Running shoe choice: stiff or flexible midsoles for performance?

When it comes to choosing a running shoe best matched to a runner’s needs, there are a number of factors to consider. Among these are anticipated training mileage, intensity and frequency. Equally important are the runner’s training background and personal/anthropomorphic characteristics, such as body mass, foot type (flat foot or high arch) and preferred running gait. These factors are critical because shoe choice needs to be properly matched to these characteristics in order to prevent or minimize the risk of injury.

Selecting a running shoe

Although there have been a number of recent studies comparing minimalist running shoes with conventional shoes (see this article as an example), there is still relatively little information available about the fundamentals of running shoe design and construction, and how this affect both performance and injury risk. This is no doubt largely due to the vast number of different shoe designs on the market, and the fact that even within a manufacturer’s model range, shoe design often changes year on year. Indeed, many runners report they feel overwhelmed by the ever-changing choices!

Typically, runners choose a running shoe on the basis of how it is marketed by the manufacturer (eg motion/pronation control, cushioning, suitability for racing etc), advice given by experienced staff at retail outlets and of course, how it feels on the foot when tried out for a quick test run! The design and construction of running shoes typically do not play a significant role in determining whether a shoe is suitable. This perhaps is understandable; if the shoe is marketed as fulfilling a functional brief and suitable for a certain type of runner, and if it feels good on the foot, why would its construction be relevant?

Running shoe construction and running economy

It turns out that when running performance is paramount, there are certain elements of running shoe construction that really do matter. One of the most important ways that shoe construction can influence performance is in relation to ‘running economy’. Running economy (sometimes called muscle economy or muscle efficiency) refers to how efficient the muscles involved in forward propulsion during running are in terms of oxygen usage during sub-maximal exercise (ie not flat out). The higher the running economy of muscles during exercise, the less oxygen is needed to propel a runner at a given sub-maximal speed.

Because endurance running events such as marathons and triathlons are not run flat out all the way, but at a sustainable sub-maximal speed, running economy actually plays a very important role in determining overall performance. Moreover, studies unequivocally demonstrate that elite runners have much higher levels of running muscle economy than their amateur or recreational counterparts. In other words – muscle economy and high levels of endurance go hand in hand.

To give you an idea of the importance of running economy, one study of collegiate cross-country team members discovered that just two factors – maximum aerobic capacity (VO2max) and running economy - could account for 92% of the variance in performance during an 8000m running race⁽¹⁾. Also, running economy has been successfully used as a key parameter (along with oxygen uptake figures) to predict marathon pace in elite runners⁽²⁾.

Furthermore, research investigating the greater performance levels of elite Eritrean runners compared to European runners possessing the same maximum aerobic capacity found that it was the superior running economy of the Eritreans that was entirely responsible for their better performances⁽³⁾. And just in case you run away with the idea that running economy is only important to highly trained or elite runners, bear in mind that research on recreational runners has found that running performance is very closely related to running economy⁽⁴⁾. In other words, higher levels of running economy in amateur runners is also very strongly associated with superior running performance.

Sole stiffness and running economy

One of the aspects of shoe construction that has attracted the recent attention of sports scientists is something called ‘longitudinal bending stiffness’, or LBS for short. LBS refers to how easy or hard it is to bend the midsole of a running shoe along its length (see figure 1). A high level of LBS means that more force and energy is required to bend the shoe along the long axis during the propulsion phase - where the runner pushes off with the rear leg, raising the heel while the toes remain in contact with the ground. Providing this bending is elastic in nature, greater LBS can act as an energy store, returning energy during the next stride. This in theory could help energy losses during footstrike, thereby helping runners to run faster and more efficiently.

Figure 1: Longitudinal bending stiffness