Associations Between the Chippaux–Smirak Index and Clinical Foot Assessments in Young Adults: A Cross-Sectional Study

Background/Objectives: The Chippaux–Smirak Index (CSI) measures relative midfoot plantar contact, whereas the six-item Foot Posture Index (FPI-6), navicular drop test (NDT), and resting calcaneal stance position (RCSP) assess static foot posture, change in navicular height, and calcaneal alignment, respectively. This study examined associations among these measures, agreement between CSI and FPI-6 classifications, and measurement reliability in young adults. Methods: This cross-sectional study included 350 young adults, with both feet assessed (700 feet). Two orthotists independently measured CSI, FPI-6, NDT, and RCSP. Generalised estimating equation models accounted for correlations between feet and were adjusted for age, sex, body mass index, and foot side. Agreement was assessed using exact percentage agreement and linear weighted Cohen’s kappa, and reliability using intraclass correlation coefficients (ICCs). Results: After adjustment, each increase of 10 percentage points in CSI was associated with a 2.141-point-higher FPI-6 total score, a 2.660 mm greater navicular drop, and an RCSP that was 3.240° more valgus (all p < 0.001). CSI and grouped FPI-6 classifications agreed for 82.9% of feet (linear weighted κ = 0.664, 95% CI 0.595–0.727). Single-measure interrater ICCs ranged from 0.644 for NDT to 0.884 for CSI. Digital remeasurement of CSI demonstrated excellent intrarater reliability (ICC(A,1) = 0.989). Conclusions: Higher CSI values were associated with a more pronated static foot posture, greater navicular drop, and a more valgus RCSP. Incomplete agreement between CSI and FPI-6 classifications indicates that these methods provide related but distinct information and should be interpreted according to the specific foot characteristic assessed.