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[[File:Paula Radciffe NYC Marathon 2008 cropped.jpg|right|thumb|300px|Paula Radcliffe, the holder of the women's world record for the marathon (2:15:25) has gained much of her improvement through greater Running Economy.]][[Running Economy]] is how much energy it takes you to run. The better your economy, the faster and further you can run. Running economy is obviously determined to some extent by biomechanical efficiency, especially [[Cadence]]. There is some evidence that biochemical changes may have a significant impact on running economy. For instance, slow twitch [[Muscle|muscles]] s require less oxygen for the same level of work as fast which muscles do, and burning fat requires more oxygen than carbohydrates.
=Why Should You Care?=
Running Economy can vary by as much as 30% between runners of a similar [[VO2max|V̇O<sub>2</sub>max]]<ref name="Daniels-1985"/>. The two charts below show the [[VO2max|V̇O<sub>2</sub>max]] and running economy of Paula Radcliffe over a 10 year period<ref name="Jones2006"/>. Over that time Paula Radcliffe's race performance dramatically improved even though her [[VO2max|V̇O<sub>2</sub>max]] did not. This suggests that for elite athletes at least, improvements in running economy are critical. It's generally believed that Running Economy is more "trainable" than [[VO2max|V̇O<sub>2</sub>max]] in athletes that have been training for years<ref name="Moore2016"/>.
* An undergraduate thesis showed no difference in running economy with subjects wearing shoes with 0mm drop, 4mm drop, and their usual running shoes (12-14mm)<ref name="Brown2013"/>.
* In one of the more bizarre bits of research, a comparison of running shoes and stiletto high heeled dress shoes (4.5cm and 7cm heels) showed that the running shoes were more economic<ref name="GuLi2013"/>.
==Shoe Flexibility==
There is some evidence that less flexible shoe may improve running economy<ref name="OhPark2017"/><ref name="Roy-2006"/>. However, there appears to be a "Goldilocks effect" where a little bit of additional flexibility improves running economy, but too much will actually have the opposite effect. It also appears that the amount of stiffness that is "just right" varies considerably from runner to runner, though this may be related to the runner's weight with heavier runners benefiting more. Converting the stiffness used in the studies to ordinary running shoes is not clear, and the two studies use different methods for measuring the stiffness. More importantly, the stiffer shoes were not because of thicker foam or more rubber outsole, which are the normal causes of inflexibility, but the addition of carbon fiber plates to highly flexible shoes. These carbon fiber plates are likely to be far more springy than typical inflexible running shoes. The first study used the Adidas Adistar Comp and compared it with modifications that increased the stiffness by 2.0x or 2.5x. In the graphs below you can see that the lighter runners have reduced running economy with stiffer shoes, while heavier runners do much better. However, even for a given weight, there is still wide variation.
[[File:RE and Stiffness2006.jpg|center|thumb|200px|The change in running economy for two different shoe stiffness's plotted against runner's weight.]]
The second study used the Reebok ZQuick which has a grooved sole to improve flexibility as its baseline, and then modified the shoes with various thickness carbon fiber plates. The ZQuick had a stiffness of only 1.5 Nm/rad, and the plates increased this to 10, 25, 32, and 42 Nm/rad (one runner tried a plate with 57 Nm/rad). The test shows a clearer sweet spot or goldilocks zone with the moderate thickness plates. For all the plates, there were some runners that did worse than in the bare shoes, and some runners that did better in even the thickest plate.
[[File:RE and Stiffness2017.jpg|center|thumb|200px|The change in running economy against stiffness, with the average as the black circles and the individual results as gray circles.]]
==Other Shoe Characteristics==
There are a handful of studies that have looked at other shoe characteristics, there is insufficient information to reach any conclusions.
* A study has suggested that shoe comfort affects running economy, with the most comfortable shoes having the greatest economy<ref name="LuoStergiou2009"/>, but due to the many differences in the shoes used, the study seems too flawed to be of much use.
* A comparison of identical shoes, one made with EVA foam and the other with Adidas' softer "Boost" foam indicated that the softer foam had a 1% better running economy<ref name="WorobetsWannop2014"/>.
A short (30 minute) high intensity run does not change economy<ref name="Morgan-1990"/>, but economy deteriorates during a marathon run, possibly due to muscle damage and the need for greater neurological muscle activation to produce the force required to maintain pace<ref name="KyrPullinen2000"/>. However, a marathon run does not significantly change running biomechanics such as ground contact time<ref name="NicolKomi2007"/>.
=Glycogen depletion & Fat Burning=
Because burning fat requires more oxygen than carbohydrate, the switch to fat burning due to [[Glycogen]] Depletion means more oxygen is required. This does not directly change running economy if it's measured as power output, but it does if it does increase the energy required remains similarpercentage of [[VO2max|V̇O<sub>2</sub>max]] which is often how Running Economy is evaluated<ref name="KyrPullinen2000"/>. However, breathing rate becomes higher to supply the required oxygen, and the amount of oxygen extracted in each breath is lower, further driving the breathing rate.
=Running Biomechanics=
There are a number of biomechanical features of running that are relatively easily measured that may impact Running Economy.
=References=
<references>
<ref name="OhPark2017">Keonyoung Oh, Sukyung Park, The bending stiffness of shoes is beneficial to running energetics if it does not disturb the natural MTP joint flexion, Journal of Biomechanics, volume 53, 2017, pages 127–135, ISSN [http://www.worldcat.org/issn/00219290 00219290], doi [http://dx.doi.org/10.1016/j.jbiomech.2017.01.014 10.1016/j.jbiomech.2017.01.014]</ref>
<ref name="Squadrone-2009">R. Squadrone, C. Gallozzi, Biomechanical and physiological comparison of barefoot and two shod conditions in experienced barefoot runners., J Sports Med Phys Fitness, volume 49, issue 1, pages 6-13, Mar 2009, PMID [http://www.ncbi.nlm.nih.gov/pubmed/19188889 19188889]</ref>
<ref name="Wierzbinski-2011">The separate effects of shoe mass and cushioning on the energetic cost of barefoot vs. shod running. Wierzbinski, Corbyn. University of Colorado at Boulder. Departmental Honors Thesis. http://digitool.library.colostate.edu///exlibris/dtl/d3_1/apache_media/L2V4bGlicmlzL2R0bC9kM18xL2FwYWNoZV9tZWRpYS8xMTkyODM=.pdf</ref>