It begins as a persistent, irritating pain in the foot or lower leg, then becomes more intense, perhaps swollen, and soon a runner knows he is being sidelined by one of the most common injuries in the run: a stress fracture. These tiny cracks in the bone can disrupt the training for months or even end a sporting season.
A segment of the multi-billion dollar wearables industry aims to save potential victims from this fate, but an engineering professor at Vanderbilt University has encountered a major problem: devices are measuring the wrong thing.
Working with a local racing club, an orthopedic specialist who advises the NFL Players Association and a team of engineers at Vanderbilt, Assistant Professor of Mechanical Engineering Karl Zelik found that sensors measure only the impact of the foot tapping on the pavement – which is virtually all of them – tell users little about the forces in the bones that lead to stress fractures.
His research confirmed that the vast majority of bone strength actually comes from muscle contraction, not foot impact, a finding largely ignored by both the wearables industry and many scientific studies.
Zelik's research, appearing today in the peer-reviewed journal PLOS One and titled "Soil reaction force metrics are not strongly correlated with tibial bone load when performed at speeds and slopes: Implications for science, sport, and wearable technology," provides the clearest and simplest demonstration of the underlying problems of existing tools and predominant methods. evaluation of bone stress and risk of injury.
"We have examined recent scientific literature and found that more than 50 scientific publications each year report or interpret their results based on this incorrect assumption that the ground reaction force is representative of the loading of the internal structure – stress in the body's bones and muscles "said Zelik, a former athletics athlete. "Measuring the ground reaction force may be convenient, but it's a wrong signal."
The wearable pressure and accelerometer sensors already available on the market can help monitor the risks of bone stress injuries, but only if they combine information on the ground reaction force and the strength of the muscles pulling against the bone. In general, you can not presume that increases in soil reaction force indicate increased bone stress, said Emily Matijevich, Ph.D. in Mechanical Engineering. student in Zelik's lab and herself an avid runner.
Matijevich performed the laboratory work the study describes, testing 10 runners along a series of speeds and slopes.
"We used high-speed motion capture cameras to track the movement of the runners and a special force measurement mat to record the ground reaction force under your feet," she said. "We then combined these signals using biomechanical algorithms to estimate the compressive strength experienced by the tibial bone at the stem, a common place for stress fractures. In almost all cases, we found that the ground reaction forces were not strongly correlated with tibial bone load ".
In several cases, lower ground reaction forces have meant more stress on the tibia, a finding opposite to what most athletes believe and against as most existing wearables work.
This research began two years ago when Vanderbilt University's assistant professor of orthopedics, Leon Scott, who is on the health and safety committees of the NFL Players Association, asked Zelik a simple question: Could wearable sensors be used effectively to prevent clinical stress fractures every day?
Matijevich, Zelik and Scott are now exploring new ways to monitor bone stress noninvasively and have recently filed a patent application for a system that fuses multiple sensor data to estimate the tibial load of both muscle contractions and reaction forces from soil. They are looking for business partners to develop this new wearable technology and explore applications for recreational runners, military cadets and elite athletes.
Scott said the combination of wearable sensors and new algorithms the team is developing gives a much better picture of bone stress, with the potential to help runners lower their chances of injury.
"There is so much you can do when the game is going because these are high speed injuries, but we can do something about stress fractures during training and conditioning," Scott said. "Right now, we do not have great tools to tell us what's going on with bones, other than experience and anecdote, and unfortunately, a lot of people are failing."
Materials provided by University of Vanderbilt. Original by Heidi Hall. Note: Content can be edited for style and size.