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Abstract

Carbon fiber plate insoles (CFPI) are designed to increase footwear stiffness and may improve elastic energy utilization during human locomotion. However, most research has focused on CFPI effects in running and sprinting, particularly endurance running, with limited evidence examining their impact on plyometric performance that relies heavily on the stretch-shortening cycle (SSC), a key mechanism underlying many sports movements.  Because SSC involves a rapid eccentric-to-concentric transition that stores and releases elastic energy, the drop jump provides a practical model to assess SSC efficiency with and without CFPI. PURPOSE: To examine the effects of CFPI on SSC efficiency and jump performance during drop jumps in recreationally trained adults. METHODS: Eight participants (n = 8, age = 25.5 ± 6.7 years, height = 1.75 ± 0.04 m, body mass = 76.52 ± 8.74 kg.) performed a series of drop jump trials under two conditions: standard footwear (control) and footwear with CFPI (VKTRY, Milford, CT). Each participant performed five drop jumps from 18-inch (approx. 46 cm) box onto a force platform (Bertec Corp, Columbus, OH, USA), sampled at 1000 Hz. Best two trials among five trials for each condition were utilized for further statistical analysis using SPSS version 28 (IBM Corp, Armonk, NY). A paired t-test was used to compare peak vertical ground reaction force (GRFv; N), ground contact time (GCT; s), jump height (JH; m), reactive strength index (RSI; m/s), vertical leg stiffness (VLS; kN/m), and jump momentum (JM; Ns). RESULTS: GCT was significantly lower when jumping with CFPI (0.36 ± 0.13 s) compared with control (0.40 ± 0.13 s; p = 0.04). There were no significant differences in either the first or second landing peak GRFv (p = 0.65 and p = 0.71, respectively), jump height (0.34 ± 0.09 m vs. 0.35 ± 0.10 m; p = 0.92), VLS (16.66 ± 9.70 kN/m vs. 17.82 ± 7.63 kN/m; p = 0.52), or jump momentum (405.98 ± 64.50 Ns vs. 404.58 ± 57.06 Ns; p = 0.77). In addition, RSI showed a non-significant trend favoring CFPI (1.10 ± 0.53 m/s vs. 0.98 ± 0.45 m/s; p = 0.099). CONCLUSION: CFPI significantly reduced ground contact time during drop jumps without changing jump height, peak GRFv, or vertical leg stiffness, suggesting a faster amortization-phase transition (i.e., improved rebound timing) rather than increased force or power output. Furthermore, RSI showed a non-significant trend favoring CFPI, further supporting a possible efficiency-related effect. Future studies should examine trained athletes and evaluate whether different plate stiffness and material properties influence plyometric performance.

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