The Science of Running Shoes
The design of most running shoes does not match the available science, and understanding this will help you know What to Look for in Running Shoes. 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
- 2 Impact & Injury
- 3 Pronation, Arch Height & Injury
- 4 Running Shoes & Impact
- 5 Running Shoes & Pronation Control
- 6 Running Shoes & Achilles Strain
- 7 Running Shoes & Injury Rates
- 8 Shoes and Running Economy
- 9 Heel Counters
- 10 Minimalist & Barefoot Running
- 11 Personal Observations
- 12 References
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.
There are various ways of evaluating impact, and not all studies use the same metric or are not clear on which metric is used.
- Active Peak is the greatest force or acceleration detected during foot strike.
- 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.)
- Where the impact is measured from may be important. Impact can be measured as force between the foot and the ground using a pressure plate, 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.
- A study of three runners found that 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.
- The impact forces of 210 notice runners were evaluated for the symmetry of their impact forces and their subsequent injury rates. The injured runners had greater symmetry than the uninjured, and there was no difference between the impact forces on the injured side compared with the uninjured side. (However, see the results on the companion study below).
- Another study of the 210 notice runners mentioned above found that Loading Rate was related to the injury rate of male (but not female) runners, though the female runners had a higher Loading Rate than the males. However, when time spend running was considered, the relationship between Loading Rate and injury disappeared.
- A review of the available research in 2007 found no relationship between impact and injury rates.
- A review of the available research in 1992 found no evidence that injury rates were related to hard or soft surfaces. (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.
- 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.
- A study of 20 female runners with a previous history of unilateral tibial stress fractures showed they had high rates of impact, but not greater peak impact than matched controls. 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. 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.)
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 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. 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. 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.
- A study of 93 runners that compared three hardness shoes (40, 52, & 65 Shore) showed that the softer shoes had the greatest impact peak. 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. The 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.
- 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. 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.
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.
- 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.
- A study comparing the minimal Vibram FiveFingers with Hoka Minimax shoes suggests there may be more Achilles tendon stress in minimal shoes. However, the group size was small (n=12), and there's no indication that the runners had experience with either type of footwear.
7 Running Shoes & Injury Rates
- 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.
- A study of 372 recreational runners assigned them randomly either a neutral or motion controlled shoe and found that those in the neutral shoe had a higher injury rate. The study is unusual in that it blinded both the subjects and the testers to the type of shoe used. The shoe type is not identified in the study, other than to say they were commercially available and modified so the runner would not know the brand or type. Both types of shoe were a 10 mm drop, and the motion control shoes had a Medial Post made of both firmer foam and a plastic insert. The study found that those in the neutral shoe had nearly twice the injury rate of those in the motion control shoes. When the subjects were divided by their Pronation, the study found that the injury rates were only different in those runners with Pronation (~25% of both groups). The study also noted that the greatest predictor of injury is a history of prior injuries, and both groups had ~75% of previously injured runners.
- An intriguing study suggests of 264 runners preparing for a marathon found that runners who swapped between multiple shoes had lower injury rates than those who only used a single pair at a time. However, the data is self-reported and there were other differences between the groups, such as the multiple shoe runners having run more half-marathons than the single shoe group.
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 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.
- 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, and another found there were significant differences between the movement of the heel and the movement of the heel counter.
- One study found that cutting the bottom of the heel counter away reduced Running Economy by 2.4%, 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.
- 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).
10 Minimalist & Barefoot Running
Most research looks at factors that might be related to injury risk, rather than injury rates directly. The one study so far that compared barefoot/minimalist running and injury rates found broadly similar injury rates. This suggests that minimalist or barefoot running neither increases nor reduces the risk of injury. 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. 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.
- The barefoot runners had more foot injuries, but fewer injures elsewhere, most noticeably in the hip.
- 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.)
- 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.
11 Personal Observations
While this page is dedicated to the current scientific research, I do want to add a few personal observations as a counterpoint.
- My testing of Running Sensors has made me realize how many ways there are of measuring the impact forces of running. I believe that this warrants much deeper scientific research.
- Impact can be measured as simple acceleration, or as "jerk" which is the rate of change of acceleration. In some situations, the human body can adapt to a continuous level of acceleration much better than it can to a rapidly changing level of acceleration. This can be observed when an aircraft takes off, and the initial buildup of acceleration seems quite dramatic, but after 10-20 seconds the steady acceleration is harder to notice.
- The impact forces can be measured on the ground, on the shoe, on the tibia (lower leg), or on the torso. Each location is likely to have a different result.
- I've observed some potentially interesting patterns during my initial testing using TgForce, RunScribe, MilestonePod, Moov Now, and Wahoo TICKR Run.
- When use a forefoot foot strike the impact measured on my shoe is typically a little higher than when I heel strike. However, the impact measured on my tibia is vastly lower. When heal striking, the impact on my shoe is typically in the 7-9g range, and 5-7g on my Tibia. When I land on my forefoot the impact on my shoe is 8-12g but the Tibial impact is 2-4g.
- When comparing Maximalist shoes with minimally or cushioned shoes I'm typically finding that the impact measured on my shoe is a little higher with the more cushioned shoes, but the Tibial impact is virtually the same regardless of cushioning. (There is a very slight suggestion that more cushioned shoes have fractionally lower Tibial impact.)
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