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

Abstract

In an attempt to improve their distance-running performance, many athletes race with carbon fiber plates embedded in their 20 shoe soles. Accordingly, we sought to establish whether, and if so how, adding carbon fiber plates to shoes soles reduces athlete 21 aerobic energy expenditure during running (improves running economy). We tested 15 athletes during running at 3.5 m/s in four 22 footwear conditions that varied in shoe sole carbon fiber plate bending stiffness. For each condition, we quantified athlete aerobic 23 energy expenditure and performed biomechanics analyses, which included the use of ultrasound imaging to examine soleus muscle 24 dynamics in vivo. Overall, increased footwear bending stiffness lengthened ground contact time (p=0.048), but did not affect ankle 25 (p≥0.060), knee (p≥0.128), or hip (p≥0.076) joint angles or moments. Additionally, increased footwear bending stiffness did not affect 26 muscle activity (all seven measured leg muscles (p≥0.146)), stride averaged active soleus volume, (p=0.068) or aerobic power 27 (p=0.458) during running. Hence, footwear bending stiffness does not appear to alter the volume of aerobic energy consuming muscle 28 in the soleus, or any other leg muscle, during running. Therefore, adding carbon fiber plates to shoe soles slightly alters whole-body 29 and calf muscle biomechanics but does not improve running economy. 30 31 In competitive athletics, marginal differences distinguish champions from their competitors. For instance, if any of the top-five 2016 33Olympic women's marathon finishers ran 0.51% faster, they would have been crowned Olympic champion. Such miniscule 34 differences highlight the importance for athletes to optimize all factors that influence race performance. One way to further optimize 35 athletic performance is to don the best footwear. Using footwear that reduces athlete aerobic energy expenditure at a given running 36 speed (improves their running economy) can augment distance-running performance by decreasing their relative aerobic intensity. 1-3 37An established method of improving footwear to augment athlete distance-running performance is to reduce its mass. 1,2,4,5 Based on 38 literature values, if an aforementioned Olympic marathoner re-raced in shoes that were 100 g less than their original footwear, they 39 would have expended aerobic energy at an ~0.8% slower rate, 5 run the marathon ~0.56% faster, 6 and taken the gold medal back to 40 their home country. 42A longstanding footwear technology that has recently polarized the running community is the use of carbon fiber plates in shoe soles. 7 43Despite the rampant use of carbon fiber plates in athletics, 8-10 many seek to regulate the use of these plates in distance-running 44 footwear based on the notion that they provide wearers an 'unfair advantage' over competitors without such technology. 11 These 45 views persist even though it is inconclusive whether adding carbon fiber to shoe soles improves running economy. [12][13][14][15][16] To date, two 46 studies have reported that...

Increasing the midsole bending stiffness of shoes alters gastrocnemius medialis muscle function during running

“…In our study, there were no group effects of MBS on E run . This is in accordance with some studies 34,38,39 but contrary to other studies 11,22 . More importantly, however, when Control was compared to Best (i.e., plated condition, which resulted in lowest E run ), an average improvement in E run of 1.97% was observed in the best plated shoe condition.…”

“…Peak soleus force increases, however, were negligible. The effects of high running speeds on GM and soleus forces become evidently important when considering that participants in Beck et al 's study 34 were running at a constant speed of 3.5 m s −1 ; whereas, our participants ran at 90% of individual sLT, which was for only 3 out of 17 participants below 3.5 m s −1 .…”

“…This not only demonstrates that subject-specific performances responses exist when the stiffness of shoes is increased, but also that performance improvements are much greater (i.e., 2.5-fold) when the individual effects are taken into account compared to group differences. Additionally, it shows that although there may 11,22 or may not 34,38,39 be significant group differences (e.g., Control vs. Stiff vs.…”

“…A recently published article investigated the effects of running with differently stiff carbon fibre plates on soleus muscle fascicle dynamics and running economy 34 . The authors found no systematic differences in soleus fascicle pennation, force, length, velocity, or stride-average soleus active muscle volume between stiffness conditions.…”

“…The authors found no systematic differences in soleus fascicle pennation, force, length, velocity, or stride-average soleus active muscle volume between stiffness conditions. Beck et al 34 therefore concluded that running with carbon fibre plates may not improve running economy and long-distance running performance. There were some methodological differences between Beck et al 's study and ours that need to be considered when comparing the findings.…”