Elsevier

Journal of Biomechanics

Volume 53, 28 February 2017, Pages 127-135
Journal of Biomechanics

The bending stiffness of shoes is beneficial to running energetics if it does not disturb the natural MTP joint flexion

https://doi.org/10.1016/j.jbiomech.2017.01.014Get rights and content

Abstract

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.

Introduction

Stiffened shoes are known to enhance running economy, which is defined as the oxygen consumption rate (VO2) during submaximal running (Roy and Stefanyshyn, 2006). Roy and Stefanyshyn reported reduced metabolic costs for specific shoe bending stiffnesses and inefficient running with stiffer shoes. Additionally, they speculated regarding the existence of an optimal shoe stiffness for running economy. Despite the significantly reduced metabolic costs observed in trials with stiffer shoes, no significant differences in either the mechanical work performed by lower-limb joints or the muscular activation level were identified (Roy and Stefanyshyn, 2006); thus, the determinant of the local minimum of metabolic cost remains unknown.

Previously studies of the effect of stiffened shoes on running mechanics have focused on MTPJ mechanics (Chen et al., 2014, Oleson et al., 2005, Tinoco et al., 2010, Toon et al., 2011, Willwacher et al., 2014). The elasticity of stiffened shoes enhances resistive torque during MTP flexion and reduces the negative work performed by the MTP muscle–tendon component (Roy and Stefanyshyn, 2006, Stefanyshyn and Fusco, 2004, Stefanyshyn and Nigg, 2000, Willwacher et al., 2013). The elastic restoring force of the bent shoes assists in forward propulsion during the extension phase (Willwacher et al., 2013). At higher bending stiffness levels, the center of pressure moves forward during the push-off phase, increasing the joint moment arm (Willwacher et al., 2014) and decreasing the ground reaction forces (GRFs). With regard to decreased ground push-offs at high shoe stiffnesses, a previous study reported that two distinct running strategies may be used to maintain steady running. One group showed increased ankle torque to compensate for a longer moment arm, whereas the other group exhibited an increased stance duration without significant changes in the ankle joint torque (Willwacher et al., 2014). These results support the idea that complementary changes in the lower-limb joint torques are required to maintain steady running in stiffer shoes. Because different muscle groups are involved in the changes in joint torque actuation, the metabolic cost of running may vary (Jackson and Collins, 2015, Roberts and Belliveau, 2005, Taylor, 1994, Umberger and Rubenson, 2011). Based on previous findings of individual-dependent forefoot stiffness (Oleson et al., 2005, Willwacher et al., 2014), we speculated that the effect of shoe stiffness on the gearing function at the MTPJ would depend on the subjective natural MTP flexion and torque.

In this study, we examined the subject-dependent effect of shoe stiffness on the metabolic cost. We hypothesized that the restriction exerted by stiffened shoes on the subject-dependent natural MTP flexion would lead to complimentary changes in the lower-limb joint torques and running energetics.

Section snippets

Methods

To test our hypothesis, we examined the changes in joint kinematics, kinetics, and metabolic cost as a function of the shoe bending stiffness. Subjects participated in four test sessions over four to five weeks. The sessions were as follows: anaerobic threshold test session (session 1), adaptation session (session 2), joint data measurement session (session 3), and metabolic cost measurement session (session 4). The aim of each session and the measurements collected are presented in Table 1. To

Results

A moderate scaling of the GRFs and a significant decrease in the peak GRF occurred in the A–P direction compared to the control trials (Fig. 2A–C). However, the stance duration increased as the stiffness increased, particularly during push-off (Fig. 2A, B and D). A decrease in the GRF combined with the increased stance duration did not significantly change the linear impulse (Fig. 2E) or the linear momentum of the body׳s CoM at touchdown or toe-off (Fig. 2F).

Most subjects showed reduced MTP

Discussion

In this study, we showed that the elastic insole assisted running propulsion and reduced the MTP muscle–tendon effort (Fig. 4D) and the metabolic cost by approximately 1.1±1.2% (Fig. 7B). 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

Conflicts of interest

None.

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (#2012-0002002).

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