Purpose: Shear loading rates have not been investigated in runners with a mid/forefoot strike (FFS) versus rearfoot strike (RFS). The purpose of this study was to compare three-dimensional ground reaction forces (GRF) and loading rates (LR) during impact in habitual RFS (hRF) and habitual FFS (hFF) strikers.
Methods: Thirty competitive runners performed 10 overground running trials with both foot strike styles. Peak three-dimensional and resultant GRFs and instantaneous LRs during impact were compared.
Results: Vertical LR significantly decreased for hRF using a FFS (RFS: 148+/-36, FFS: 98+/-31 BW/s) but was similar for hFF running with either foot strike (FFS: 136+/-35, RFS: 135+/-28 BW/s). Posterior impact forces were present during FFS but not RFS, and posterior LR was significantly greater for both groups during FFS (-58+/-17 versus -19+/-6 BW/s). Medial impact forces were also present during FFS but not RFS, and medial LR was significantly larger for both groups during FFS (-21+/-7 versus -6+/-6 BW/s). Interestingly, hFF had greater impact peaks and LRs in all directions compared to hRF during FFS. This may be explained by hFF using a smaller strike index (hFF: 62+/-9%, hRF: 67+/-9%; P=0.02), which was significantly inversely related to vertical LR and impact peak.
Conclusion: Peak resultant and vertical LRs are not ubiquitously lower when using a shod FFS versus RFS despite an absence of resultant and vertical impact peaks. Furthermore, there were impact peaks in the posterior and medial directions, leading also to greater LRs in these directions during FFS. Therefore, transitioning from RFS to FFS in traditional running shoes may not offer long-term protection against impact-related running injuries since hFF running with a FFS demonstrated many GRFs and LRs similar to or greater than RFS.
(C) 2014 American College of Sports Medicine