Effects of shoe bending stiffness on running economy

Abstract

In this study, we analyzed how the stiff shoes help a runner reduce metabolic cost and an optimal bending stiffness of shoes was also suggested to get best running efficiency. One hypothesis is that the stiff shoes store elastic energy when its flexion and reutilize such energy during push-off so it help runners reduce the metabolic energy. Too stiff shoes, however, might disturb the natural MTP flexion so unnatural motion and over-work were occurred. We focused on the MTP joint stiffness when MTP joint flexion was maximized (red dot in Fig. 1B and 1C), and stiffer shoes than this stiffness need larger ankle torque. Otherwise, MTP joint flexion would be disturbed and decreased. As a result, runners did not increase their ankle torque to bend their stiff shoes, so MTP flexion was decreased with stiffer shoes (Fig. 2B). Their ankle moment arm was increased and push-off impulse was decreased with nearly same ankle torque. Over-work at hip joint to compensate this loss of propulsion was found (Fig. 3B). Such reduced MTP flexion limits the thresholds for reutilization of the elastic energy, so the optimal shoes stiffness could be obtained by stiffening the shoes until the shoes stiffness was similar to MTP joint stiffness when its maximal flexion (Fig. 4A and 4B). As hypothesized, stiffer shoes store larger elastic energy, but too stiff shoes restrict their natural motion. An optimal shoes stiffness could be compromised in consideration of the maximizing the elastic energy without the disturbed joint motion and resultant over-work.