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