The aim of this study was to provide reference pressure data, divided into anatomical regions, for a typically developed population across the LT phase of gait as a comparative tool for clinical populations. Data was also presented for two pathological case studies with abnormal joint morphology and joint mobility, and compared relative to the reference data to determine the clinical utility of examining the LT phase. The results indicated the importance of the LT phase of gait by highlighting the clear differences during this phase for the two case studies presented. For case study one, the clubfoot participant, there were clear differences between the leading and trailing foot for all variables, with the trailing foot appearing to affect foot pressures in the leading step. For case study two, the flatfoot case study, pressures in the trailing foot were consistent with TD children although the leading foot medial and lateral pressures were on average 1.4 times higher than TD children. The greatest differences seen in mid-foot pressures occurred following the LT phase.
Typically developed population
For the TD population, our participants produced forces in the LT phase consistent with those presented in the literature [14, 16]. In agreement with previous findings, we observed increased values for the medial forefoot during push-off in the trailing step across all pressure and force parameters [14, 16]. In the leading step, we saw a balanced hindfoot, producing equal forces and pressures in the first 25% of LT, before a slight increase in pressure medially. It has been suggested in the literature that following a typically balanced heel strike, pressures progress laterally through the midfoot, with normal participants producing peak lateral-medial force indexes of approximately 1.6 during midstance (ie. increased lateral forces compared to medial) . This slight medial shift in our TD participants may have been due to the transfer of load from the opposing limb originating from the medial side of the limb. After toe-off (Fig. 1A) there was a pressure shift laterally again during midstance. Midfoot contact was first seen at approximately 40% of the LT phase, which was indicative of a clear heel strike before flatfoot is achieved. Despite not being evident in the LT phase data, there was a medial shift in the forefoot across the second half of the stance phase (Fig. 1A).This is characteristic of typically developed gait [11, 12].
In the contact area graphs, total forefoot contact area was greater than total hindfoot contact area. This was to be expected given the anatomy of the plantar surface of the foot. As the same amount of force is being applied to a smaller contact area in the hindfoot, there was an increase in mean pressure for the hindfoot. The differences in peak pressures, from forefoot to hindfoot, were not as large. We suspect that this was due to the medial forefoot’s large contact area, as most of the pressure was concentrated over the first metatarsal and hallux, producing similar peak pressures to a region with a smaller contact area. It is important to note that contact areas were reported in absolute values. Overall, the TD pressures observed in our participant cohort was in agreement with previous findings across the entire footprint [14, 16].
Case study one: clubfoot
The contact area and force graphs for the trailing foot revealed a lateral bias for case study one. Contact area for both medial and lateral regions in the trailing forefoot were slightly larger at the start of the LT phase, perhaps due to an increase in double-limb support duration (Table 1), and or the larger stature of this participant compared to the mean TD participant. Observation of the force graphs revealed that the clubfoot participant demonstrated increased lateral forefoot forces and reduced medial forces in the first 30% of the LT phase, which was due to hindfoot-tibia adduction, and forefoot-tibia inversion of the trailing foot (see Supplementary Figure 3). Lateral dominance was more pronounced in the mean pressure and peak pressure graphs, with peak pressures in case study one doubling those seen in the TD population. Further, increased peak pressures were evident medially, however these occurred under the second and third metatarsal head rather than the first metatarsal and hallux. These increased peak pressures occur only in the first 65% of the LT phase, before overlapping the TD participant band. Early peak pressures and decreased forces during LT resulted in decreased ankle power at push off (see Supplementary Figure 4).
The largest differences seen in the contact area graphs was in the lateral hindfoot, where peak contact area was two and a half times the magnitude of the typical band. We also observed an increase in the contact area of the medial hindfoot at 30% of the LT phase, likely due to increased inversion during swing (see Supplementary Figure 3) . This would have also led to a large lateral contact area at initial contact, followed by an increased medial contact during loading. Unsurprisingly, the lateral contact area remained larger across the entire LT phase. Case study one also exhibited an earlier and larger contact area in the midfoot due to the combined kinematic impacts of the deformity at ankle and foot joints (i.e. hindfoot-tibia equinus and inversion; see Supplementary Figure 3), resulting in a loss of heel strike and flat foot at initial contact. There were also increases in force, mean pressure and peak pressure at the lateral hindfoot of the leading step compared to TD participants (Figs. 3E, 4E and 5E). In contrast with the contact area graphs, there was a decrease in force and mean pressure for the medial hindfoot (Figs. 3D and 5D). The pattern of the pressure curves was also different with a smooth inverted U shape peaking at an average of 45% of the LT phase observed for all graphs, again highlighting the lack of a prominent heel strike in the hindfoot. The unloading observed at the end of the LT phase for all variables is not seen in the TD bands for the hindfoot and is possibly a stabilising adaptation in the clubfoot participant . Finally, the lateral hindfoot graphs for the clubfoot participant more closely resembled those for the medial hindfoot. We hypothesise that this is due to the large increase in contact area for this region.
Case study two: flat foot
Case study two presented with increased velocity and a slightly increased percentage of time in double limb support. The forefoot of the trailing step for the case study two demonstrated a similar pattern for the majority of LT metrics with considerable overlap with the TD graphs. There was a tendency however, toward medial forefoot bias for contact area and force. Indeed, typical profiles are generally observed at the forefoot during double support in flat foot patients . The largest differences for all reported variables was during the LT phase at the level of the medial hindfoot, where significant increases in force (23% increase), mean pressure (28% increase) and peak pressure (66% increase) were evident. Increased hindfoot-tibia eversion seen at heel strike in the foot model kinematics verified these force and pressure findings (see Supplementary Figure 5). Increased forces were also seen in the lateral hindfoot and midfoot, with midfoot forces continuing to increase after the completion of LT (Fig. 1B). This was supported by the hindfoot-tibia kinematics, which revealed greater eversion compared to the TD band following the LT phase (see Supplementary Figure 5). This may have been due to insufficient contributions from the intrinsic foot muscles responsible for the storage and return of elastic energy within the arch  or structural changes such as calcaneal inversion and talar adduction . Midfoot forces could also continue to increase across stance as a consequence of the medial forefoot bias seen during the LT phase. Differences in mean and peak pressure curves compared to force showed a large peak at approximately 25% of LT followed by a gradual decline ending within the TD band for the mean pressure and just above the TD band for peak pressure. This provides support to the premise of a ‘heavy’ heel strike in case study two compared to TD participants, resulting in a slightly increased power absorption at the ankle and knee (Supplementary Figure 6). Compared to contact area profiles, pressures begin to normalise as contact area increases above the TD band. The lateral hindfoot followed a similar pattern in the peak and mean pressures, but to a lesser extent. This medial shift is supported by centre of pressure (COP) data that has been reported in the literature [37, 41, 42]. Specifically, COP lines in previous studies have reported to be medially deviated in flat foot participants [39, 41], and as such we would expect to see a medial shift across the whole foot. Typically, increases in force have been documented in the mid- and forefoot of flatfoot participants during stance, but not in the hindfoot . A focus on the LT phase highlights the increases in medial force and pressure in the hindfoot which may have not been presented previously.
Finally, the midfoot of the flatfoot case study followed a similar pattern to the TD population, as contact is not observed until 30% of the LT phase. This means that case study two demonstrates a defined heel strike before foot flat occurs. Once foot flat occurred however, a steeper incline across all four variables compared to TD participants was observed. The greatest increases were seen in contact area and force. Not only was a larger portion of the plantar surface in contact with the ground, but force was also being applied to it. Peak pressures showed the smallest differences to the TD cohort, suggesting that the increased forces seen are evenly dispersed across the plantar area. During double limb support the opposing limb was still supporting a portion of the load, resulting in the midfoot being less deformed and therefore producing lower pressures. During single limb support, however, force in the midfoot continued to increase, peaking at 45% of the leading foot’s stance phase (Fig. 1C). We suspect that whilst the greatest midfoot differences in the flatfoot case study occurred after termination of the LT phase, it may have been due to the hindfoot-tibia inversion present during LT which could be corrected with an orthotic.
The pressure reported for the flat foot case study is in good agreement with kinematic data presented in Supplementary figure 5 and in the literature [36, 44], suggesting that plantar loads analysis that incorporates anatomical landmark projections might be sufficient for clinical interpretation when full lower limb kinematics is not feasible. Access to both 3D motion capture systems and plantar load devices may be confined to hospital based gait laboratories and clinical research facilities. Nonetheless, our findings suggest that collection of a minimal marker set and plantar loads may allow for a more economical option for clinical evaluation of foot motion when a fully equipped laboratory is not available. It’s important to acknowledge that our TD cohort was relatively small for a normative database, with a large age range of participants at various stages of development and gait maturity. Additionally, the case studies do not necessarily represent the range of foot impairments seen in CTEV and flatfoot patients. Nonetheless, our data does provide evidence of the importance of examining the LT phase in clinical management. Future studies should investigate the impact of gait maturation on regional pressure and force metrics, using a larger dataset that can be evaluated with cluster analysis, to determine whether age specific normative reference data is indicated. It would also be beneficial to produce complete data sets for various clinical populations. It is likely not feasible to review all force, area and pressure profiles in focus on the LT phase in a clinical review. Keijsers and colleagues  report high correlations between reported peak pressure and mean pressure variables in a typical population, suggesting reporting fewer parameters is sufficient. We therefore recommend that normalised force and peak pressures should be the first point of reference. These variables show how force is transferred between regions and highlight areas of concern for pressure sores and ulceration. In summary our findings highlight (i) the importance of isolating data collected during double limb support and (ii) examining the LT phase has the potential to provide clinically meaningful information for intervention planning.