BRINGING SCIENCE TO TREATMENT

Injury prevention: shoe facts, shoe fiction

How does running shoe design and construction affect injury risk, and how does this affect advice on shoe choices that clinicians give to their injured runners? Andrew Hamilton looks at what the recent research has to say.

Running is one of the most popular sports and fitness activities across the globe. However, while it is a simple sport and one that is readily accessible, it carries a significant risk of injury. Reported injury rates vary, but a comprehensive review study estimated rates of running-related injuries in athletes between 19% and 79%(1). Although there has been plenty of research investigating training-related risk factors like distance, frequency, training intensity, the knowledge about footwear design and related risk factors is still quite limited.

Shoe construction

When it comes to an analysis of running injury risk, factors such as training mileage, intensity, and frequency are relatively easy to study; these are well-defined parameters, which are easily compared across different studies and populations. Shoe construction and design, however, is a different matter. Although there have been several recent studies comparing minimalist running shoes with conventional shoes (a topic for another time), there is still relatively little information about the fundamentals of running shoe design and injury risk.

This shortage of data is primarily due to the vast number of different shoe designs on the market. Compounded with the fact that within a manufacturer’s model range, shoe design often changes from year to year. Indeed, many runners report they feel overwhelmed by the possible choices. Adding to the confusion are the proclamations by shoe manufacturers of better stability and motion control, lower impact forces, and more efficient running gait, while the plain truth is that many running (and walking) related injuries still occur. Moreover, despite the changing technology, running injury incidence has NOT changed noticeably over the last few decades(2). So what factors in shoe construction are relevant for injury risk?


Cushioning and impact forces

Well-cushioned shoes are commonly believed to reduce injury risk. The theory is that absorbing and dissipating forces generated by footstrike (which can easily be three times the runner’s body weight), reduces the impact forces, and also the injury risk, transmitted to the lower limb. But how true is this? In one review study on this topic, Canadian scientists analyzed data accumulated over 25 years on impact forces and foot pronation and the development of running-related injuries(3).

They found many contradictions in the experimental results and the established theories and concluded:

Theoretical, experimental and epidemiological evidence on impact forces shows that impact forces are NOT important factors in the development of chronic and/or acute running-related injuries.”

Instead, they postulated that impact forces during footstrike serve as input signals, which help produce ‘muscle tuning’ shortly before the next contact with the ground. This tuning helps to minimize soft-tissue vibration and reduce joint and tendon loading. In other words, increasing cushioning and lessening footstrike impact interferes with the body’s ability to alter and adapt its neuromuscular response and maintain its preferred joint movement path for a given movement task.

Further evidence that increased heel/midsole cushioning is not a panacea for reducing running injury risk comes from a Finnish study published last year (2018)(4). Researchers compared impact loading and ‘spring-like’ mechanics of running using a conventional running shoe and a highly-cushioned maximalist shoe at two training speeds (10kmh and 14.5kmh). What they discovered was that the highly-cushioned maximalist shoes indeed altered the spring-like running mechanics – but amplified rather than attenuated impact loading. This surprising outcome was more pronounced at 14.5kmh, where ground reaction force impact peak and loading rates were 10.7% and 12.3% greater respectively in the maximalist shoe compared to the conventional shoe. The researchers attributed the higher impact loading with the maximalist shoes to a stiffer leg during landing compared to that of running with the traditional shoes, which explains why shoes with more cushioning do not protect against impact-related running injuries.

Cushioning and plantar pressure

The evidence suggests that extra heel and midsole cushioning doesn’t reduce general running injury risk. However, is there an argument for increased cushioning in conditions such as plantar fasciitis, which is aggravated by pressure applied to the foot sole? In a study by Australian scientists, researchers investigated in-shoe plantar pressure loading and comfort in 22 athletes while running in two popular neutral-motion cushioned running shoes recommended for athletes with cavus feet – the ‘Asics Nimbus 6’ and the ‘Brooks Glycerin 3’(5).

Compared with a control shoe, both the cushioned running shoes significantly reduced peak pressure and the total pressure loading during each stride (by 17% to 33%). However, pressure reduction was not uniform; the Brooks Glycerin most effectively reduced pressure beneath the whole foot and forefoot while the Asics Nimbus most effectively reduced rear foot pressure. Also, both of these shoes reduced force at the forefoot by 6% and increased it at the midfoot. These results suggest that neutral-cushioned running shoes are effective at reducing plantar pressures in athletes with cavus feet, justifying their general recommendation over standard shoes. Though, the regional differences in measured pressure reduction indicate that neutral-cushioned running shoe recommendation should shift from being categorical to being based on the location of any injury or elevated plantar pressure.

Heel height and injury

Running shoes are available in a wide range of heel-to-toe drops (the height difference between the forward-most and rear-most parts of the inside of the shoe, where the foot rests). While this heel-to-toe drop influences the footstrike pattern in runners, there’s been little research on its effect on injury risk. However, a 2016 study tracked 553 recreational runners for six months and sought to answer the following(6):

  1. Does the heel-to-toe drop of standard cushioned running shoes influence injury risk?
  2. Is there a relationship between training volume, heel-to-toe drop, and injury risk?

The runners were divided into three groups; the runners in each of these three groups were given shoes with different heel-to-toe drops to run in: group A – a 10mm drop, group B – a 6mm drop and group C – a 0mm drop (ie level from heel to toe). The results showed that for more frequent runners, the 10mm heel-to-toe drop shoes were the safest; with the 0mm and 6mm heel-to-toe drop shoes increasing injury risk. However, in occasional runners averaging less than one training session per week, this risk was reversed (0mm or 6mm heel-to-toe drop being safest).

Lateral stiffness

An important but often overlooked factor in shoe design is lateral stiffness – how resistant the shoe is to torsional twisting around the toe-to-heel axis (see figure 1). A very recent study of 1025 military cadets investigated the relationship between torsional stiffness and lower extremity musculoskeletal injury(7). The study groups wore shoes with minimal, moderate, or extreme torsional stiffness. The researchers tracked the cadets for nine weeks and found that wearing shoes with a moderate level of torsional stiffness was significantly less likely to cause any lower extremity injury (49%), including an overuse lower extremity injury (52%). These findings suggest that shoes need to have enough torsional stiffness to accommodate foot movement while preventing excessive movement (ie, too little stiffness). Since lateral stiffness declines as shoes wear, runners should be aware that high-mileage shoes might significantly increase injury risk.

Figure 1: Depiction of lateral (torsional) stiffness in running shoes

Lower lateral stiffness increases ease of rotation in the direction of arrows.


Midsole bending stiffness and metatarsal/plantar loading

In addition to torsional stiffness, midsole stiffness (the ease with which the heel can lift while toes remain on the ground) also affects injury risk. Canadian researchers investigated if running in a shoe with more midsole stiffness redistributes lower-limb joint load (compared to a control shoe)(8). Thirteen recreational runners ran on a treadmill at 7.8mph under two shoe conditions while motion capture and force platform data were collected:

  1. Shod in commercially available running shoes.
  2. Shod in the same shoes with carbon fiber inserts to increase midsole bending stiffness.

The results showed that running in the stiff condition (with carbon fiber inserts) resulted in significantly heavier loading at the metatarsophalangeal (MTP) joint, while reducing loading at the knee. The larger MTP joint plantar flexion moment occurred as a result of increased vertical ground reaction force at the instant of peak power, along with an earlier onset of MTP joint plantar flexion velocity. The obvious implication is that runners with a history of MTP joint or plantar injury should avoid shoes with high levels of midsole stiffness.

Closure mechanism/lacing

Another commonly overlooked but essential factor is the shoe closure mechanism. A Belgium study found that runners seeking to avoid an injury should select shoes that fit well with a suitable closure mechanism, which ensures a comfortable and snug fit(9). This factor was deemed MORE critical for injury prevention than selecting a running shoe from a gait analysis or by relying on recommendations from shoe store staff!

What constitutes the best closure mechanism? Chinese researchers compared similarly constructed running shoes with either conventional shoelaces or an elastic/Velcro fastening system(10). They found that the elastic-covered running shoes had a lower perceived comfort rating in terms of shoe length, width, heel cup fitting, and forefoot cushioning. The elastic-covered running shoes also recorded higher peak plantar pressure on the lateral side of the forefoot and larger maximum rear-foot pronation. By contrast, the laced shoes helped runners obtain a better shoe fit, increased comfort, and decreased maximum pronation and plantar pressure.

The lacing pattern also influences comfort and stability. Research shows that compared with a regular six-eyelet technique (or using even fewer eyelets), seven-eyelet lacing results in a significant enhancement of perceived stability without differences in perceived comfort(11). This is relevant as many runners fail to utilize all the available eyelets when lacing their shoes (see figure 2). Encourage runners to experiment with different lacing patterns to find one that feels most snug and comfortable.

Figure 2: Seven-eyelet lacing

Many runners do not use all the available eyelets for lacing (as shown here – 6 rather than 7).


Last but not least – regular renewal!

Materials in running shoes deteriorate over time, which means that the ability of shoes to optimally control foot motion decreases as the miles add up. However, most runners are unable to detect when to call time on their running shoes, with apparent implications for injury risk. An excellent study on this topic investigated whether the forces experienced by a runner’s foot increase after a certain mileage (400 miles/640kms) and whether runners were able to detect changes in heel cushioning properties from subjective ‘feel’(12).

Researchers measured the in-shoe plantar pressures and vertical forces of fifteen runners’ shoes for when brand new, and again after 160, 320, 480, and 640kms. After running 480km, the shoes provided a 16% to 33% lower amount of cushioning in the heel region of the midsole. The durometer measurements concurred, showing a corresponding decrease in cushioning ability (increase in hardness). However, the runners were NOT able to detect these changes – ie, the shoes “felt the same,” even though they had lost significant amounts of cushioning capacity(12).

With that in mind, clinicians should educate runners to replace their shoes on a regular ‘miles clocked up’ basis and not to rely on subjective feel to determine when shoes need replacing. Runners must keep a tally of mileage over time. For an objective measure of wear, recommend the purchase of a durometer (used to measure midsole hardness/cushioning as well as other aspects of wear such as torsional and midsole stiffness).


Summary of take-home points for clinicians

It’s important for clinicians to know the facts about shoe construction and injury risk because a large percentage of consumers believe correct shoe choice prevents injury(13). Athletes will rely on your expertise to choose the best shoes. Use the opportunity to educate them on the best selection for their needs and other injury prevention strategies such as appropriate training load and exercise. Some points to remember:

  • Increased cushioning does NOT relate to reduced injury risk and should not be the ‘go-to’ option for runners seeking reduced injury risk. The exception is for runners suffering from plantar fasciitis and related conditions; however, any increased cushioning needs to reduce pressure in the appropriate region of the sole.
  • A heel-to-toe drop of around 10mm reduces injury risk in regular runners, but a flatter shoe may be more suitable for occasional runners.
  • Lateral torsional stiffness should be moderate – neither excessive nor too low.
  • Those with a history of plantar or MTP injury should avoid shoes with high levels of midsole stiffness.
  • Lacing is the best closure mechanism, and seven-eyelet lacing is recommended. Runners should be encouraged to experiment with lacing patterns to maximize comfort and stability.
  • Regular shoe renewal is a must – based on mileage, not subjective feel.

References

  1. Br J Sports Med. 2007;41(8):469–480
  2. Mil Med. 2015 Mar; 180(3):321-8
  3. Clin J Sport Med. 2001 Jan;11(1):2-9
  4. Sci Rep. 2018 Nov 30;8(1):17496
  5. Am J Sports Med. 2008; (36)11 2139-46
  6. American Journal of Sports Medicine 2016 44: 11, 2933-2940
  7. Am J Sports Med. 2019 Oct;47(12):2853-2862
  8. J Sci Med Sport. 2019 Nov;22(11):1272-1277
  9. J Foot Ankle Res. 2019 Aug 17;12:43
  10. J Sports Sci. 2011 Feb;29(4):373-9
  11. Res Sports Med. 2010 Jul;18(3):176-87
  12. Int J Sports Phys Ther. 2017 Aug;12(4):616-624
  13. Int J Environ Res Public Health. 2019;16:3766
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