6 million spent yearly on new recruits who cannot complete basic combat training because they sustain stress fractures.

Learning to run the right way, for scores of runners who sustain overuse injuries a year, could literally keep them on the road to good health but it could also save the U.S. military nearly $6 million spent yearly on new recruits who cannot complete basic combat training because they sustain stress fractures.

Dr. Philip Crowell, biomechanics team leader with the Human Research and Engineering Directorate (HRED) at the U.S. Army Research Laboratory, helped develop what could turn into a training program for runners, inside and out of the military, who are at risk for stress fractures. That's what a potential outcome from a 2009 study he conducted as part of a University of Delaware collaborative effort with physical therapy professor Dr. Irene Davis, director of the institution's Running Injury Clinic in Newark. The study focused on seeing if runners could reduce the factors that lead to stress fractures through gait retraining.

Today, Crowell is presenting his findings in journals and meetings with various groups to draw attention to the study in hopes of finding transition partners, first, within the Army. He said this is a fairly new area of research that doesn't exactly fit into the type of human factors research that has traditionally been done by HRED.

Findings from their study, he said, could open the door to deeper understanding of the effectiveness of gait retraining as a means for reducing the risk of stress fractures.

"The retraining methods could be used by athletic trainers or physical therapists to train people to run in a way that may prevent stress fractures. It could also be used by athletic trainers or physical therapists to help injured runners rehabilitate and learn to run in a way that may reduce their risk of re-injury," said Crowell. "The model can be used by researchers to examine strain rates on the tibia during activities such as walking, walking while carrying a load like a backpack, running uphill or downhill, running at various speeds, and running over and around obstacles. The model can also serve as the basis of comparison for more sophisticated finite element model that will be developed."

Stress fractures in bones are overuse injuries that occur as a result of repeated loading on the bone. Repetitive loading from activities such as marching and running causes microdamage to the bone. Normally, bones can repair themselves. However, if the microdamage occurs faster than it can be repaired, or if the normal repair mechanisms are disrupted, the microdamage can accumulate and result in a stress fracture.

Crowell said the study was designed to determine if runners can use real-time visual feedback from an accelerometer to achieve immediate reductions in tibial acceleration and vertical-force loading rates. The study also aimed to determine if multiple training sessions would allow subjects to reduce their lower extremity loading and to maintain those reductions for one month, and to determine if reduced lower extremity loading associated with the gait retraining program resulted in reductions in the loading on the tibia itself.

As part of the study, runners were pre-screened to identify those at higher risk for tibial stress fractures, which considered runners' training regimen, diet, running surfaces, fitness level, and bone structure, for example. Recent evidence suggests, though, that stress fractures of the tibia are related to acceleration of the tibia and vertical force loading rates that occur as the foot impacts the ground.

Subjects then ran on a customized, instrumented treadmill for eight control sessions, followed by eight retraining sessions. During retraining, an accelerometer, an electromechanical sensor, placed just above the subject's ankle measures the acceleration of the tibia. Output readings from the accelerometer appear on a monitor in front of the treadmill, showing runners their tibial acceleration relative to a threshold prescribed by the researchers.

A line placed across the output monitor, or display, provides a target goal for subjects.

"We have a database of 125 runners who have not had stress fractures. We measured peak tibial acceleration as those runners ran across the laboratory at approximately 3.7 meters per second. For that group, the average peak tibial acceleration was 6 g [g = acceleration due to gravity] and the standard deviation was approximately 3 g. Then, we screened runners looking for those with peak tibial acceleration that was one standard deviation above the average (or 9 g). This was roughly the peak tibial acceleration measured on another group of runners who had sustained stress fractures, so that put those people at increased risk of sustaining a stress fracture," he said.

"When we brought subjects in for their first retraining session, I looked at the peak tibial acceleration that was displayed on the monitor in front of them. Then I adjusted the line across the screen so that it was at approximately half of their peak value. This gave them a goal to work for and if they achieved it or at least got close, they would be running with peak tibial acceleration that was near the average for the group of 125 uninjured runners."

Runners could, in effect, tune into the rhythms of their running style and make mental notes correlating their gait to performance on the monitor, as outputs from the accelerometer. Runners were expected to modify their running gait in such a way as to keep their acceleration peaks below the line but they weren't told exactly what to do, or not exactly how to do it. But, by focusing in on their rhythms, pace, and how their feet landed on the treadmill, they were able to get the readings below the line.

"Humans are pretty good at adjusting their running gait to accommodate different types of terrain, and each person has a slightly different way of doing it. So, we felt it would be best to let the subjects determine what adjustments to make to their gait in order to land more softly and reduce their peak tibial acceleration," Crowell explained.

During the first handful of retraining sessions, subjects ran on the treadmill for about 10 to 15 minutes and were able to see accelerometer readouts on the monitor. Slowly, that information was taken away from them so by the time they entered the seventh and eighth sessions, which lasted about 30 minutes, they were able naturally regulate their gait to reduce their tibial acceleration.

In one case, a runner who was a heel-striker changed to a forefoot strategy which meant she'd land more on the ball of her foot. Others made more of a conscious effort to roll on the whole length of the foot and then push off.

In their pilot study, four of the five runners were able to adjust their running style in just one session using feedback queues they got from their own bodies to reduce tibial acceleration. In the retraining study, 10 new subjects, provided with real-time feedback, were instructed to run "softer" and try to reduce their tibial shock. They achieved reductions in peak tibial acceleration (approximately 50 percent) vertical loading rates (roughly 30 percent), and impact peak (20 percent) and were able to continue showing this adjustment one month after the study concluded.

"We gradually reduced the amount of feedback so that the subjects would pick up on internal cues that would help them maintain reductions in peak tibial acceleration. This is something that has been shown to be successful in motor learning studies," he said.

As part of this study, Crowell created a simple model of the tibia to examine the strain rates in tibia. The model considered the tibia as a hollow cylinder. Before and after training, researchers collected data from runners to estimate muscle forces, joint reaction forces and moments, and inertial forces and moments that were entered into the models.

For the model, he added muscle forces where the muscle would be attached to the tibia; muscles can create very large forces during activities such as running. This mathematical model was run through simulation software to ultimately see stresses, strains, and strain rates at the medial and posterior sides of the mid-tibia and the distal third of the tibia-all points around the tibia that are common for stress fractures. The results of the model showed that strain rates on the tibia were reduced as a result of the retraining program.

"I calculated strain rates from data collected before subjects entered the retraining program. I also calculated strain rates from data collected one month after they finished the program," said Crowell. "The results showed that the strain rates were decreased after the retraining program, and they were decreased in regions where stress fractures are common."

Crowell said the model could also be used to examine stresses, strains, and strain rates for data collected under various other conditions like speed, slope, fatigue, and even footwear. "Prospective studies are needed to determine if the gait retraining program can reduce the occurrence of stress fractures. If the retraining program can reduce the occurrence of stress fractures, it may be beneficial to establish gait retraining programs in physical therapy clinics, fitness centers, and military training facilities as a means of preventing stress fractures."


Last Update / Reviewed: April 18, 2011