Normal foot loading parameters and repeatability of the Footscan® platform system

Foot pressure measurement systems for quantitative gait analysis have become increasingly popular in research and clinical practice [1]. Such measuring systems can be used for distinguishing between normal and pathological gait [2], designing foot orthoses and in-shoe wedges [3, 4], classifying the foot types [5], and assessing the success of corrective foot surgery [6]. An ideal plantar pressure system should have the advantages of convenient use, comfort, economy, sanitation, safety, small occupation area, easy disassembly and transportation, highly accurate measurement, and satisfactory repeatability [7]. Since an increasing number of clinical decisions and treatment strategies are made based on the data collected by the plantar pressure systems, the knowledge about repeatability and normal reference values of the devices is critical before usage.

Presently, there are several brands of pressure measuring systems used in the clinic, including the in-shoe measurement systems (Novel Pedar®, TekScan F-Scan®, RS-Scan Insole®, WalkinSense®, and IBV Biofoot®) and platform systems (Novel Emed®, TekScan MatScan®, Medicapteurs S-Plate®, and the RS-Scan Footscan®) [8]. Most of these have proved to be reliable tools for quantifying the dynamic plantar pressure [7‐16]. Some researchers utilized the “intraclass correlation coefficients (ICCs)” and/or “coefficients of variation (CVs)” as the evaluation criteria and found that the WalkinSense®, MatScan®, Pedar®, and Emed® systems demonstrated the acceptable reliability and repeatability, respectively [1, 8‐10, 15]. Maetzler et al. [7] and Putti et al. [11, 14] took the “coefficient of repeatability” as the evaluation index and certified the repeatability of the Pedar® and Emed® systems, respectively. Although some clinical studies have been reported based on the Footscan® platform system, hitherto, only little information is available on the performance characteristics of this system. In 2010, Low et al. [17] reported an excellent reliability of the Footscan® pressure insoles. In addition, de Cock et al. [18] investigated the temporal characteristics of foot roll-over during jogging with the Footscan® platform and reported an adequate reliability while measuring the temporal parameter.

However, to the best of our knowledge, neither any of the previous publications have addressed the repeatability of the Footscan® platform system comprehensively, nor the ranges of the normal plantar pressure values have been identified for the healthy foot during level walking using this system. Thus, the present study was designed to assess the repeatability of the Footscan® platform system and establish a reference range for foot loading parameters, which can assist with the identification of pathological conditions.

Plantar pressure measurement is a clinical tool for assessing foot pathology, which has been regarded as an integral component while formulating the patient’s intervention plans [29]. The Footscan® platform system is commonly employed in the research and clinical setting, and therefore, it is essential to determine the repeatability of this system and identify the standard pressure values.

The mid-gait and two-step protocols are the commonly used methods to collect the foot pressure data [20, 30]. In some other plantar pressure system reliability studies, the mid-gait protocol was adopted [7, 14]. The researchers believed that the mid-gait was an optimal representative of the normal gait [14], and the participants were allowed extra time to acclimatize themselves to the mid-gait protocol to improve the quality of measurement [7]. Compared with the mid-gait protocol, the two-step protocol was simpler and time-saving [20, 31], and thus, it might be more suitable for patients with severe gait or coordination problems and those who experience difficulty in accomplishing a prolonged plantar pressure test [31]. However, some authors reported that the two-step protocol produced longer CT [20, 30, 32] than the mid-gait protocol. In addition, Wearing et al. [30] reported that the two-step protocol elicited reductions in both the PP and MaF beneath the heel. Hence, the two-step protocol might not resemble the natural gait [7]. In the present study, all the participants were healthy and capable of ambulating independently; thus, to record a natural gait, the mid-gait protocol was applied. Van der Leeden et al. [20] reported that a minimum of three measurements were sufficient for obtaining a consistent average. In the present study, three representative trials were recorded per testing session.

Herein, we assessed the repeatability of the Footscan® platform system for the 50 parameters of interest by calculating the ICCs and CVs. Considering the values of ICCs, every dynamic parameter analyzed showed moderate to good repeatability. For the intra-session repeatability, the majority (86%, 43/50) of the parameters had good repeatability (ICCs > 0.75), and the mean CV values for PP, CT, CA, PTI, and MaF were 17.1, 7.8, 6.7, 17.7, and 17.1%, respectively. For the inter-session repeatability, all the parameters showed good repeatability, and the mean CV values for PP, CT, CA, PTI, and MaF were 11.5, 4.5, 4.5, 14.2, and 13.0%, respectively. Several other studies demonstrated the intra- and inter-session repeatability of different pressure measuring systems, and the results were comparable to those observed in the present study with ICCs > 0.75 [17] and CVs < 20% [8, 33]. As is well-known, human gait is a rhythmical oscillation, and the foot steps are not identical in every gait cycle [34, 35]. Therefore, the level of ICCs and CVs achieved in the present study is clinically acceptable, which suggests that the Footscan® system is repeatable. In addition, we found that the inter-session repeatability was higher than the intra-session repeatability owing to the inter-session measurements being calculated with an average of three trials. Therefore, using a single trial to capture a participant’s foot loading parameters is not sufficient, and multiple trials should be averaged to decrease the variability of gait, as physiological fluctuations between trials are inevitable [23].

It is worth noting that the T2–5 and MF zones exhibited lower ICCs and higher CVs than the other zones in the variables that were analyzed. The findings were consistent with those from previous studies that used the Emed® and MatScan® platform systems [8, 15] and the Pedar® in-shoe system [9], which indicated the poorest repeatability in T2–5 and MF zones. The authors attributed the greater variability to the inherent variability in these regions during gait and relative smaller force and pressure exerted upon the T2–5 and MF zones [8, 15]. The present results supported the explanation. We found that areas with lower PP and MaF, such as the T2–5 and MF zones, showed lower repeatability than the more loaded regions, such as the M2, M3, MH, and LH zones. These findings are clinically important because the regions of the foot with high plantar pressures are good indicators of potential injury [36, 37]. Therefore, a higher repeatability in these zones is highly desirable for clinical applications [15]. In addition, the information elicited from the analysis of the plantar pressures and forces under the T2–5 and MF zones should be treated with caution.

In the present study, we also identified the ranges for PP under the healthy foot. Clinically, PP is the most relied upon plantar pressure parameter [7]. In the current investigation, the higher PP values were found under the M2, M3, and MH regions, and the lower ones were found under the T2–5 and MF zones. These findings were in agreement with the previous reports using other plantar pressure platform systems [14, 26, 38]. Some authors reported that the T1 zone exhibited the biggest PP [7], while we found the highest PP value under the M2 zone, closely followed by that of the M3 zone. Different observation results of the PP distributions could be attributed to different divisions of the foot, softwares used for analysis, participants, test protocols, experimental conditions, sensor characteristics, and measuring technologies [14, 38].

It’s important to note that the mean PP values obtained in this study using the Footscan® system are lower than that from the other studies [7, 14, 26]. In 1996, Davis et al. [39] recommended collecting the plantar pressure data with sensors that have dimensions ≤6.36 mm × 6.18 mm. Subsequently, Urry et al. [40, 41] used a pressure platform with a sensor size of 5 mm × 6 mm to determine the accuracy of the footprint CA measurements and the geometric indexes derived from the footprints. The studies reported that platforms with smaller sensor size and greater spatial resolution might produce a more accurate measurement of the footprint parameters [40, 41]. Another study by Urry et al. [42] recommended using platforms that have sensors of 5 mm × 5 mm or less. They believed that platforms with larger sensors would provide an underestimation of the peak pressure. According to the manufacturer’s manual, the sensor dimensions of the Footscan® platform were 7.62 mm × 5.08 mm. The relatively larger sensors and lower spatial resolution (2 sensors/cm²) of the Footscan® platform might affect the accuracy of the measurements. This reminds us that the Footscan® system might be more appropriate for the comparisons of conditions using the same system rather than situations where absolute values are required to determine the clinical condition of the participants [17].

Consistent with previous studies using other plantar pressure platform systems [7, 14, 38] and in-shoe system [11], CT was longest in the metatarsal regions, and the M3, M4, and M2 zones were the top 3 regions showing long CT. The metatarsal heads bore weight for 68.5–82.8% of the stance time, which is comparable with previous studies using the platform [7, 14] and in-shoe systems [9, 11].

CA is an important plantar pressure variable, and the combination of CA and PP can provide a lot of information for the prediction of potential damage. In the current study, CA was highest under the heel region (MH + LH zones), followed by the MF zone. Meanwhile, the smaller CA was recorded under the metatarsal regions which may lead to higher PP [14]. These results are consistent with previous studies using the platform [7, 14] and in-shoe systems [9, 11].

PTI of the whole stance phase reflects the integrated effects of pressure and time, which is related to foot pains [43, 44] and skin problems such as diabetic foot ulcers [45]. Monitoring the PTI may serve as a valuable strategy for the early prediction and prevention of the pathological conditions. The PTI values found in this study were higher under the M2 and M3 zones, and lower under the MF and T2–5 zones, supporting the findings of previous investigators who used the platform [7, 14] and in-shoe systems [9, 11].

MaF is a commonly used dynamic plantar pressure parameter. In the present study, the higher MaF values were found under the MH, M3, LH, and M2 zones, and the lower ones were found under the T2–5 and T1 zones. In addition, the T2–5 and MF zones exhibited higher intra- and inter-session CVs than the other zones for the MaF. These findings were in agreement with previous reports [8], which used the TekScan MatScan® platform system.

Nevertheless, there are some limitations of this study that should be pointed out. First, this study is limited by a small number of participants, which might reduce the reliability of the results. Second, all the participants in the present study were young healthy adults, and thus, our findings cannot necessarily be extrapolated to other clinical populations. Future investigations should focus on the repeatability of the plantar pressure measurement in patients with gait problems. Third, the selection of a representative step and manual corrections to the masked zones were subjective; the need for a standardized method is required. Finally, since different systems have different performance characteristics, the range of foot loading parameters identified in the current study cannot be considered when using other brands of systems.

The results of this study indicate that the Footscan® system is a reliable instrument for assessing the dynamic plantar pressure distributions during barefoot level walking in healthy participants. The system displayed satisfactory repeatability for the selected parameters that are commonly used in clinical investigations. The ranges of PP, CT, CA, PTI, and MaF values have been analyzed and can be used to assist with the identification of cases with foot problems. Moreover, the ranges should be used cautiously, and the overall clinical situation must be taken into consideration when making clinical judgments and treatment recommendations.