Plantar pressure during gait: norm-referenced measurement for Brazilian healthy adults using the Footwork Pro ® System

Introduction: The measurement of plantar pressure is an important component in the evaluation of the locomotive system. However, the absence of norm-referenced measurement poses limitations to its use. Objective: To verify the influence of gender on plantar pressure during gait in healthy adults and to propose norm-referenced measurement that may be used as a reference for monitoring. Methods: The study included 353 healthy participants (158 females and 195 males), aged between 20 and 64 years, and with a normal foot posture. Using a pressure platform, the peak plantar pressure and pressure-time integrals were measured in three areas of the foot: forefoot, midfoot, and hindfoot. Results: Both indicators of plantar pressure showed no significant differences between genders (p ≤ 0.05). Higher peak plantar pressure was found in the forefoot region, while a higher pressure-time integral was found in the hindfoot region. Percentile distribution values were made available for the data set of females and males. Conclusion: The available norm-referenced measurement may be used to identify pathological gait parameters, monitor the efficacy of therapeutic interventions, and detect individuals in need of referral for a more sophisticated and detailed evaluation.


Introduction
The measurement of plantar pressure distribution during gait analysis is an essential biomechanical tool to identify normal and pathological patterns of human locomotion. 1 In this particular, it becomes relevant to analyze the foot function given its participation as a support point to dissipate and transfer reaction forces on the ground. 2 Measurement instruments capable of measuring the foot function through the plantar pressure use imaging technology, platforms, and sensory insoles. 2 In this case, pressure platform systems allow for the direct measurement of the vertical component of the ground reaction forces and the load distribution on the plantar surface with acceptable accuracy and reproducibility. 3,4 Although its clinical applicability is related to monitoring foot pathologies 5 and evaluating the effectiveness of orthosis use, 6 surgical interventions, 7 and physical rehabilitation, 8 the value of plantar pressure analysis as a diagnostic and screening tool for possible health dysfunctions remains uncertain. The lack of uniformity of measurements, sensitivity among different instruments, masking forms of foot areas, 9 and, mainly, scarce propositions of norm-referenced measurement constitute some of the barriers for its appropriate use in rehabilitation procedures.
To ensure the suitable proposition of norm-referenced measurement, it is necessary to investigate the influence of gender on plantar pressure. Previous studies have tried to identify possible differences between men and women in the distribution of plantar pressure during gait analysis; however, their findings have been inconsistent. 10

Procedures
Before the data collection, the participants filled out a structured questionnaire to gather demographic data and determine their current health status. Bodyweight and height measurements were taken to calculate the body mass index (quotient between the measure of body weight in kilograms and height in squared meterskg/m 2 ) and identify the foot posture index. 23 The foot posture index consisted of six criteria for the postural positioning of the foot, with the individual in orthostatic. Through observation, the researcher assigned a score from -2 to +2 to each of the criteria, thus ranging the total sum from -12 to +12. The foot posture was then classified as highly pronated (score from 10 to 12), pronated (score from 6 to 9), neutral (score from 0 to 5), supinated (score from -1 to -4), and highly supinated (score from -5 to -12). 23 The foot posture index demonstrated acceptable reproducibility for both the final score and the single scores. 24

Plantar pressure measurements
The plantar pressure measurements were performed during gait analysis using the FootWork Pro® system (AM According to the manufacturer's specifications, the required participants' information was entered into the electronic system, and the equipment was calibrated for each measurement.
We used a three-step gait initiation protocol, 25 which showed a satisfactory agreement with the midgait protocol as to the measurement of the dynamic indicators of plantar pressure. 26 The participants were first familiarized with the protocol by performing numerous trials until they understood the steps involved in the evaluation. The plantar pressure was measured using the AutoMask function of the FootWork Pro 2.9.1 software in three plantar regions: hindfoot (0-30% of the total foot length), midfoot (30-60% of the total foot length), and forefoot (60-100% of the total foot length). 28 Once the plantar regions were determined, when necessary, manual adjustments were performed by the investigator to ensure the quality of the measurements.   Table 1 shows the data regarding the anthropometric characteristics of the participants.   Notably, the three masks did not show statistically significant differences between gender for the peak plantar pressure and time-pressure integral. As for the peak plantar pressure, both genders showed higher values in the forefoot mask and expressively lower values in the midfoot mask. As for the pressure-time integral,

Statistical treatment
We tested the data for normality using the Kolmogorov-Smirnov test. Upon confirmation of the normality requirement for data distribution, we calculated the mean and standard deviation values.
We used the Student's t-test on independent samples to detect statistical differences between genders in the measurements of the dynamic plantar pressure. Norm-referenced measurement of the peak plantar pressure and the pressure-time integral were however, the hindfoot mask showed higher values, while the midfoot and forefoot masks showed less expressive differences.
As no statistically significant differences have been found between the two genders, Table 3

Discussion
To our understanding, the current study advanced previously conducted efforts that sought to establish norm-referenced measurement for plantar pressure indicators during gait analysis. [18][19][20] Though some studies have reported normative data, they were obtained through in-shoes baropodometry systems or extensive platforms, rendering their use in smallspace environments infeasible. Moreover, it is worth noting that these studies were limited to specific ages and did not take the influence of foot posture into account. Furthermore, the lack of standardization of the measurement units, as well as different algorithms for calculating the variables, may result in inappropriate interpretations of the plantar pressure measurements.
In this study, no statistical differences were found between men and women in the plantar pressure data, contradicting previous findings. 11,29 However, one should note that these earlier studies investigated children and the elderly. Differences in the foot structure may explain the differences observed between genders.
However, especially for adults, it is expected that the The current study showed no differences in both plantar pressure indicators by gender, which supports an earlier paper that showed a strong linear dependence between peak plantar pressure and the pressuretime integral in healthy subjects. 33 Therefore, the discriminatory feature of the latter cannot be ascertained.
We suggest, however, further studies, especially with outcomes related to pathological conditions, to verify this discriminatory capacity, as well as the relevance of its association to the plantar peak pressure. The study is not without potential limitations. First, the non-performance of random sampling techniques, sample size calculation, and self-report of health status may have resulted in a methodological bias. Besides, the Footwork Pro® plantar pressure measurement system lacks evidence regarding the conformity of its plantar pressure values with those produced by other devices, which may make eventual comparisons infeasible.
Finally, potential confounding variables such as ankle range of motion, muscle strength, and plantar sensitivity were not controlled. We believe, however, that these variables were not affected due to the inclusion criteria of the participants.

Conclusion
This study sought to fill a gap in the literature regarding the availability of normative plantar pressure values during gait analysis in healthy adults. We first analyzed eventual differences in plantar pressure between male and female subjects and, based on the similarity identified, processed the data of both genders together to propose the norm-referenced measurement.
Clinicians and researchers can use the suggested normreferenced measurement to compare pathological parameters of gait analysis based on data from apparently healthy individuals, measure the efficacy of therapeutic interventions, and detect individuals in need of referral for more sophisticated and detailed assessment.

Authors' contribution
FRPGR was responsible for the study design, data interpretation, and manuscript writing. DPG was responsible for the study design, data analysis and interpretation, and manuscript review. Both authors approved the final version.