Reliability of the Dynamic Foot Index for Observational Assessment of Foot Motion During Gait

Reliable clinical tools for observational assessment of dynamic foot motion during gait are limited. Although three-dimensional motion analysis is considered the reference standard, its cost and complexity restrict routine clinical use. The Dynamic Foot Index (DFI) is a simple, low-cost observational tool designed to assess foot motion during gait.

This methodological study investigated the intra- and inter-rater reliability of the Dynamic Foot Index (DFI) in healthy adults. Forty-nine participants (98 feet) walked barefoot on a treadmill at a self-selected speed while being recorded from a posterior view using a smartphone camera. Two experienced physical therapists independently analyzed video recordings of one complete stride per limb, classifying foot motion across three stance-phase subphases (loading response, midstance, and push-off) using a three-point ordinal scale. Intra-rater reliability was assessed after a 30-day interval. Weighted Kappa coefficients with 95% confidence intervals and absolute percentage agreement were calculated.

Intra-rater reliability ranged from moderate to substantial across gait subphases (weighted Kappa = 0.54-0.80), with higher agreement observed during loading response and push-off. Inter-rater reliability ranged from fair to substantial (weighted Kappa = 0.28-0.68), with lower values during midstance. Absolute agreement ranged from 65% to 92%, depending on the subphase and limb assessed.

The Dynamic Foot Index (DFI) demonstrated moderate to substantial intra-rater reliability and variable inter-rater reliability across stance-phase subphases in healthy adults. These findings support the use of the Dynamic Foot Index (DFI) as a clinically feasible observational tool for longitudinal assessment of foot motion during gait, particularly when applied by the same evaluator.