Enhanced ground reaction force analyses reveal injury-related Biomechanical differences in runners

This three-part study investigated alternative pre-processing techniques to better understand the differences in patterns of ground reaction force (GRF) and load rate (LR) among runners with running-related injury (RRI). 534 runners were assessed on an instrumented treadmill with 3D kinematic data capture. Participants were classified as "injured" or "uninjured" and "rearfoot" (RF) or "non-rearfoot" (non-RF) strikers. The raw net GRF is normalized by time and then averaged across at least ten steps for the left and right foot; a double Gaussian characterizes the biphasic double-mass-spring model of running gait. Six parameters from the Gaussians were used to describe the relative differences and shape change based on injury condition. LRs were calculated using a central difference numerical derivative of the raw normalized net force data. 32% of runners reached peak negative LR (unloading) within the first 20% of stance. Injured RF strikers had 18% higher peak positive LR and 6% shorter time to peak than uninjured RF strikers (p < 0.05). Injured non-RF strikers showed peak negative LR 10% earlier in normalized stance, with a 10% shorter interval between positive and negative peaks (p < 0.05). The magnitude and timing of impact and active phases of GRF waveforms differed in runners with history of tibial stress fractures and current Achilles tendinopathy (p < 0.05). LR and LR timing are important in relation to specific RRI. These alternative pre-processing methods may help improve mechanistic understanding of GRF and LR and identify gait retraining foci for specific injury diagnoses in the future.