The current investigation is a complementary analysis of plantar pressure and comfort data obtained during gait analysis of various LWI designed for knee OA. A summary of the joint kinematic and kinetic outcomes for these insoles were reported in Tse et al. (12), which found standalone medial arch supports increased the KAM. Similar findings of increased KAM with medial arch supports have been reported (22,23), despite Hinman et al. (23) reporting non-significant increases in a small sample with large variability in KAM change. For this reason, standalone medial arch supports were excluded from the current analysis of plantar pressure outcomes. However, outcomes across all insole conditions are reported in Appendix 1 of supplementary materials.
2.1. Participants
A convenience sample of healthy adults from the university and surrounding community were recruited via electronic and print media and word of mouth. Exclusion criteria for study participation included any musculoskeletal injury or pain, orthotic insole use in the twelve months prior to physical screening, and any history of neurological conditions impairing gait. Ethics approval for this study was received from the institutional Clinical Research Ethics Board. All participants received written and verbal explanations of the details prior to providing written consent for study enrolment.
2.2. Orthotic Insoles
Four pairs of sulcus length orthotics were custom-fabricated for each participant, using three-dimensional laser volumetric casting by a Canadian Board-Certified Pedorthist (Fig. 1). Non-contoured insoles fabricated from ethyl-vinyl acetate foam (EVA) (Shore A stiffness 55) included a neutral 3mm flat control (FLAT) and 5° lateral wedge (WEDG). Two pairs of custom contoured arch support insoles were fabricated from the volumetric casts: (1) variable-stiffness (V-ARCH) was constructed with plastazote foam laterally (Shore A stiffness 70) and EVA medially (Shore A stiffness 20), (2) uniform-stiffness (U-ARCH) was constructed with EVA (Shore A stiffness 55). Two supported-LWI conditions were created by affixing each custom arch support to the top of the WEDG: WEDG+V-ARCH and WEDG+U-ARCH. All insoles were covered with a full-length piece of neoprene and secured into a standardized sandal during all walking trials (Fig. 1).
2.3. Procedure
All participants were randomly assigned a study limb of interest and fitted with standardized sandals to match their foot dimensions. The sandals had a neutral heel to toe drop, Velcro straps to secure them to the feet, and removeable footbeds, into which orthotic insoles could be fitted. Prior to collecting data during walking in each insole condition, participants were encouraged to acclimate to the insole and sandals by walking freely and resolving any fit abnormalities. The FLAT condition was tested first, and the five remaining insole conditions were systematically randomized for all participants. A walking trial was deemed successful if the walking speed was within 5% of the average walking speed during walking with FLAT (determined using photoelectric timing gates), and if the study limb foot struck the ground fully within the confines of a floor-embedded force platform. A minimum of five successful walking trials along a 10 m walkway were recorded in each insole condition.
2.4. Data Collection and Reduction
Plantar pressure was measured bilaterally using flexible shoe-embedded sensors (F-Scan, Tekscan, Boston, MA, USA) at 100 Hz. All sensors were trimmed to match the sandal size and secured to the neoprene topcover with double-side tape to ensure consistent sensor placement among insole conditions (Fig. 1). Each sensor detects pressure as an array of sensing units, with a surface area resolution of 0.258 cm2 per sensing unit. Only data collected during the stance phase of gait were processed and analyzed. For each calculated outcome, the average of five successful walking trials was calculated to represent its value in each insole condition.
Pressure data recorded by each sensing unit was filtered using a zero-lag 4th order low-pass Butterworth filter with a 25 Hz cut-off. For faulty sensing units that detected non-physiological pressure, its data were replaced with the time-series average of its eight neighbouring sensing units. A semi-automated masking program was used to segment the foot into seven plantar regions: hallux, forefoot (medial, central, lateral), midfoot, rearfoot (medial, lateral) (Fig. 2). All masks were visually checked, and any erroneous regional masks were manually corrected. For each plantar region, a representative time-series pressure signal was calculated as the average pressure of all active sensing units within the region. From the regional time-series pressure data, the discrete outcomes extracted for analysis included: peak pressure (kPa), pressure-time integral (kPa*sec), and time of peak pressure (% stance). The contact area in each plantar region was calculated as the sum of active sensing units in the region multiplied by the surface area per unit.
The medial-lateral pressure index (MLPI) represents the medial-lateral plantar pressure distribution with respect to the midline of the foot (heel centroid to 2nd toe) throughout stance phase (Fig. 2). For each frame of data, MLPI was calculated as the perpendicular distance between the centre of pressure (CoP) and the midline of the foot, normalized to the foot width (24). Positive and negative values indicate a CoP that is lateral or medial to the midline of the foot, respectively. For each walking trial, the mean MLPI and area under the MLPI curve (AUC) during the first and second halves of stance phase were calculated separately.
Participants rated insole comfort on a 15-point global rating of change scale (25). Using the FLAT insole condition as the reference comparator, +7 and -7 represented a change in comfort that was maximally “improved” or “reduced”, respectively. Zero represented no change in comfort (equivalent to FLAT). Following completion of walking trials in all insole conditions, participants selected one insole as their most preferred.
2.5. Statistical Analysis
Normality was evaluated via visual inspection of histograms and supplemented with a Shapiro-Wilk test. Homogeneity of variance was evaluated via Mauchly’s test of sphericity. For discrete outcomes of MLPI, regional plantar pressure, and comfort change, repeated measures analyses of variance were used to test for differences between four insole conditions (FLAT, WEDG, WEDG+V-ARCH, WEDG+U-ARCH). Significant main effects of insole condition were followed-up with post hoc Tukey’s HSD pairwise comparisons. Statistical calculations were completed with jamovi version 1.6 (26), at an alpha level of a = 0.05.