Larger root mean square deviations were found for all four foot deformity categories compared to healthy controls, indicating abnormalities in the plantar pressure patterns of people with HMSN. Detailed evaluation of the complete plantar pressure patterns revealed decreased plantar pressure underneath the rearfoot in the case of a plantar flexed position of the first metatarsal, increased plantar pressure underneath the lateral side of the foot in the case of a correctable or uncorrectable hindfoot varus, and decreased plantar pressure underneath the distal part of the second/third metatarsals in all foot deformity categories. The fifth metatarsal head pressure ratio was the best discriminant plantar pressure ratio to differentiate between healthy controls and people with HMSN and between the four foot deformity categories. Center of pressure trajectories were more lateral throughout the stance phase in the case of a correctable or uncorrectable hindfoot varus compared to healthy controls. In the anterior-posterior direction, the center of pressure trajectory was more anterior during loading response and midstance and more posterior during terminal stance for all HMSN subgroups compared to healthy controls. Hence, this study revealed spatially and temporally distinct plantar pressure patterns for each of the four foot deformity categories in people with HMSN.

The current study provided a deeper insight into the plantar pressure patterns of people with HMSN by evaluating plantar pressure patterns for each of the four foot deformity categories separately. Although clear differences in plantar pressure pattern were seen between the HMSN subgroups, all subgroups showed an increased pressure underneath the midfoot, which is in line with previous findings [5,6,7,8]. Prior studies that evaluated pressure underneath the rearfoot showed inconsistent results [5,6,7,8], while our study revealed decreased pressure underneath the rearfoot in people with HMSN. This discrepancy could possibly be explained by differences in foot deformity since previous research did not clearly distinguish between different foot deformity categories in the analysis of the plantar pressure measurements [5,6,7]. Furthermore, the pressures in the previous studies were not normalized for total pressure [5, 6, 8] and were, therefore, highly influenced by body weight.

As a general measure of abnormality, we introduced the RMSD of a plantar pressure pattern from the mean plantar pressure pattern of healthy controls. The RMSD increased with increased severity of foot deformity, i.e. the lowest RMSD was found for healthy controls, followed by the neutral subgroup, the valgus subgroup, and the correctable varus subgroup, while the highest RMSD was found for the uncorrectable varus subgroup. Therefore, the RMSD seems to be a powerful outcome measure to identify abnormalities in the plantar pressure patterns of people with HMSN. While the RMSD provides an overall view of the severity of a foot deformity, the plantar pressure ratios provide insight into the location of the abnormalities. The fifth metatarsal head pressure ratio appeared to be a strong discriminant to differentiate between healthy controls and people with HMSN and between the foot deformity categories. This ratio is especially capable of capturing pressure abnormalities of foot deformities with a varus position of the hindfoot since the fifth metatarsal head pressure ratio was significantly different between healthy controls, the neutral subgroup, the correctable varus subgroup, and the uncorrectable varus subgroup. Hence, we propose to use the RMSD, as a measure of overall abnormality of a plantar pressure pattern, in combination with the fifth metatarsal head pressure ratio, as a measure of varus deformity of the hindfoot, as outcome measures to evaluate foot impairments in people with HMSN.

A varus deformity of the hindfoot caused a more lateral center of pressure trajectory throughout the stance phase, which is in line with previous research [17]. Furthermore, a more anterior foot landing and a reduction in the forward progression of the tibia resulted in differences in the anterior-posterior center of pressure trajectory between healthy controls and all HMSN subgroups. However, only a few differences were found between the HMSN subgroups. Furthermore, in the anterior-posterior direction, it is difficult to interpret the results since it is unknown whether the reduction in forward progression of the tibia is caused by foot deformities (pes equinus) or by a compensation strategy for calf muscle weakness. Moreover, in the medio-lateral direction, a shift in the center of pressure trajectory between (sub)groups was found, but the course of the center of pressure trajectory over the stance phase was similar for the (sub)groups, indicating that the temporal investigation of plantar pressure seem to be less crucial for the evaluation of foot deformities in this population. Hence, center of pressure trajectories seem to be a less valuable outcome measure for the evaluation of foot deformities in people with HMSN.

Future research to establish the responsiveness of the proposed outcome measures to surgical interventions is recommended. Plantar pressure measurements have been used before as outcome measures for surgical interventions in people with HMSN and foot deformities [7, 17,18,19]. However, in most studies the number of foot areas that were studied was limited to five: the rearfoot, lateral and medial midfoot, and lateral and medial forefoot [7, 18, 19]. This makes it impossible to identify differences in the plantar pressure distribution underneath the distal part of the metatarsals, whereas the current study highlights the importance of studying the plantar pressure underneath the metatarsal heads separately for the evaluation of foot impairments in people with HMSN. Therefore, the proposed outcome measures could improve the evaluation of surgical interventions in people with HMSN.

Center of pressure velocity, which is an indicator of walking velocity [13], was significantly lower for the HMSN subgroups compared to healthy controls. Previous research found decreased plantar pressure underneath the heel and medial part of the forefoot and increased pressure underneath the midfoot and lateral part of the forefoot with decreasing walking velocity [20, 21]. This suggests that the differences in plantar pressure distribution as found in the current study can partially be explained by differences in walking velocity between the (sub)groups. However, the studies described above investigated the effect of walking velocity on plantar pressure patterns within subjects, whereas the influence of walking velocity on plantar pressure patterns between subjects has shown to be limited [10]. Moreover, center of pressure velocity was not significantly different between the HMSN subgroups, while clear differences in plantar pressure distribution were found between these subgroups.

A limitation of our analysis method is that it requires information on the complete foot shape to be able to normalize the plantar pressure patterns and to define the foot areas. As a consequence, seven feet had to be excluded from analysis because of incomplete footprints due to severe equinus deformities. However, it is expected that without the exclusion of these severe foot deformities the differences in plantar pressure patterns between (sub)groups would have been even larger. Additionally, the number of subjects in the uncorrectable varus and valgus subgroups were limited because these foot deformities are less common in people with HMSN compared to the other foot deformity categories. Furthermore, each foot was analyzed separately in this study because of differences in foot deformity between the feet of a person, which was the case in 14 people that were included. This is considered a limitation of this study since the plantar pressure pattern of one foot is influenced by the other foot. Another limitation of this study is the classification of the foot deformities based on clinical examination. This clinical classification does not allow the foot impairments to be studied on a continuous scale, while the plantar pressure measurements do. Therefore, in future studies, it would be interesting to investigate whether plantar pressure ratios and RMSD values could be used as continuous variables to examine foot impairments.