## 4 Discussion
(1998), an ICC of 0.90 may be considered the minimum level acceptable to be used in individuals [46]. The relatively reduced reliability in HVA seems to be a common issue among measurement methods, as previous studies also reported relatively low reliability for this parameter [17]. Other studies reported low to moderate reliability for HVA. Use of goniometry, for instance, had moderate values (ICC = 0.81) [47], whereas footprint analysis for the foot angle (not specifically HVA) showed low ICC values (0.33-0.78) [48]. The reliability of the SIBPAIR-based measurement protocol for HVA was comparable to that using X-ray imaging (ICC = 0.89) and very close to the minimum level of 0.9 for use in individuals, indicating a significant improvement over most existing non-ionizing measurement methods. For parameters that were directly defined based on 2D footprint or pressure distributions in the literature, namely the three arch indices (AI, SAI, and CSI), the current approach results also gave high intra-rater, inter-rater, and inter-session reliability with ICC values greater than 0.90 and low SEM. These results were comparable with most previously published methods and were even better than some methods. The AI measured using footprints and plantar pressure distributions was reported to have high intra-rater (ICC = 0.96-0.99) and inter-session (ICC = 0.94) reliability [11; 49; 50], while high intra-rater (ICC = 0.96) and moderate inter-rater (ICC = 0.80) reliability was reported for SAI [51; 52], and the corresponding ICC values for the CSI were 0.99 and 0.79, respectively [51]. These previous studies showed that based on pressure platform and footprint data, AI could be a highly reliable parameter, but the SAI and CSI were only moderately reliable when measured by different raters, and tended to be underestimated [51]. The current results show that the SIBPAIR-based method overcomes the limitations of existing methods based on footprint or plantar pressure, achieving high measurement reliability for all the sole parameters considered in the current study. The current study reports the test-retest reliability of a new experimental protocol based on the SIBPAIR method implemented on the FAST machine. The results showed that the new method was capable of giving accurate and reliable measurements regardless of the rater's experience or time of measurement. It will be useful for foot assessment and subsequent applications, such as design and manufacture of customized orthoses or shoes. It may also be useful for the study of morphological differences between dominant and non-dominate feet. The current study was limited to normal foot morphology. Further studies may include a reliability assessment on subjects with a greater range of foot deformities, e.g., severe hallux varus, clubfoot, etc. Given the high reliability and low SEM for most of the parameters, a direct validation of the measurements against measurements made by using medical images will be needed for future applications that required accurate data for decision-making, such as in a clinical setting. Further studies may also include more parameters based on the reconstructed weight-bearing 3D sole surface measured by the current approach, such as the distances of the first and fifth metatarsophalangeal joints to the sustentaculum tali, great toe height, distal end of the first toe and instep height [15; 16; 53; 54], and may determine their test-retest reliabilities. One senior and one junior rater were used in the current study following previous protocols [35; 36; 37]. Further studies may include raters with different levels of experience for a more complete assessment of the effects of rater experience.