by Elizabeth Hofheinz, M.P.H., M.Ed.
A team from Harvard University’s Department of Physical Medicine and Rehabilitation has delved into the physics of running in order to examine ground reaction force variables between healthy and injured runners—both as a group and within specific common injuries.
Their study, “Impact-Related Ground Reaction Forces Are More Strongly Associated With Some Running Injuries Than Others,” was published in the September 11, 2020 edition of The American Journal of Sports Medicine.
Caleb Johnson, Ph.D., apost-doctoral fellow at the Spaulding National Running Center at Harvard Medical School, told OSN, “The general mission of our lab, and running clinic, is to advance the prevention and treatment of running-related musculoskeletal injuries. More specific to our lab, our main interest is in the biomechanical factors that contribute to running-related injury. One of these factors that has been fairly well published on is vertical ground reaction force loading rates, or the rate of overall loading to the lower extremities when a runner contacts the ground with each stride of running.”
“However, as noted in a recent review by van der Worp et al. (British Journal of Sports Medicine 50.8 (2016): 450-457), there have been inconsistencies in the literature regarding the association of loading rates and running injuries. With this study, we wanted to explore a potential reason for this inconsistency; the grouping of all types of running injury together, a frequent practice in the literature. Essentially, we wanted to see if specific injuries are more/less associated with loading rates, and therefore grouping all types of injury together has the potential to wash out these injury-specific associations.”
Looking at 125 runners with patellofemoral pain, tibial bone stress injury, plantar fasciitis, Achilles tendinopathy, or iliotibial band syndrome, the researchers compared them to 65 healthy controls. Both groups undertook an instrumented treadmill assessment at a self-selected speed. The impact-related ground reaction force variables they examined were: vertical average and instantaneous load rates, posterior and medial/lateral instantaneous load rates, and vertical stiffness at initial loading.
“The overall takeaway,” said Dr. Johnson, “was that specific injuries, patellofemoral pain and plantar fasciitis, were driving associations between running-related injury and vertical ground reaction force loading rates. Along with this, we developed Receiver Operator Characteristic Curves (ROC curves) to attempt to identify general and injury specific thresholds for vertical loading rates, above which the number of injured vs uninjured runners appeared to increase significantly. We found that levels of 58-67 BW/s (vertical instantaneous load rates) were the best separators of injured vs uninjured runners for specific injuries. Last, we did not find any associations between loading rates in the other components of the ground reaction force (anterior-posterior and medial-lateral) and injury.”
“One surprise was that we did not find an association between vertical loading rates and tibial bone stress injuries, which has been reported fairly consistently in previous literature. One potential reason for this is that we included all bone stress injuries, along the spectrum of early grade stress injuries to full stress fractures. Most previous literature has focused on only stress fractures. A second surprise was that we found several associations with vertical stiffness during initial loading, an estimate of the amount of vertical forces being absorbed by the lower extremities during the initial loading phase of stance. This variable has been reported on frequently in past literature, so it is something to look further into in future research.”
Reduced vertical loading rates=healthier feet?
“This work provides evidence that there is a role of reducing vertical loading rates on the management of several specific running injuries; plantar fasciitis and patellofemoral pain. Further, we provide specific values that appear to be good targets to reduce vertical loading rates below, in this effort. There are a number of gait retraining methods for reducing the rate of vertical loading that have been well supported in previous literature, including foot strike angle modification and biofeedback with wearable sensors.”
“It is also important to note that while these variables were associated with injury, based on mean differences and high odds ratios for the thresholds, the area under the curve for ROC curves and sensitivity/specificity of the selected thresholds were not overly high. Along with the fact that we only looked at ground reaction force variables, and with a retrospective design, our work cannot support the notion that vertical loading rates are the dominating cause of running-related injuries. We can only say that it appears to be a contributing factor.”