The Science of Running Shoes
The design and selection of running shoes does not match the available science. The commonly held beliefs follow this logic: Runners get injured due to impact and excessive Pronation, running shoes reduce impact and pronation, and therefore running shoes reduce injury. Unfortunately, every part of this rationale seems to be flawed. There are other aspects of shoe design, such as the raise heel or arch support that have even less evidence to support them.
- Injuries due to impact. There is surprisingly little evidence that impact forces cause injuries, and there is even some evidence that lower impact forces are associated with higher injury rates. It's been suggested that excessive impact can result in injury, while more moderate impact can produce important adaptations that are necessary for improved performance.
- Injuries due to over pronation. The science around Pronation and injury rates is quite mixed. Part of the problem is science does not generally look at pronation directly, but uses arch height with the assumption that low arches pronate more. There is some evidence that high or low arches have slightly higher injury rates, or that different arch heights have different patterns of injury.
- Running shoes reduce impact. There is good evidence that increased cushioning does not reduce impact forces. Runners who normally run in shoes will have higher impact when initially running barefoot, but after adaptation the impact forces are actually lower without shoes.
- Running shoes reduce pronation. Motion control shoes (the highest level of anti-pronation) only reduce pronation by about 1.5% when compared with a simple cushioned shoe. It seems unlikely that this is enough to produce any real-world effect.
- Running shoes reduce injury. There is no evidence that running shoes reduce injury rates. Assigning shoes based on arch height does not change injury rate, nor is there any indication that more cushioned shoes have a lower injury rates. There is some evidence that motion control shoes cause greater leg pain and more training days lost, and this applies to all arch types.
- Raised heel. Another common feature of running shoes is a raised heel, which is intended to reduce the strain on the Achilles tendon. However there is little evidence that the raised heel actually reduces the strain on the Achilles tendon, and no evidence that the raised heel actually reduces Achilles tendon injuries.
- Arch support. Often running shoes have a raised area under the arch that is intended to provide support. I found no evidence to support this idea.
- Barefoot running. The reduced impact seen with barefoot running led many people (myself included) to believe that this would in turn result in lower injury rates. However, there is no evidence that barefoot runners have a lower injury rates. More importantly, there is a growing body of evidence to suggest that the transition to barefoot running is associated with a high injury risk.
1 The Myth of Running Shoe Types
There is good evidence to support the widely held belief that injury rates among runners are quite high, with estimates of injury rates varying between 20% and 80% of runners. It is widely assumed that impact forces and excessive pronation cause running injuries, and that running shoes are designed to alleviate these problems. This leads to the common recommendation that different types of shoes should be recommended based on a runners arch height. In fact, REI, Zappos, Runners' World, and Road Runner Sports all include this advice.
2 Impact & Injury
The relationship between impact and injury is less clear than one might suppose. It has been suggested that while excessive impact can result in injury, lower levels of impact are the stimulus for improved strength and performance (See Supercompensation.) 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.
- The impact forces at the heel are not related to the forces at common injury sites such as the ankle, Achilles, or knee..
- A study of 131 runners showed that injury rates were highest in those with the lowest impact levels.
However, there is also some evidence of a relationship between higher impact and 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.
- A study of five female runners who had previously had a stress fracture showed higher peak impact forces than subjects without stress fractures.
- 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.
3 Pronation, Arch Height & Injury
The evidence for the correlation between pronation and injury is rather mixed. This is compounded by the use of arch height as a proxy for pronation.
- An analysis of 29 studies showed that high or low arched feet had slightly higher risk of injury than normally arched feet.
- There is also evidence for the opposite conclusion, where high or low arched feet have a lower risk of injury.
- One study found that while injury rates are the same for different arch heights, the location of the injuries varies with arch height.
- Another study found that while injury rates are similar for different arch heights, those with low arches had more expensive injuries. (This was a study in the military, where such expenditure is more easily tracked.)
- A year-long study of 927 novice runners showed no correlation between arch height and injury rates.
- A study of 1597 runners found that those with the lowest arches were 2.7x more likely to have knee (patellofemoral) pain than those with the highest arches. (Note that this study used navicular drop as an indicator of pronation, but other factors contribute significantly to navicular drop.)
- A retrospective study found that arch height was not different between runners who had previously been injured and those that had never been injured.
4 Running Shoes & Impact
There is good evidence that increased cushioning does not reduce impact. However, runners who normally run in shoes have greater impact forces when running barefoot, but this is reversed with barefoot experience.
5 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 meta-analysis of 5 studies showed that motion control shoes can reduce pronation when compared with barefoot or simple cushioned shoes, but only by about 2%.
- A study compared a Motion Control shoe (MC) with a Cushioned shoe (CT) with 20 high arched (HA) and 20 low arched (LA) runners. The motion control shoe was the New Balance 1122 and the cushioned shoe was the New Balance 1022. The change in pronation (in degrees) is shown below.
- A study of 10 male runners compared "normal" running shoes with and without a 10 degree orthotic wedge showed the orthotic reduced pronation by 6.7 degrees.
- A study of 25 inexperienced, over-pronating female runners looked at differences in pronation in motion control and cushioned shoes, before and after a 1.5 Km (~1 mile) run. These runners only averaged 2.1 Km (1.3 miles) per week and had pronation of more than 6 degrees. The Motion Control shoes reduced pronation by 3.3 degrees before the run, but after just this short run the Motion Control shoes made no difference. The motion control shoes were Adidas Supernova Control and the cushioned shoes were Adidas Supernova Cushion. The results are shown below:
|Before 1.5 Km run||13.9||10.6||3.3|
|After 1.5 Km run||17.7||17.7||0|
- A study of 10 experienced rear foot runners were tested with shoes of varying heel flare. This heel flare is how much wider the heel is at the bottom than the top, and the flared heels reduced pronation from 12.6 to 11.1 degrees (1.5 degree decrease) when compared with any heel without any flare. In practice, it's rare for a shoe to be this narrow at its base, and other studies have not shown this effect.
- The heel counter is intended to link the heel of the foot to the shoe, but a study found that a rigid heel counter did not prevent slippage within the shoe any better than a flexible heel counter. Also, the pronation of the foot can be twice as large as the pronation when measured on the shoe.
- A study of 7 people compared pronation when stepping down from a platform in shoes and when barefoot. The shoe was the Adidas Response Cushion and the platform was 4 inches/10 cm high. Pronation with shoes was less (17.9 degrees) than when barefoot (20.5 degrees). However, because the reduction was so small, the study concluded that it was impractical to alter pronation with this type of footwear.
6 Running Shoes & Achilles Strain
A common feature in running shoes is for the heel to be thicker than the forefoot, something that is commonly called "drop". In the 1980's a drop of 12-15mm was recommended to prevent Achilles tendon and calf injuries, but there is little evidence to support this:
- No studies have shown raised heels reduce Achilles (or other) injuries.
- Shoes with the different levels of a drop do not change the range of motion of the ankle during running.
- A study of five runners, each running in five different shoes with heel heights of 2.1-3.3cm (5.0 to 9.5 degrees) did not support the idea that heel height changes stress on the Achilles' tendon.
- A study looked at 30 runners that were either assigned a minimal drop shoe (4mm) or were trained to adopt a midfoot strike (MFS) pattern. The minimal drop shoe reduced heel impact, but the MFS training had no effect.
- A study looked at 12 Rear Foot Strike runners using 16 combinations of midsole thickness and drop. The lower drop shoes had a more midfoot strike pattern, but the thickness had no impact. (Ground contact time was greater with lower drop shoes.)
- A study of 12 male runners looked at foot strike for shoes with 0mm, 4mm, and 8mm of drop, plus barefoot. 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.
7 Injury Rates & Shoes
- A study of 247 runners over 5 months showed no difference in injury rates between firm and softly cushioned shoes.
- Three studies compared evaluated the idea that shoe type should be determined by arch height. Runners were put into two groups, with one group assigned shoes based on the shape of the arch, and the other group just assigned a stability shoe regardless of their arch. These studies found no difference in injury rates. The studies were done by the US Army (2168 men, 951 women), Air Force (1955 men, 718 women), and Marine Corps (840 men, 571 women).
- A study of 81 women training for a half marathon were randomly assigned cushioned, stability and motion control shoes. The cushioned shoe was a Nike Pegasus, the stability was Nike Structure Triax, and the motion control was Nike Nucleus. The runners were then analyzed based on their arch height.
- The study found that the motion control shoe was associated with the highest levels of pain while running for all foot types, though the difference was only significant for the neutral and pronated foot types.
- All the highly pronated runners wearing the motion control shoe missed a training day due to pain, the highest proportion of any of the subgroups.
- The neutral runners had higher levels of pain in the neutral shoe than the stability shoe. The pronating runners had higher levels of pain in the stability shoe than the neutral shoe. This is the opposite of most recommendations for shoe and foot type.
- Note that while the overall sample size was reasonable (81), each individual subgroup was quite small (5 to 18 runners) and variation within subgroup of results was large. The subgroups also varied significantly in weight, BMI, age, and years of running experience.
8 Shoes and Running Economy
Main article: The Science of Running Economy
Studies have consistently shown that heavier shoes reduce running economy. Each 100g/3.5oz added to the weight of each shoe reduces running economy by about 1%. 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. Note that running shoes provide less cushioning in colder temperatures.
9 Minimalist & Barefoot Running
Most research looks at factors that might be related to injury risk, rather than injury rates directly. I found no studies that evaluated barefoot or minimalist running and injury rates. So while barefoot and minimalist running tends to have lower impact, it's unclear if this will have any bearing on injury rates. Of greater concern is some compelling evidence that the transition to barefoot or minimalist footwear is correlated with higher injury rates, especially stress fractures in the foot.
- A review of 23 studies found moderate evidence for higher Cadence and lower impact, but noted a lack of high quality evidence. Examples include:
- There are some instances of Metatarsal Stress fractures in runners who had changed to minimalist shoes, with no other changes in their training habits.
- A study of 99 runners were randomly assigned a traditional cushioned shoe (Nike Pegasus), partial-minimalist shoes (Nike Free 3.0 V.2) or minimalist shoes (Vibram 5-Finger Bikila). The runners had a minimum of 5 years' experience and had no injuries in the previous 6 months. The runners took part in a 12 week training program in which they gradually adopted their assigned footwear. They increased their time in the assigned footwear from 10 min (19%) in week 1 to 115 min (58%) in week 12.
- Runners in the traditional shoes had a lower incident of injury (#4/32 runners) than the partially minimalist (#12/32 runner) or minimalist (#7/35 runners).
- The only statistically significant difference in pain scores for the shoe conditions was in shin and calf pain, with runners in the partially minimalist and minimalist shoes having greater pain scores than the traditional shoes. However, the underlying data is a little more complex. Below are shown the pain scores before and after the trial for each shoe type. As you can see, the pain goes up by the greatest percentage for the partially minimalist, but this rise is from a much lower initial level. The absolute level of pain is lowest for the partially minimalist condition, making this study tricky to interpret.
|Baseline||7.3 (9.7)||4.9 (6.8)||5.3 (11.8)|
|12 weeks||18.4 (13.9)||13.9 (9.6)||28.8 (22.3|
- A study looked at bone marrow edema in 36 experienced runners transitioning to Vibram FiveFingers (VFF) shoes. The runners were randomly assigned VFF or their normal running shoes, with the VFF runners gradually transitioning based on the recommendations of Vibram at that time. Only 1 of the 17 runners in the control group showed signs of a bone marrow edema, compared with 9 of the 19 VFF runners.
- In 2014, Vibram settled a lawsuit that they made false and unsubstantiated claims that their FiveFingers shoes could reduce injury rates.
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