Running shoe technology: the cost of carbon

Improving RE

Developing high levels of RE requires an efficient running technique honed through years of training. However, innate qualities such as low body mass and high levels of tendon stiffness (which help with energy return between strides) are also important. In addition, research over the past 20 years or so has shown that strength training can significantly boost running economy (see this article for a fuller discussion)^(6,7). All of these of course depend on the runner, but there’s something else to factor in - running shoe design and construction.

The mechanical properties of a running shoe can and do impact the RE of the runner using them. For example, minimalist shoes (ie with reduced shoe mass and heel drop) have been shown to yield significant improvements in RE (around 1-4%) compared to conventional heavier shoes with thicker EVA (ethylene vinyl acetate) midsoles⁽⁸⁾. Another design factor that affects RE is a shoe’s midsole characteristics. Wunsch et al. showed that a leaf spring-structured midsole improved RE by around 1%⁽⁹⁾, while a midsole material composed of a stiff thermoplastic polyurethane (TPU) has been used to enhance energy return during running – potentially increasing RE by as much as 4.1%^(10,11).

Shoe stiffness in the real world

High levels of RE are desirable for excellent running performance but not all runners are naturally efficient, especially those who are novices or amateurs. So given the above, should all runners seek out shoes that enhance RE – for example shoes with stiff midsoles that give high levels of energy return? The intuitive answer is ‘yes’ but there are some questions that remain to be answered. One in particular concerns the real-world use of these high-stiffness shoes. That’s because while they tend to produce RE benefits when tested on the treadmill by highly trained/elite runners under controlled conditions in a laboratory, much of the data has been gathered from short-duration running tests, lasting just a few minutes or so. A valid question therefore is whether these shoes automatically translate in better performance when used over longer distances by amateur rather than elite runners?

New research

To try and answer this particular question, a team of French researchers has carried out a study on the stiffness of running shoe midsoles, and the effect of RE, running biomechanics, and fatigue over the half-marathon distance⁽¹²⁾. Published earlier this month in the journal ‘Medicine and Science in Sports and Exercise’, the researchers set out to investigate how a shoe with a high degree of the longitudinal bending stiffness (LBS) impacted performance of well-trained but amateur runners in a half marathon trial compared to a standard shoe with average LBS.

LBS refers to the stiffness in the forward-backward direction (ie not lateral stiffness found in stability shoes). A shoe with high LBS requires more force to bend the shoe as the foot pushes off the ground, but less energy is lost during footstrike. In this study, the high-LBS shoe was constructed with a stiff carbon plate insert in the midsole, which is a lighter alternative to TPU, albeit more expensive. The normal LBS shoe was of the same overall design but had no such insert, with the midsole being constructed using conventional methods.

What they did

Thirteen well-trained, amateur male runners (with half-marathon time averaging around 1 hour 40 minutes) performed two half-marathons on two separate occasions. On one occasion, they wore the high-LBS shoes with a carbon plate insert, while on the other, they wore standard-LBS shoes. Each runner performed both half marathons at the same relative pace, which was matched individually to their own fitness level. This pace was 95% of the speed of their second ventilatory threshold. In plain English, the runners ran at a pace just below the point where the effort start to feel really hard, and where breathing rate increases markedly (ie just remaining in the zone where a prolonged effort is possible).

Before and after both the half-marathons, tests were performed to see whether/how the shoes had impacted aspects of performance over the 13.1 miles. These tests were:

·        RE, measured at a speed of 12kmh.

·        The amount of force developed by the ankle plantarflexor muscles (muscles in the feet used to push heel off the ground during running gait).

Meanwhile, observations were also carried out during both half-marathons. These were:

·        Running kinematics (to see whether the running biomechanics had changed to any degree).

·        RE, measured at whatever pace each runner was maintaining during the half marathon.

·        Perceived comfort of the shoes.

The results from the two shoe trials were then compared to see what differences emerged.

What they found

The first finding was that (as predicted), running in the high-LBS shoe during the pre- and post- half-marathon testing at 12kmh resulted in better RE; RE was improved by around 1% when the high-LBS shoes were worn compared to the standard-LBS shoes. However, when running over the half-marathon distance itself, the apparent RE advantage of the high-LBS shoes disappeared. An analysis of running biomechanics found that foot contact time with the ground increased by 3% when the high-LBS shoes were worn, even though the amount of foot flexion prior to push off (ie height of the heel above the ground) was diminished by around 9%.

Moreover, the runners also reported that the high-LBS shoe was perceived as less comfortable and more fatiguing than the standard shoe – not just as the distance increased, but at all times during the half marathon. This subjective observation was confirmed by post- half-marathon testing; when the high-LBS shoe was worn, the amount of force developed by the ankle plantarflexor muscles had fallen by 20.0%, whereas wearing the standard shoes resulted in a decline of only 13.3%. In their summing up, the authors stated that their results suggest the addition of carbon plates alone to increase LBS did not necessarily provide a significant advantage for half-marathon performance.

Practical implications

These findings call into question the assumption that increasing LBS in order to improve RE automatically leads to performance gains. While it’s true that adding the curved carbon plates in the running shoes slightly improved RE during short running bouts at low intensity (ie during testing), this improvement was not observed during a half-marathon run. The researchers noted that the increased fatigue in the foot muscles when using the high-LBS shoes didn’t seem to change running gait significantly, but did reduce comfort, which may have produced subtle variations in running biomechanics – enough to wipe out any RE gains from the plates.

What does this mean for amateur or recreational runners choosing a new pair of running shoes? The main conclusion to draw is that runners shouldn’t be seduced in believing that high-LBS stiffness shoes are always better, or that if a shoe has fancy carbon plates in the midsole, it’s going to definitely result in performance gains. Yes, high-LBS shoes can result in better RE during lab testing, but there’s far more to shoe design than simply inserting a stiff carbon plate into the midsole. Besides, there are other ways of increasing energy return in running shoes – eg the use of more elastic-like compounds in key areas of the shoe. In particular, the issue of shoe comfort shouldn’t be neglected; the above study suggests that poor comfort may result in biomechanical adjustments to your running style, which is detrimental to RE.

Finally of course, runners should never neglect the importance of injury prevention. While good levels of energy return are desirable, it is imperative to choose shoes that best accommodate your individual running style so that future injuries can be avoided. No matter how high-tech a shoes is, your running performance will suffer greatly if you need to take time off to recovery from injury because that shoe wasn’t suitable for your own needs. And no amount of increased LBS will be able to compensate for lost training time!

References

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