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* Peak Impact is the greatest force seen during the initial landing.
* Loading Rate is how rapidly the forces build up and can either be averaged over parts of this section of the graph or an instantaneous peak can be used. (That would be peak rate of change of impact, not peak impact.)
* In additionWhere the impact is measured from may be important. Impact can be measured as force between the foot and the ground using a pressure plate, impact the acceleration of the foot/shoe, or the acceleration of the tibia. * Impact is sometimes normalized to body weight, but not always.* Not all studies have evaluated impact when the subjects have had time to adapt to a particular shoe (or lack of shoes). It seems reasonable to me that a runner's impact levels will be different in unfamiliar footwear.
There is evidence that the impact seen in running does not result in injury:
* Impact forces are not related to injury rates in epidemiologic studies<ref name="Nigg-1997"/>.
* A review of the available research in 2007 found no relationship between impact and injury rates<ref name="van Gent-2007"/>.
* A review of the available research in 1992 found no evidence that injury rates were related to hard or soft surfaces<ref name="van Mechelen-1992"/>. (Of course, you can't assume too much about the impact rates from the surface.)
However, there is also some evidence of a relationship between higher impact and stress fractures, but not other types of injury:
* A study that compared 20 runners who had never been injured with 20 runners that had prior injuries found that peak impact rates were higher in those that had been previously been injured<ref name="Hreljac-2000"/>.
* A study of five female runners who had previously had a stress fracture showed higher peak impact forces than subjects without stress fractures<ref name="GrimstonNigg1994"/>.
* A meta-analysis of 13 studies found that while there was no correlation between rates of stress fracture and impact, there was a relationship for the rate of loading<ref name="Zadpoor-2011"/>.
* A study of 20 female runners with a previous history of unilateral tibial stress fracture fractures showed they had high rates of impact, but not greater peak impact than matched controls<ref name="Milner-2006"/>.Strangely the injured runners were no more asymmetric than the controls, with higher impact levels in both injured and uninjured legs. * A comparison of runners with previous tibial stress fractures found that the injured runners had greater braking and impact forces than the controls<ref name="ZifchockDavis2006"/>. However, the injured runners
It seems possible that Loading Rate is related to stress fractures, but it seems unlikely that impact is related to other injury types. (It's estimated that stress fractures account for 0.7% to 20% of clinical injuries<ref name="Fredericson-2006"/>.)
=Pronation, Arch Height & Injury=
[[File:ImpactAndInjuryBahlsen1988.jpg|none|thumb|300px| A graph of peak vertical impact force and the frequency of running-related injuries<ref name="Bahlsen1988"/>.]]
=Running Shoes & Impact=
There is good evidence that increased cushioning does not reduce impact, and may even increase it.* A study of 14 runners using three different midsole harnesses (25, 35, 45 Shore) at three different speeds showed no difference in impact measured with a force plate for the different shoes<ref name="Robbins-1990NiggBahlsen1987"/>. The impact did increase with increasing pace, and based on my shoe reviews the three shoes are relatively soft (most are ~45 Shore, with some Hoka shoes going as soft as 35 Shore). * Comparing shoes with the same midsole hardness but fore/heel heights of 0mm/4mm, 8mm/12mm, 16mm/20mm, plus barefoot found there were no impact changes between the shod conditions<ref name="WrightNeptune1998HamillRussell2011"/>. The runners only had 5-10 minutes to adapt to each condition, and it's unclear if any of the runners had barefoot experience. The impact for the barefoot condition was lower than shod, but the runners changed to a forefoot landing when barefoot. * There was no difference found in impact forces between two shod conditions where one type of shoe provided 50% more cushioning than the control shoe<ref name="ClarkeFrederick2008"/>.* A study of 93 runners that compared three hardness shoes (40, 52, & 65 Shore) showed that the softer shoes had the greatest impact peak<ref name="NiggBahlsen1987Garcia AznarBaltich2015"/>. HoweverBased on my measurements of running shoes, runners the range of firmness in this test goes from somewhat soft to remarkably firm. The impact ranged from 1.70x Body Weight (BW) for the hardest shoe, to 1.64x BW for the medium and 1.54x BW for the hardest. The impact was measured using a force plate and used the impact peak, not the active peak (see diagram above) which is why the impact is much lower than other studies that report 2.0-2.4x BW. The study found that running increased their joint stiffness in the softer shoes, which may be the cause of the greater impact. * Runners who normally run in shoes have greater impact forces when running barefoot, but this is reversed with as a runner becomes adapted to being barefoot experience<ref name="Robbins-1990"/><ref name="Divert-2005"/><ref name="Robbins-1987"/>. As noted above, the interaction of impact and injury rates is unclear.
=Running Shoes & Pronation Control=
The evidence indicates that even Motion Control shoes can only reduce pronation by around 1.5%, which is unlikely to be enough to make any real-world difference.
* A study of 12 male runners looked at foot strike for shoes with 0mm, 4mm, and 8mm of drop, plus barefoot<ref name="ChambonDelattre2013"/>. The barefoot condition was midfoot strike rather than rear foot strike in the shoes. The different drop conditions were not significantly different, though there was a trend towards more rear foot strike with the 8mm drop than with 0mm and 4mm drops.
* A drop of 15mm or 7.5mm did not produce a significant reduction in Achilles tendon stress<ref name="Dixon-1998"/>.
=Running Shoes & Injury Rates & Shoes=
Several studies have found there is no evidence to support the idea that running shoes can reduce injury rates<ref name="RichardsMagin2009"/><ref name="van Gent-2007"/><ref name="van Mechelen-1992"/>.
* A study of 247 runners over 5 months showed no difference in injury rates between firm and softly cushioned shoes<ref name="TheisenMalisoux2013"/>.
<ref name="Divert-2005">C. Divert, G. Mornieux, H. Baur, F. Mayer, A. Belli, Mechanical comparison of barefoot and shod running., Int J Sports Med, volume 26, issue 7, pages 593-8, Sep 2005, doi [http://dx.doi.org/10.1055/s-2004-821327 10.1055/s-2004-821327], PMID [http://www.ncbi.nlm.nih.gov/pubmed/16195994 16195994]</ref>
<ref name="Robbins-1987">SE. Robbins, AM. Hanna, Running-related injury prevention through barefoot adaptations., Med Sci Sports Exerc, volume 19, issue 2, pages 148-56, Apr 1987, PMID [http://www.ncbi.nlm.nih.gov/pubmed/2883551 2883551]</ref>
<ref name="ClarkeFrederick2008">T. Clarke, E. Frederick, L. Cooper, Effects of Shoe Cushioning Upon Ground Reaction Forces in Running, International Journal of Sports Medicine, volume 04, issue 04, 2008, pages 247–251, ISSN [http://www.worldcat.org/issn/0172-4622 0172-4622], doi [http://dx.doi.org/10.1055/s-2008-1026043 10.1055/s-2008-1026043]</ref>
<ref name="NiggBahlsen1987">B.M. Nigg, H.A. Bahlsen, S.M. Luethi, S. Stokes, The influence of running velocity and midsole hardness on external impact forces in heel-toe running, Journal of Biomechanics, volume 20, issue 10, 1987, pages 951–959, ISSN [http://www.worldcat.org/issn/00219290 00219290], doi [http://dx.doi.org/10.1016/0021-9290(87)90324-1 10.1016/0021-9290(87)90324-1]</ref>
<ref name="Jorgensen1990">U. Jorgensen, Body load in heel-strike running: The effect of a firm heel counter, The American Journal of Sports Medicine, volume 18, issue 2, 1990, pages 177–181, ISSN [http://www.worldcat.org/issn/0363-5465 0363-5465], doi [http://dx.doi.org/10.1177/036354659001800211 10.1177/036354659001800211]</ref>
<ref name="Altman-2016">AR. Altman, IS. Davis, Prospective comparison of running injuries between shod and barefoot runners., Br J Sports Med, volume 50, issue 8, pages 476-80, Apr 2016, doi [http://dx.doi.org/10.1136/bjsports-2014-094482 10.1136/bjsports-2014-094482], PMID [http://www.ncbi.nlm.nih.gov/pubmed/26130697 26130697]</ref>
<ref name="ZifchockDavis2006">Rebecca Avrin Zifchock, Irene Davis, Joseph Hamill, Kinetic asymmetry in female runners with and without retrospective tibial stress fractures, Journal of Biomechanics, volume 39, issue 15, 2006, pages 2792–2797, ISSN [http://www.worldcat.org/issn/00219290 00219290], doi [http://dx.doi.org/10.1016/j.jbiomech.2005.10.003 10.1016/j.jbiomech.2005.10.003]</ref>
<ref name="HamillRussell2011">Joseph Hamill, Elizabeth M. Russell, Allison H. Gruber, Ross Miller, Impact characteristics in shod and barefoot running, Footwear Science, volume 3, issue 1, 2011, pages 33–40, ISSN [http://www.worldcat.org/issn/1942-4280 1942-4280], doi [http://dx.doi.org/10.1080/19424280.2010.542187 10.1080/19424280.2010.542187]</ref>
<ref name="Garcia AznarBaltich2015">Jose Manuel Garcia Aznar, Jennifer Baltich, Christian Maurer, Benno M. Nigg, Increased Vertical Impact Forces and Altered Running Mechanics with Softer Midsole Shoes, PLOS ONE, volume 10, issue 4, 2015, pages e0125196, ISSN [http://www.worldcat.org/issn/1932-6203 1932-6203], doi [http://dx.doi.org/10.1371/journal.pone.0125196 10.1371/journal.pone.0125196]</ref>
<references/>
[[Category:Science]]
[[Category:Injury]]