Bending Stiffness of Running Shoes and Running Energetics

Hot on the heels of my last post on the running shoes for the sub-2hr marathon we now have a study that looks at running energetics and the bending stiffness of the shoe. I don’t want to re-litigate all the issues surrounding running shoes and running economy, so they can be read here for background. This study looked at the stiffness of the running shoe. It is assumed that the stiffer a shoe is, the greater lever arm there could be for propulsion. While this sounds great, it could be at the expense of interfering with natural foot function, such as limited first metatarsophalangeal joint dorsiflexion will impair the function of the windlass mechanism. This means that comprises may need to be made to get the optimal level of stiffness for the individual and it is highly likely that the right amount of stiffness will need to be tuned for each individual runner to get the optimum amount. This is part of what I speculated that Adidas and Nike might be doing in their quests for the sub-2 hr marathon. Here is the study:

The bending stiffness of shoes is beneficial to running energetics if it does not disturb the natural MTP joint flexion
Keonyoung Oh, Sukyung Park,
Journal of Biomechanics; Available online 18 January 2017
A local minimum for running energetics has been reported for a specific bending stiffness, implying that shoe stiffness assists in running propulsion. However, the determinant of the metabolic optimum remains unknown. Highly stiff shoes significantly increase the moment arm of the ground reaction force (GRF) and reduce the leverage effect of joint torque at ground push-off. Inspired by previous findings, we hypothesized that the restriction of the natural metatarsophalangeal (MTP) flexion caused by stiffened shoes and the corresponding joint torque changes may reduce the benefit of shoe bending stiffness to running energetics. We proposed the critical stiffness, kcr, which is defined as the ratio of the MTP joint (MTPJ) torque to the maximal MTPJ flexion angle, as a possible threshold of the elastic benefit of shoe stiffness. 19 subjects participated in a running test while wearing insoles with five different bending stiffness levels. Joint angles, GRFs, and metabolic costs were measured and analyzed as functions of the shoe stiffness. No significant changes were found in the take-off velocity of the center of mass (CoM), but the horizontal ground push-offs were significantly reduced at different shoe stiffness levels, indicating that complementary changes in the lower-limb joint torques were introduced to maintain steady running. Slight increases in the ankle, knee, and hip joint angular impulses were observed at stiffness levels exceeding the critical stiffness, whereas the angular impulse at the MTPJ was significantly reduced. These results indicate that the shoe bending stiffness is beneficial to running energetics if it does not disturb the natural MTPJ flexion.

Nothing in the methods or analysis jumps out at me as being problematic. The study used the Reebok ZQuick shoe as the control and then five different stiffnesses of carbon plates to alter the bending stiffness. A key finding of the study was this:

In this study, we showed that the elastic insole assisted running propulsion and reduced the MTP muscle-tendon effort and the metabolic cost by approximately 1.1 % ± 1.2 %. However, at a certain level, this strategy became ineffective because the added elasticity restricted the natural flexion of the MTPJ. Specifically, a high bending stiffness of the insole disturbed the flexion of the MTPJ and reduced the transfer of the ankle joint torque to the ground push-off by lengthening the moment arm

This is key: “at a certain level” … looking at the table in the full paper on that critical stiffness that “certain level” varies substantially from runner to runner. In other words, the optimal shoe for running energetics is subject specific and needs to be tuned.

As always, I go where the evidence takes me until convinced otherwise ….and this is more evidence supporting the concept of tuning shoes to the indiviusdal runner.

Oh, K., & Park, S. (2017). The bending stiffness of shoes is beneficial to running energetics if it does not disturb the natural MTP joint flexion Journal of Biomechanics DOI: 10.1016/j.jbiomech.2017.01.014

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