Adding carbon fiber to shoe soles does not improve running economy: a muscle-level explanation

In an attempt to improve their distance-running performance, many athletes race with carbon fiber plates embedded in their shoe soles. Accordingly, we sought to establish whether, and if so how, adding carbon fiber plates to shoes soles reduces athlete aerobic energy expenditure during running (improves running economy). We tested 15 athletes during running at 3.5 m/s in four footwear conditions that varied in shoe sole carbon fiber plate bending stiffness. For each condition, we quantified athlete aerobic energy expenditure and performed biomechanics analyses, which included the use of ultrasound imaging to examine soleus muscle dynamics in vivo. Overall, increased footwear bending stiffness lengthened ground contact time (p=0.048), but did not affect ankle (p[≥]0.060), knee (p[≥]0.128), or hip (p[≥]0.076) joint angles or moments. Additionally, increased footwear bending stiffness did not affect muscle activity (all seven measured leg muscles (p[≥]0.146)), stride averaged active soleus volume, (p=0.068) or aerobic power (p=0.458) during running. Hence, footwear bending stiffness does not appear to alter the volume of aerobic energy consuming muscle in the soleus, or any other leg muscle, during running. Therefore, adding carbon fiber plates to shoe soles slightly alters whole-body and calf muscle biomechanics but does not improve running economy.