Negative Load Rates In Ground Reaction Forces During Running By Footstrike Type And Injury Status

Controlled negative load rates facilitate smooth transitions between the stance and swing phases in running gait. Conversely, early, and erratic maximal unload rates indicate inefficiencies in loading the full reaction force. Maximum positive and negative load rates (LR; force loading & unloading) and timing during stance are not commonly reported in gait analytics, limiting a greater understanding of the dynamic control of ground collision in the context of foot strike type and injury status. PURPOSE: To determine the LR of ground reaction forces (GRF) during stance phase of running gait among rearfoot (RF) and non-RF strikers and by injury status (current or recent history). METHODS: Runners (N = 535; 52% female; 35.5 ± 15.2 yr; 22.5 ± 3.2 kg/m2) ran on an instrumented treadmill at self-selected speed at 0% grade for 10 min, GRFs were sampled at 1200 Hz. A 7-camera 3D motion capture system collected data at 120 Hz. The numerical time derivative of GRF provided LR; initial peak positive and negative LR, and time between each peak were determined. Mean and standard deviation windows for LR were plotted. The footstrike type was confirmed by kinematic calculation and video review. RESULTS: Figures A-F show LR for all runners and are separated by injury status. First, RF strikers produced 27% greater peak positive and 19% greater negative LR in early stance compared to non-RF strikers with a 34% shorter time between these two loading peaks and a tighter standard deviation range (Figs A, D, respectively; p < .05). Second, injured runners with RF strike demonstrate the shortest time between the positive and negative loading peaks of all groups (p < .05). Finally, injured runners with RF strike produced the greatest initial peak positive LR and greatest negative LR (p < .05). CONCLUSION: Reporting of positive and negative loading should be considered in running gait analytics to provide insight into the stability of the initial impact peak in GRFs relative to injury status and foot strike type.