Effect of Midsole Compliance on Oxygen Consumption and Perception of Effort and Comfort during running

Introduction Advanced Footwear Technology (AFT) enhances running economy, which is partly attributed to midsole foam properties such as high resilience (the ability of a material to absorb and recover energy under elastic deformation) and high compliance (the tolerance of a material to deformation). While compliant midsoles are known to improve running economy over non-compliant ones, the impact of further increasing compliance in already compliant midsoles remains unclear. Also unclear is the effect of increased compliance on the perception of effort and comfort during running, as this might transform a feeling of cushioning into one of instability. This study aimed to address these issues by comparing three current AFT models with similar resilience but varying compliance: Nike AlphaFly 2 (NAF), Nike VaporFly 3 (NVF), and On Cloudboom Echo 3 (CBE), with the NAF being the most compliant shoe, and thus the one with the highest level of energy return (11.1J) compared with both the NVF (6.5 J) CBE (6.0 J). Methods Sixteen well-trained runners (age 31 ± 5 years, height 178 ± 9 cm, body mass 66 ± 10 kg, body fat 14 ± 4%, V̇O2peak 59 ± 4 ml･kg-1･min-1) performed, on different days, sub-maximal running for 6 min at 16 km·h-1 (80 ± 7% V̇O2peak) on a treadmill and a 400-m track. Treadmill tests included two runs each in NAF and NVF, while track tests included three runs each in NAF, NVF and CBE, with shoe order varied systematically. Gas exchange was continuously monitored while perceived exertion and comfort were rated post-run using a 100mm visual analogue scale. Spatiotemporal data, including impact loading, ground contact time, and cadence, were assessed using accelerometry. Results The NVF improved running economy compared with the NAF (-0.8 ± 0.3 ml･kg-1･min-1, P < 0.05) and CBE (-0.7 ± 0.2 ml･kg-1･min-1, P < 0.05). These findings were corroborated by lower heart rate and ventilation with NVF, present during both treadmill and overground running. However, there was no correlation between the shoe differences seen on both surfaces. No significant differences were found between the shoes concerning perceived effort and comfort. Participants experienced lower impact magnitudes in the NVF (5.4 ± 1.5 g) compared with the NAF (5.6 ± 1.5 g, P < 0.05) and CBE (5.5 ± 1.5 g, P < 0.05). No changes in spatiotemporal data were associated with the differences in running economy between the shoes. Discussion/Conclusion These findings indicate that improvements in running economy with AFT are not a matter of endlessly pursuing increased compliance and energy return. Furthermore, perception seems to be unaffected by higher midsole compliance when different shoe models are tested, suggesting that many other factors are at play. As perception of exertion did not differ between shoes despite noticeable differences in physiological variables, it remains to be seen whether such minute differences are relevant for performance, or whether perhaps longer trials are needed to detect differences in exertion.