Within-day and between-day reliability of a FBG-based smart sock system for measuring active toe flexion displacement of the hallux

This study examined the test-retest reliability of hallux flexion displacement measured using a smart sock system with embedded fiber Bragg grating (FBG) sensors. Thirty female participants consisted of 15 hallux valgus (HV) patients and 15 control participants were recruited. Maximum active hallux flexion displacement was measured twice on each participant in the first visit; the same procedures were repeated 7 days later. Intraclass correlation coefficients (ICC2,1) and standard error of measurement (SEM) were applied to test within-day and between-day reliability. Paired-samples T-test was performed to compare the displacements between trials. Results showed almost perfect within-day reliability for both HV and control groups (ICC = .984 and .977, respectively) with small SEM (both 0.5 cm). However, fair to moderate between-day reliability was found (.323 and .438, respectively). Significant differences were found between repeated measurements taken on the same day (mean difference = 0.3 cm, p = .023) and on different days (mean difference = 1.6 cm, p = .027), though the effect size was small. The poorer between-day reliability is likely due to the inconsistency in fitting the sock onto the foot. Future optimization of the prototype design is called for to improve the fitting consistency of wearable sensors onto patients.