Habitual non-rearfoot strike pattern attenuates impact loading during level and uphill running but not during downhill running.

This study aimed to investigate the effects of uphill running (UR) and downhill running (DR) on impact loading and lower extremity biomechanics in runners with different habitual foot strike patterns (FSPs). Twenty-one habitual non-rearfoot strike pattern (NRFS) runners and 21 habitual rearfoot strike pattern (RFS) runners were recruited to run at 12 km h-1 at slopes of 10%, 0%, and -10% in a random order with their preferred FSPs on the instrumented treadmill. A motion capture system was used to synchronously collect marker trajectories. During UR and DR, 19% and 14% of the habitual RFS runners transitioned to NRFS, but the habitual NRFS runners kept using NRFS. NRFS runners had a lower vertical instantaneous loading rate during level running (LR) and UR but a lower vertical average loading rate only during UR than RFS runners. NRFS runners had lower knee and higher ankle impulse and cumulative load than the RFS runners. Habitual FSP is adaptable during graded running. NRFS attenuated impact loading during LR and UR, but not during DR, where its main biomechanical effect may be a redistribution of load from the knee to the ankle rather than an overall reduction in impact loading.