Why Barefoot Runners Never Win

Andy, I' m short on time right now, so I'll be brief. You always want the shoe to be as light as possible to reduce metabolic cost. The literature suggests that there is a range of surface stiffness values (Ksurf) over which the metabolic cost of running has been shown to be reduced. Ksurf is defined by stiffness of the footwear and the stiffness of the ground analogous to springs in series. So, if you wanted to maintain a fairly constant Ksurf, you'd decrease the shoe sole stiffness on harder terrain and increase it on softer terrain. Ultimately, you are probably looking to match the natural frequency of the CoM to the surface.

 

OK I think I follow but one last question please.

If you had 2 mid-soles of different elastic modulus, but otherwise identical and the environmental surface stiffness is kept constant would the one with the larger modulus be more efficient?

 

It really depends on the stiffness of the environmental surface, but generally reducing the surface stiffness (Ksurf) appears to decrease metabolic cost, so in answer to your question, no.

 

Andy:

Yes, all the research seems to say that for every 100 grams of mass added to the feet, the metabolic cost of running increases by approximately 1%. In addition, I have long contended that the increase in metabolic cost seen with less mass on the foot (i.e. less mass at the end of the lower extremity) is most likely due to the increase in moment of inertia of the lower extremity that comes with mass added to the foot. However, to my knowledge, this hypothesis has not been researched before.

During the forward recovery phase of running [the forward recovery phase of running is the equivalent of the swing phase of walking where the foot is off the ground and rapidly being swung forward] any increase in moment of inertia of the lower extremity will increase the muscular work required to accelerate the limb forward which, in turn, will increase the metabolic cost of running.

As an aside, this is exactly why Oscar Pistorius had such an unfair advantage over his able-bodied opponents in the Olympics. The mass of his carbon fiber blades was less than half that of an able bodied runner's legs which decreased the moment of inertia of his lower extremity. The result was that Pistorius could shorten his forward recovery phase signficantly and greatly increase his stride frequency for probably less metabolic energy than his able-bodied opponents. In this way, the barefoot running debate, with no mass on the foot, and the Oscar Pistorius vs able-bodied runner debate, with a 58% reduction in "leg" mass, are very similar when it comes to the discussion of moment of inertia of the lower extremity and metabolic efficiency of running.

As far as surface stiffness vs running shoe midsole stiffness for running on harder surfaces such as asphalt, concrete or hard-packed dirt, the optimum running shoe midsole will be one that is light and springy. For running on grass or soft trails with low surface stiffness, running with a thinner soled shoe or even barefoot would likely be the most efficient, but the runner would need to have tough enough plantar foot to run barefoot in these conditions. Finally, when running in sand with much lower surface stiffness, there is no use to having a shoe other than to protect the foot from obstacles. Therefore, on sand or on the bearch, barefoot would again likely be the most efficient metabolically.

One last point, when considering running shoe midsole design, including thickness and compliance, in order to optimize running efficiency, body weight is a huge factor to consider, and not just surface stiffness. A lighter runner will simply not compress the "spring" of the midsole as much as a heavier runner so lighter runners will require less thick and more compliant shoe soles. However, a heavier runner, in too thin or too compliant of a running shoe midsole, will "bottom out" the "midsole spring" which, in effect, will make the midsole harder and less compliant for them. Therefore, the mass of the runner should always be taken into account when recommending the optimum running shoe for them.

Hope this helps.:drinks