Rearfoot and midfoot or forefoot impacts in habitually shod runners.

PURPOSE: Shear loading rates (LR) have not been investigated in runners with a mid- or forefoot strike (FFS) versus rearfoot strike (RFS). The purpose of this study was to compare three-dimensional ground reaction forces (GRF) and LR during impact in habitual rearfoot strikers (hRF) and habitual forefoot strikers (hFF) strikers. METHODS: Thirty competitive runners performed 10 overground running trials with both foot strike styles. Peak three-dimensional and resultant GRF and instantaneous LR during impact were compared. RESULTS: Vertical LR significantly decreased for hRF using an FFS (RFS = 148 ± 36 body weight [BW]·s(-1), FFS = 98 ± 31 BW·s(-1)) but was similar for hFF running with either foot strike (FFS = 136 ± 35 BW·s(-1), RFS = 135 ± 28 BW·s(-1)). Posterior impact forces were present during FFS but not during RFS, and posterior LR was significantly greater for both groups during FFS (-58 ± 17 vs -19 ± 6 BW·s(-1)). Medial impact forces were also present during FFS but not during RFS, and medial LR was significantly larger for both groups during FFS (-21 ± 7 vs -6 ± 6 BW·s(-1)). Interestingly, hFF had greater impact peaks and LR in all directions compared with 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. CONCLUSIONS: Peak resultant and vertical LR 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 LR 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 because hFF running with an FFS demonstrated many GRF and LR similar to or greater than RFS.