Effects of carbon fiber insole on lower-extremity muscle activation and wearing comfort during treadmill running

Background and Objective: Although the role of shoes on sports performance and injury has been extensively examined, only a few studies investigated the effects from insoles. Recent studies on carbon fiber insoles (CFI) on athletic performance reported that CFI could improve sports performance by reducing energy loss and increasing energy return. However, there are scarce reports on the effects of CFI on muscle fatigue and wearing comfort. While stiffer CFI insoles are superior in energy loss reduction and energy return, they could increase more muscle activation to absorb the shock or provide more propulsive force to push CFI, leading to increased muscle fatigue and discomfort. Therefore, this study aims to evaluate the effect of CFI on lower-extremity muscle activation and wearing comfort during treadmill running. Methods: Three types of insoles were compared, namely CFI, CFI with cushioning (CFIC), and a benchmark commercial insole (COM). Fifteen healthy young men participated in the experiment. Each participant wore the same sports shoe, three different insoles in a random order, and ran on a treadmill at a speed of 10 km/h for 5 minutes. Surface electromyography signals of four lower-extremity muscles (Rectus femoris, Tibialis anterior, Biceps femoris, and Gastrocnemius medialis) were recorded in real-time for measuring muscle activation. After completing a trial run with each experimental insole, the participants provided their subjective ratings on perceived insole stiffness, energy support, overall comfort, and fatigue. Analysis of variance (ANOVA) and post-hoc grouping analysis were conducted to statistically evaluate the effects from three different insoles. Paired t-test was performed to compare CFI and CFIC and explore any cushioning effect. Results: In terms of lower-extremity muscle activation, even though CFIC and COM showed no significant difference for all four lower-extremity muscles, CFI induced a marginally significant increase of 1.5% on Gastrocnemius medialis (p = 0.063) and a significant reduction of 0.7% on rectus femoris (p=0.011) than COM. For the subjective ratings, both CFI and CFIC were significantly stiffer than COM (p < 0.001), but there were no significant differences in overall comfort, energy support, and fatigue. Compared with CFI, CFIC significantly reduced Gastrocnemius medialis muscle usage by 2.1% (p = 0.012) and was marginally less stiff (p = 0.102).Conclusions: This preliminary study showed that the carbon fiber insole CFI induced higher calf muscle usage and was perceived to be stiffer during treadmill running, which could contribute to provide the propulsive force for better sports performance. Carbon fiber insole with cushioning (CFIC) can help to relieve muscular fatigue. Further research should be conducted to examine the carbon insole effects on sports performance and long-term muscle activation and perceived feelings.