# Influence of Footwear Features on Oxygen Consumption and Running Economy: A Review
## 4 Discussion
### Longitudinal Bending Stiffness  
The insertion of elements that increased the longitudinal bending stiffness making the midsole more rigid to flex was the latest tool added to modern running footwear that could have an influence on running economy [36]. This increased bending stiffness of the shoe can be carried out through a stiffer material in the midsole or by inserting a carbon plate.  
In the study by [PERSON] et al., the first method--providing greater rigidity to the midsole--was used [54]. In this study, tests on a running track with footwear were carried out by combining the following materials: material that returned a lot or little energy with another material that provided bending stiffness or not, creating four footwear conditions. After testing the different shoe conditions on 19 subjects, no significant differences were found for any combination of materials.  
In a study published in 2017, a control shoe was compared with four shoes with carbon plates of different stiffness [34]. In order to provide a different stiffness to each sole, they took as a reference in this study what they called \"critical stiffness\" and defined it as the joint moment index of the first metatrasophalangeal joint to reach the maximum flexion of this joint. Thus, there were five levels of stiffness in the midsole: control shoe; a carbon plate with rigidity similar to critical stiffness; a plate stiffness between the control shoe value and critical stiffness; a carbon plate with a stiffness that was twice the critical stiffness; and shoes where the stiffness of the plate was between the values of the critical stiffness and the maximum stiffness. After carrying out a running test in each shoe condition, they obtained a favorable result in favor of the shoes that contained the carbon plate with a rigidity similar to the critical stiffness (\(p<0.05\)). This comparative study assessed a control shoe with two shoes that included a carbon plate [55]. The shoe conditions presented in this study were: control shoe, stiff shoe and very stiff shoe. After performing the corresponding tests with each shoe, results were shown in a graph featuring a U-shaped curve where there was a lower oxygen consumption in 11 of the 13 subjects in the rigid shoe compared to the control shoe (\(p=0.014\)). [PERSON] et al. compared a control shoe with a bending stiffness of 8.1 N/mm with a shoe with a carbon plate in its midsole that provided a stiffness of 23.1 N/mm [53]. After testing the 18 participants on a 200 m athletic track, the authors did not obtain any difference between the two shoe conditions although 13 of the 18 subjects showed lower oxygen consumption on the rigid shoe. Thanks to the separation of the participants into two groups based on whether there was an improvement or not, the limit to consider a better running economy was established in a decrease in oxygen consumption of at least 0.5% for rigid shoes compared to control shoes. When the differences were studied by separating the groups, they found benefits in the group of 13 subjects who responded to the carbon plate.  
There seems to be a consensus on the insertion of the carbon plate in the midsole of the shoe; however, it is important to note that no one stiffness was ideal and, instead, the value seemed to depend on each subject.