A convenience sample of healthy individuals with no congenital, or acquired pathology of the nervous or musculoskeletal systems, no deformities or serious injuries of the pelvis or lower limbs and feet, no self-reported lower limb/foot pain were included in the study. This study was approved by the Ethical Committee of the Faculty of Physical Culture, Palacký University in Olomouc, Czech Republic (reference number 3/2018) and all participants provided written informed consent prior to data collection.

Participants walked at a self-selected walking speed along a 15 m walkway whilst wearing a correctly sized pair of ProTouch Drop Shot trainers (IIC-INTERSPORT, Bern, Switzerland) under two conditions; no orthosis and a pair of foot orthoses with a medial heel bar, positioned under the sustentaculum tali which aimed to minimise foot eversion during the stance phase (see Fig. 1). The OFM and CAST foot kinematics were captured simultaneously at 200 Hz using an eight camera Vicon Vantage V5 (Oxford Metrics, UK) motion capture system.

Diagram illustrating medial (top) and lateral (bottom) aspects of the medial heel bar

The CAST lower limb model was used to measure lower limb kinematics [18]. Anatomical markers were placed on the anterior and posterior superior iliac spines, medial and lateral femoral epicondyles, and medial and lateral malleoli. The shank segment was defined proximally by the knee joint center, estimated from the medial and lateral femoral epicondyle markers, and distally by markers placed on the medial and lateral malleoli. Clusters of non-collinear markers were placed on the lower leg (i.e. shank) and the thigh. Marker placement for both foot models is displayed in Fig. 2. Retroreflective markers for the OFM were positioned according to Stebbins et al. [7] with a following modifications. Instead of a marker wand positioned on the posterior aspect of the calcaneus, the lateral calcaneus (HF2) marker was used for hindfoot tracking, the sustentaculum tali marker was moved inferior to fit on the shoe surface, and the hallux was not tracked in this study due to not being able to place a marker on the hallux under shod conditions.

Diagram illustrating the lateral (left) and posterior-medial marker positions of the Calibrated Anatomical System Technique (CAST) multi-segment foot model and the Oxford Foot Model (OFM) applied simultaneously. CAL1 and CAL2 represents the inferior and superior posterior aspect of calcaneus; HF1 and HF2 represents the medial and lateral hindfoot; STAL represents the sustentaculum tali; LCAL represents the lateral aspect of the calcaneus (at the same distance from the most posterior point as STAL); MMAL and LMAL represents the medial and lateral malleoli; P1MT, P5MT, and P3MT represents the base of the 1st and 5th metatarsals, and between the base of the 3rd and the 4th metatarsals, respectively; D1MT and D5MT represents the medial first and lateral fifth metatarsal heads; TOE represents the mid-point of the distal heads of the 2nd and 3rd metatarsals; FF1 and FF2 were not used for the analysis

The proximal and distal CAST MFM hindfoot was defined by the medial and lateral malleoli (MMAL, LMAL) and sustentaculum tali (STAL) and lateral aspect of the calcaneus (LCAL, at the same distance from the most posterior point as STAL [7]) markers, respectively. The hindfoot was tracked using markers positioned on the medial and lateral hindfoot (HF1, HF2) in addition to the marker positioned inferior on the posterior aspect of the calcaneus (CAL1), and STAL and LCAL markers. The proximal and the distal forefoot were defined by the STAL and LCAL markers, and the first and the fifth metatarsal markers (D1MT, D5MT), respectively. The forefoot was tracked using markers positioned on the base of the 1st and 5th metatarsals, and between the base of the 3rd and the 4th metatarsal (P1MT, P5MT and P3MT, respectively). The two most anteriorly placed markers (FF1, FF2) were not used by either model in this analysis (Table 1).

A single static trial with participant standing with a comfortable relaxed posture was captured and used to calculate local coordinate systems of the segments. Anatomical markers were removed after the static trial and all tracking markers remained in place during both conditions. For each participant, twenty-five trials were collected, of which seventeen trials had complete marker trajectories for all markers, and were used for the analysis. The two orthotic conditions were randomized by participants picking different coloured balls from a bag which represented the with and without orthotic conditions. A five-minute familiarization and wash-out period was used between each condition.

Kinematic data were processed in Vicon Nexus 2.8 and exported to C3D format. Heel strike and toe-off were identified manually based on linear acceleration, velocity and visual inspection of the heel and toe marker trajectories. The kinematic data were filtered with a fourth-order low-pass Butterworth filter with a 6 Hz cut-off frequency. The CAST lower limb model and both the CAST MFM and the OFM were applied in Visual 3D (C-motion, USA) using segment optimization pose estimation. For the shank, hindfoot and forefoot segment coordinate systems of both foot models, the motion about a medio-lateral (X), an antero-posterior (Y) and a vertical (Z) axis was plantar/dorsiflexion, inversion/eversion and abduction/adduction, respectively. The X–Y-Z Cardan rotation sequence equivalent to the segment coordinate system was used to calculate joint kinematics [23], and data were normalized to 100% of the stance phase.

Statistical analysis was carried out using spm1d package version 0.4.3 (http://www.spm1d.org/) in Python version 3.8. The D’Agostino-Pearson K2 test was used to assess the time series data normality. Data were not normally distributed; therefore, the non-parametric version of vector field analysis, statistical non-parametric mapping (SnPM) was used [24]. SnPM paired t-tests (p < 0.05), with the number of iterations set to 10,000, were used to explore the effect of the medial heel bar over the stance phase for each foot model for each participant in all three anatomical planes using observations from both feet. The hindfoot relative to shank and forefoot relative to hindfoot segments in both feet were compared between models. The segments for each foot model had comparable markers and thus hypothesised that outputs would be similar, despite having subtly different anatomical axes definitions. The stance phase was split into the first double support (DS1), single support (SS) and the second double support (DS2) phase, identified from the gait events, and statistically significant kinematic effects of the medial heel bar, represented by the suprathreshold clusters in the SnPM analysis, were compared between the two foot models for each sub-phase separately. For each sub-phase of the gait cycle, possible kinematic outcomes between foot models were; the same kinematic effect (blue), no effect for both models (turquoise), a unique effect of either the CAST MFM (green) or OFM (orange), or an opposite effect of the two foot models (red). The visual inspection of the SnPM analysis outputs showed considerable number of small, arguably clinically irrelevant suprathreshold clusters (waveform areas showing statistically significant differences). The authors could not find any data to identify minimal clinically relevant suprathreshold cluster size specific to this type of analysis, therefore, based on the visual inspection, only differences that were significant for more than 5% of the stance phase were considered meaningful in the different sub-phases and were included in the analysis (Fig. 3).

An example of statistical non-parametric mapping (SnPM) analysis conducted for both the Calibrated Anatomical System Technique (CAST) multi-segment foot model and the Oxford Foot Model (OFM) for the hindfoot coronal plane during the first and second double support sub-phases (DS1 and DS2) and single support sub-phase (SS). The upper panel shows mean kinematic waveforms for the CAST and OFM under no orthoses (blue and green, respectively) and under the medial heel bar conditions (orange and red, respectively).The lower two panels show the statistical output of the SnPM analysis with grey shaded area illustrating the suprathreshold clusters (p < 0.05). The purple circle highlights the suprathreshold cluster not included in the analysis