Nike AlphaFly. It’s not the carbon plate. New Research 2020.

A study has been released (Feb 2020, pre-publication) that looks into the physics and physiology
of carbon fiber plates in running shoes;  “Adding carbon fiber to shoe soles does not improve
running economy: a muscle-level explanation.”  (Full study HERE).



The goal of any running shoe manufacture is to help an athlete reduce the energetic cost of
running at a given speed. I.e improve running economy. Traditionally, shoe manufactures did
this by making shoes ultra-light. However, over the last few years, we’ve seen the introduction of
monster truck shoes with stacks of cushioning and carbon fiber plates.

Carbon Fiber Plated Shoes

Despite numerous world records being broken since the introduction of the Nike VaporFly in 2017
we’re yet to see conclusive evidence if the carbon fibre plate adds a performance boost, as opposed
to the cushioning that surrounds it.

Nike VaporFly 4%

As at March 2020, there have been two studies showing an
improvement in running economy with carbon plates (0.8% and 1.1%), and now four showing no
improvement.


The theory goes that carbon fiber plate increases the stiffness of the shoe, and shifts ground contact
more to the forefoot creating a greater ground reaction force. (figure 2)

Plantar flexor muscle tendons

This isn’t all good news, because there needs to be an opposite reaction force otherwise your ankle would collapse. Therefore, the soleus (calf muscle) needs to work harder than in a traditional shoe and at greater metabolic cost due to the greater ground reaction force.

The Study

15 athletes ran fro 5min at 4:45min/km (7:39 min/mile) in four different conditions.

1. Adidas Adizero Adios BOOST 2 without a carbon fiber plate
2. Adidas Adizero Adios BOOST 2 with 0.8 mm thick carbon fiber plate 
3. Adidas Adizero Adios BOOST 2 with 1.6 mm thick carbon fiber plate
4. Adidas Adizero Adios BOOST 2 with 3.2 mm thick carbon fiber plate

There was no significant difference in Limb-joint Dynamics, Stride Kinematics and Ground Reaction Forces, Muscle-tendon Dynamics, Soleus Dynamics, or Running Economy.

Discussion

Despite a 6.4-fold increase in footwear bending stiffness, the participants ran with nearly identical body, limb-joint, and calf muscle mechanics and elicited non-different running economy values.

If footwear bending stiffness does not affect running economy, why does wearing Nike footwear with carbon fiber plates embedded in their midsole improve running economy compared to wearing Adidas footwear? Nike VaporFly soles are 35-62% taller (depending on midsole location), midsole foam is roughly half as stiff (softer), and its hysteresis is 11.1% less during vertical loading and unloading (it’s bouncier).

Because both midsole foam stiffness and bouncy are associated with more economical running, Nike footwear may elicit superior running economy values than Adidas footwear due to their relatively compliant and resilient midsole foam – not increased bending stiffness from a carbon fiber plate.