Changes

* 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.

However, there is also some evidence of a relationship between higher impact and stress fractures, but not other types of injury:

* 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

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="Garcia AznarBaltich2015"/>. Based on my measurements of running shoes, 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. * Highly cushioned Minimax shoes and conventional shoes may result in more knee stress than minimalist shoes<ref name="NiggBahlsen1987Sinclair-2016"/>. HoweverThe study looked at 20 male runners and measured impact with a pressure plate while filming the leg movement. The leg movement was then used to estimate knee stress based on a model that used knee movement and angle. The Minimax shoes were Hoka, the minimalist shoes were Vibram FiveFingers, but the conventional shoes were not specified. The runners were not familiar with the non-traditional shoes, and only had 5 minutes familiarization. The study also found greater contact forces in the non-minimalist shoes. * 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"/>. * In a study somewhat related to shoes and impact, a study looked at impact and running surface and found there were not impact differences between concrete, synthetic track, natural grass, and a treadmill<ref name="FuFang2015"/>. The study used a pressure sensor in the shoe's insole and an accelerometer attached to the Tibia, with the runners wearing non-cushioned, minimalist shoes. As noted above, the interaction of impact and injury rates is unclear.

* A study comparing the minimal Vibram FiveFingers with Hoka Minimax shoes suggests there may be more Achilles tendon stress in minimal shoes<ref name="SinclairRichards2015"/>. However, the group size was small (n=12), and there's no indication that the runners had experience with either type of footwear. =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"/>. One 2015 study did show a reduced injury rate with motion control shoes.

Studies have consistently shown that heavier shoes reduce running economy<ref name="LussianaFabre2013"/><ref name="Burkett-1985"/><ref name="Sobhani-2014"/><ref name="Wierzbinski-2011"/>. Each 100g/3.5oz added to the weight of each shoe reduces running economy by about 1%<ref name="Franz-2012"/><ref name="Wierzbinski-2011"/><ref name="Frederick 1985"/><ref name="Frederick-1984"/>. Studies of cushioning and Running Economy have provided conflicting information. I believe this conflict is due to some studies using a cushioned treadmill to compare barefoot and shod conditions. Not surprisingly, if a study uses a cushioned treadmill, the cushioning provided by the shoe does not confer any additional advantage over the barefoot condition. Analyzing the research, I conclude that a well cushioned running shoe can improve Running Economy by an estimated 2-3.5% compared with a weight matched un-cushioned shoe<ref name="Franz-2012"/><ref name="Wierzbinski-2011"/><ref name="Tung-2014"/>. Note that running shoes provide less cushioning in colder temperatures<ref name="DibSmith2005"/>. There are indications that a highly flexible shoe that is modified with a springy carbon fiber plate might be more efficient than a highly flexible shoe on its own<ref name="OhPark2017"/><ref name="Roy-2006"/>. =Heel Counters=The [[Heel Counter]] is intended to link the heel of the foot to the shoe, * A study found that a rigid heel counter did not prevent slippage within the shoe any better than a flexible heel counter<ref name="Gheluwe-1995"/>. * A source of confusion in biomechanical studies is that it is much easier to measure the movement of a heel counter than the foot within. Studies can assume that changes in the movement of the heel counter reflect the equivalent changes at the foot. One study found the pronation of the foot can be twice as large as the pronation when measured on the shoe<ref name="Stacoff-2001"/>, and another found there were significant differences between the movement of the heel and the movement of the heel counter<ref name="Stacoff-1992"/>. * One study found that cutting the bottom of the heel counter away reduced [[Running Economy]] by 2.4%<ref name="Jorgensen1990"/>, a relatively large change. Personally, I suspect this reduction in economy is due to the discomfort of the modified shoe, but the results are intriguing. <br/>[[File:HeelCounterRemoved.jpg|none|thumb|150px|The removed heel counter.]]* Some runners are concerned that a rigid heel counter may irritate the Achilles tendon. I found no research to support or refute this concern, but personally I see it as relatively unlikely. I suspect that irritation of the Achilles tendon by a shoe is more likely to be due to the extreme rear of the upper coming up to high, or curving inwards to cup around the heel too far. Note that pain in this area could also be due to the irritation of the bursa, rather than the tendon (retrocalcaneal bursitis).

Most research looks at factors that might be related to injury risk, rather than injury rates directly. I found no studies The one study so far that evaluated compared barefoot or /minimalist running and injury rates found broadly similar injury rates. So while This suggests that minimalist or barefoot and minimalist running tends to have lower impact, it's unclear if this will have any bearing on neither increases nor reduces the risk of injury rates. Of greater concern is However, there remains some compelling evidence that the transition to barefoot or minimalist footwear is correlated with higher injury rates, especially stress fractures in the foot.* A study comparing 107 barefoot runners with 94 shod controls found similar injury rates<ref name="Altman-2016"/>. The barefoot runners had fewer injuries per runner, but ran less miles, so the injury rate (injuries per mile) was similar. ** The barefoot runners ran 75% of their mileage barefoot and the remainder in minimal shoes, characterized by no arch support, no reinforced heel counter and no midsole. (That's a pretty good definition of a minimalist shoe.) The barefoot runners had been barefoot for an average of 1.7 years, and 63% began running barefoot 6–12 months before the study. That's a reasonably good adaptation period and should avoid any injuries related to the transition, ** The study attempted to ensure the two groups were similar, but there are far more men in the barefoot group (88%) than the shod group (73%). ** The runners' average mileage was 25 miles/week in the shod group and 15 miles in the barefoot group. (The study required a minimum of 10 miles/week.) ** There were 346 running related injuries, just under half of them clinically diagnosed. There were significantly more injuries per runner for the shod runners, but when the injury rate is normalized by the mileage the barefoot runners have a non-significantly higher injury rate.<br/>[[File:Altman-2016-Rates.jpg|center|thumb|300px|Injury rates per runner and normalized to mileage.]]** The barefoot runners had more foot injuries, but fewer injures elsewhere, most noticeably in the hip.<br/>[[File:Altman-2016-Injuries.jpg|center|thumb|300px|Location if injuries for both groups. It's important to note this is given as the number of injuries, not the percentage normalized which is a little misleading as there are more shod runners than barefoot.]]** The barefoot runners had far more injuries to the sole of the foot (plantar surface), mostly cuts. ** Shod runners may have had far more plantar fasciitis. (This re-enforces my belief that arch supports contribute to plantar fasciitis.)