Acute effects of long-distance running on mechanical and morphological properties of the human plantar fascia.

Long-distance running (LDR) can induce transient lowering of the foot arch, which may be associated with mechanical fatigue of the plantar fascia (PF). However, this has not been experimentally tested in vivo. The purpose of this study was to test our hypothesis that LDR induces transient and site-specific changes in PF stiffness and morphology, and that those changes are related to the lowering of the foot arch. Ten male recreational long-distance runners and 10 untrained men were requested to run overground for 10km. Before and after running, shear wave velocity (SWV: an index of soft tissue stiffness) and thickness of PF at three different sites from its proximal to distal end were measured using supersonic shear imaging and B-mode ultrasonography. Foot dimensions including the navicular height were measured using a three-dimensional foot scanner. SWV at the proximal site of PF and navicular height were significantly decreased in both groups after running, with a higher degree in untrained men (-21.9% and -14.1%, respectively) than in runners (-4.0% and -6.3%, respectively). The relative change (%Delta) in SWV was positively correlated with %Deltanavicular height in both groups (r = 0.69 and r = 0.65, respectively). Multiple regression analysis revealed that %DeltaSWV at the proximal site solely explained 72.7% of the total variance in %Deltanavicular height. It is concluded that LDR induces transient and site-specific decreases in PF stiffness. These results suggest that the majority of running-induced lowering of the foot arch is attributable to the reduction of PF stiffness at the proximal site.