One of the most common injuries runners face is a stress fracture: a small crack in one of the bones, usually in the foot or lower leg. For a while now it has been thought that this was caused by the impact of the foot hitting the floor, however a study has found that this isn’t the case.
Karl Zelik, assistant professor of Mechanical Engineering at Vanderbilt University, has been working on this issue for some time. He realised that tests for wearable items like running shoes only measure the impact of feet hitting the pavement, but according to his research this information doesn’t have much - if anything - to do with the actual cause of stress fractures.
In actuality, the majority of force that acts on bones that become fractured comes from the contraction of muscles surrounding them, not from the impact of running. In fact, in some cases lower impacts between the feet and the ground were actually associated with higher amounts of stress being put on the tibia.
Professor Zelik said: “We looked through the recent scientific literature, and we found that more than 50 scientific publications each year report or interpret their results based on this incorrect assumption that ground reaction force is representative of internal structure loading - the stress on bones and muscles inside the body.”
Along with PhD student Emily Matijevich, Professor Zelik tested ten runners on a force-measuring treadmill, utilising a variety of slopes and running speeds. They combined this with biomechanical algorithms, which were used to estimate the amount of force acting on the tibia in each case.
The results showed that the forces from the ground were not correlated with the amount of force acting on the tibia. They hope to use this data to influence the wearables market, encouraging runners and other athletes to track both information about their impact on the ground and the force from muscles pulling against the bone if they want to avoid stress fractures.