Background
During functional activities such as walking, the human foot exerts a force upon the underlying surface, and in turn, a force of equal magnitude and opposite direction is exerted upon the foot. This force is commonly termed the ground reaction force [
1,
2]. Technological advances in pressure-sensing technology, enabling the quantification of the vertical component of this force and the contact area at different regions under the foot, have become commercially available for research and clinical applications. This has enabled further insight into the plantar loading characteristics of the foot during functional activities such as walking and running [
3,
4].
Elevated plantar pressures have been widely recognised as a causative factor in the development of several pedal pathologies, including the development of stress fractures [
5], plantar calluses [
6,
7] and neuropathic ulceration [
8]. Factors shown to be associated with elevated plantar pressures include forefoot deformity [
9], increased heel pad stiffness [
10] and lesser toe deformity [
11]. The analysis of plantar forces and pressures has also played an integral role in the management of lower limb disorders. Specifically, footwear modifications [
12] and redistributive insoles [
13] aimed at offloading areas of high pressure prone to ulceration have been assessed for effectiveness in patients with diabetic peripheral neuropathy.
Commercially available systems currently employed by clinicians and researchers to assess dynamic plantar pressures include in-shoe measurement systems (Novel Pedar
®, TekScan F-Scan
®, RS-Scan Insole
® and IVB Biofoot
®) and platform systems (Novel Emed
®and the RS-Scan Footscan
®) [
14]. The validity of these measurement systems has been documented throughout the literature, suggesting they are able to accurately quantify dynamic plantar loading patterns of the foot [
15‐
19]. Validity of the TekScan MatScan
® system has been reported by the manufacturer, displaying a mean percentage difference of 1.9% when compared against an AMTI force platform (TekScan Incorporated, personal communication, 26/02/2010) and has also been shown to be highly accurate in an independent study which compared several commonly used plantar pressure measurement systems [
20]. However, to the authors knowledge, no study to date has investigated the reliability of the TekScan MatScan.
® As this system is widely utilised by researchers and clinicians it is essential that its reliability is adequately established.
Therefore, the primary aim of this study was to determine the reliability of the TekScan MatScan® system in assessing plantar forces and pressures during level barefoot walking using a test-retest analysis of thirty healthy asymptomatic participants. The secondary objective of this study was to determine if the calculation of median or mean values of plantar pressure and forces yielded more reliable measurements between trials.
Results
Participant characteristics
Information describing participant characteristics is presented in Table
1. The overall mean age (SD) of participants was 28.2 (6.1) years (range 18 to 39 years) and the mean BMI was 23.7 (3.0) kg/m
2. Males comprised 60% (n = 18) of the sample and participants exhibited a relatively normal foot posture, as evidenced by a mean FPI score of 4.3 (3.3) [
23].
Table 1
Participant characteristics.
Age (years) - mean (SD) | 28.2 (6.1) |
Gender (Male/Female) | 18 M/12 F |
Height (cm) - mean (SD) | 174.0 (7.9) |
Weight (kg) - mean (SD) | 72.1 (12.0) |
Body mass index (kg/m2) - mean (SD) | 23.7 (3.0) |
Foot Posture Index (FPI-6) - mean (SD) | 4.3 (3.3) |
Reliability of mask application
Intraclass correlation coefficient values demonstrated good reliability with values ranging from 0.96 to 1.00 (Table
2).
Table 2
Intra-rater reliability of mask application, assessed with the variable maximum force.
Total | 1.00 (1.00 -1.00) |
Heel | 0.99 (0.98 - 0.99) |
Midfoot | 0.99 (0.98 - 0.99) |
MPJ345 | 0.96 (0.85 - 0.99) |
MPJ2 | 0.99 (0.96 - 0.99) |
MPJ1 | 0.99 (0.96 - 0.99) |
Hallux | 0.99 (0.98 - 0.99) |
Lesser toes | 0.99 (0.97 - 0.99) |
Intra-session repeatability
Coefficients of variation and ICCs across the three repeated trials within the one session ranged from 3 to 22% and 0.83 to 0.98 respectively for maximum force, 3 to 32% and 0.65 to 0.92 for peak pressure, and 6 to 27% and 0.49 to 0.98 for average pressure (Table
3). The midfoot region demonstrated the largest variation between trials for all three parameters.
Table 3
Intra-session reliability (coefficients of variation [CoV], and intraclass correlation coefficients [ICC]) obtained from three repeated trials.
Total | 3.4 | 0.98 (0.96 - 0.99) | 3.5 | 0.92 (0.85 - 0.96) | 5.9 | 0.98 (0.96 - 0.99) |
Heel | 4.7 | 0.97 (0.94 - 0.98) | 6.9 | 0.90 (0.83 - 0.95) | 12.9 | 0.66 (0.37 - 0.83) |
Midfoot | 22.1 | 0.96 (0.93 - 0.98) | 31.7 | 0.69 (0.45 - 0.84) | 27.4 | 0.49 (0.06 - 0.74) |
MPJ345 | 16.1 | 0.83 (0.70 - 0.91) | 10.8 | 0.82 (0.63 - 0.91) | 11.8 | 0.91 (0.84 - 0.95) |
MPJ2 | 15.0 | 0.75 (0.54 - 0.87) | 5.9 | 0.91 (0.84 - 0.95) | 11.9 | 0.78 (0.60 - 0.89) |
MPJ1 | 19.2 | 0.72 (0.49 - 0.86) | 17.2 | 0.84 (0.71 - 0.92) | 14.4 | 0.75 (0.53 - 0.87) |
Hallux | 15.2 | 0.86 (0.75 - 0.93) | 13.7 | 0.85 (0.73 - 0.92) | 15.0 | 0.88 (0.79 - 0.94) |
Lesser toes | 21.1 | 0.92 (0.83 - 0.95) | 25.1 | 0.65 (0.35 - 0.82) | 15.6 | 0.88 (0.78 - 0.94) |
Inter-session reliability - maximum force
The relative reliability between sessions when using the mean of three measurements was good for the total area, heel, midfoot, MPJ2, MPJ1 and lesser toes, as evidenced by ICCs ranging from 0.76 to 0.95, and moderate for MPJ345 and the hallux (ICCs of 0.72 and 0.71, respectively). The relative reliability between sessions when using the median of three measurements was good for all seven regions, as evidenced by ICCs ranging from 0.79 to 0.97 (Table
4).
Table 4
Inter-session reliability of maximum force (N).
Total | 665.38 (113.56) | 654.50 (121.90) | 0.92 (0.84 - 0.96) | 5.1 | 91.01 | -84.83 - 101.40 |
Heel | 400.50 (70.21) | 391.09 (88.16) | 0.87 (0.75 - 0.94) | 6.9 | 71.78 | -66.69 - 85.51 |
Midfoot | 111.31 (61.49) | 108.56 (63.06) | 0.95 (0.89 - 0.97) | 13.2 | 39.72 | -37.46 - 42.95 |
MPJ345 | 215.35 (56.88) | 214.86 (52.07) | 0.72 (0.49 - 0.85) | 13.5 | 79.53 | -80.02 - 81.10 |
MPJ2 | 149.94 (28.05) | 147.98 (31.87) | 0.76 (0.56 - 0.88) | 9.9 | 40.01 | -39.03 - 42.86 |
MPJ1 | 141.41 (41.58) | 145.43 (36.19) | 0.80 (0.61 - 0.90) | 12.3 | 47.56 | -52.86 - 44.72 |
Hallux | 96.79 (26.38) | 94.83 (25.40) | 0.71 (0.47 - 0.85) | 14.6 | 38.54 | -36.97 - 40.80 |
Lesser toes* | 52.56 (25.79) | 58.55 (27.56) | 0.92 (0.84 - 0.96) | 5.1 | 24.52 | -30.79 - 18.73 |
|
Median of three trials
|
Total | 660.58 (119.64) | 661.07 (115.03) | 0.97 (0.94 - 0.99) | 3.0 | 54.13 | -54.92 - 55.80 |
Heel | 393.54 (68.94) | 397.86 (80.02) | 0.91 (0.81 - 0.95) | 5.8 | 59.23 | -67.57 - 58.94 |
Midfoot | 105.91 (62.17) | 114.35 (65.51) | 0.93 (0.85 - 0.96) | 15.8 | 47.46 | -56.88 - 39.81 |
MPJ345 | 218.09 (59.62) | 215.45 (57.57) | 0.81 (0.64 - 0.91) | 11.8 | 70.12 | -68.16 - 73.55 |
MPJ2 | 147.20 (29.81) | 141.80 (31.68) | 0.79 (0.62 - 0.89) | 9.3 | 37.95 | -42.76 - 33.93 |
MPJ1 | 141.02 (36.68) | 141.12 (40.40) | 0.79 (0.60 - 0.89) | 12.5 | 48.25 | -49.13 - 49.03 |
Hallux | 98.48 (26.06) | 93.65 (25.89) | 0.78 (0.58 - 0.89) | 12.8 | 33.93 | -29.22 - 39.03 |
Lesser toes* | 58.45 (30.90) | 53.05 (27.07) | 0.89 (0.78 - 0.95) | 17.2 | 25.50 | -21.08 - 32.07 |
The absolute reliability of measurements between sessions was determined using the CV, MDC and 95% LoA statistics. When using the mean of three measurements, CVs ranged from 5 to 16%, the MDC ranged from 24.52 to 91.01 N and the 95% LoAs ranged from -84.83 to 101.40 N. When using the median of three measurements, CVs ranged from 3 to 17%, the MDC ranged from 25.50 to 70.12 N and the 95% LoAs ranged from -68.16 to 73.55 N.
The only region to display a significant mean difference between sessions was the lesser toes (p = 0.01 when using the mean of three measurements and p = 0.03 when using the median of three measurements), where the percentage differences for the mean and median were both 10%.
Inter-session reliability - peak pressure
The relative reliability between sessions when using the mean of three measurements was poor to moderate (ICCs between 0.51 and 0.72) for the total area, heel, midfoot, and MPJ345, and good (ICCs between 0.75 and 0.82) for MPJ1, MPJ2, hallux and the lesser toes . The relative reliability between sessions when using the median of three measurements was poor to good for the midfoot and hallux (ICCs of 0.54 and 0.72, respectively) and good (ICCs from 0.75 to 0.85) for the total area, heel, MPJ345, MPJ2, MPJ1, and the lesser toes (Table
5).
Table 5
Inter-session reliability of peak pressure (kPa).
Total | 2794.89 (382.45) | 2726.24 (254.97) | 0.58 (0.28 - 0.75) | 7.6 | 549.17 | -519.75 - 647.23 |
Heel | 2500.69 (353.03) | 2569.34 (509.94) | 0.65 (0.39 - 0.82) | 10.1 | 657.04 | -774.72 - 647.23 |
Midfoot | 912.01 (372.65) | 843.37 (343.23) | 0.72 (0.49 - 0.86) | 21.5 | 519.75 | -460.91 -588.39 |
MPJ345 | 2196.68 (362.84) | 2245.72 (451.10) | 0.51 (0.19 - 0.74) | 12.9 | 774.72 | -843.37 - 745.30 |
MPJ2 | 2637.98 (343.23) | 2598.76 (362.84) | 0.75 (0.54 - 0.87) | 6.8 | 480.52 | -451.10 - 529.55 |
MPJ1 | 1627.90 (441.29) | 1735.77 (441.29) | 0.87 (0.57 - 0.88) | 12.5 | 578.59 | -686.46 - 480.52 |
Hallux | 1833.84 (421.68) | 1775.00 (411.87) | 0.78 (0.59 - 0.89) | 10.8 | 539.36 | -480.52 - 608.01 |
Lesser toes | 990.47 (284.39) | 1019.89 (372.65) | 0.82 (0.65 - 0.91) | 14.1 | 362.84 | -421.68 - 362.84 |
|
Median of three trials
|
Total | 1117.95 (186.32) | 1186.60 (245.16) | 0.81 (0.64 - 0.90) | 3.1 | 225.55 | -225.55 - 245.16 |
Heel | 1314.09 (245.16) | 1343.51 (225.55) | 0.82 (0.66 - 0.91) | 7.3 | 490.33 | -509.94 - 578.59 |
Midfoot | 500.13 (166.71) | 500.13 (147.09) | 0.54 (0.23 - 0.75) | 23.3 | 558.97 | -608.01 - 519.75 |
MPJ345 | 1274.86 (323.61) | 1294.47 (254.97) | 0.75 (0.54 - 0.87) | 9.2 | 549.17 | -529.55 - 588.39 |
MPJ2 | 1794.61 (382.45) | 1824.03 (274.58) | 0.78 (0.59 - 0.89) | 7.3 | 529.55 | -480.52 - 578.59 |
MPJ1 | 1127.76 (264.77) | 1098.34 (225.55) | 0.85 (0.72 - 0.93) | 13.7 | 617.81 | -608.01 - 647.23 |
Hallux | 1049.31 (1255.25) | 1059.11 (245.16) | 0.72 (0.49 - 0.86) | 10.7 | 519.75 | -421.68 - 617.81 |
Lesser toes | 558.97 (147.09) | 578.59 (166.71) | 0.79 (0.60 - 0.89) | 16.8 | 441.29 | -480.52 - 411.87 |
When using the mean of three measurements, CVs ranged from 6 to 22%, the MDC ranged from 362.84 to 774.72 kPa and the 95% LoAs ranged from -843.37 to 745.30 kPa. When using the median of three measurements, CVs ranged from 3 to 23%, the MDC ranged from 225.55 to 617.81 kPa and the 95% LoAs ranged from -608.01 to 647.23 kPa.
There were no systematic differences in mean values as evidenced by paired t-tests for any of the regions assessed.
Inter-session reliability - average pressure
The relative reliability between sessions when using the mean of three measurements was good (ICCs between 0.75 and 0.88) for all regions except the midfoot, which displayed poor reliability (ICC 0.44). The relative reliability between sessions when using the median of three measurements was moderate for MPJ345, MPJ2, and the lesser toes (ICCs between 0.69 and 0.71) and good for the total area, heel, midfoot, MPJ1 and the hallux (ICCs between 0.77 and 0.88) (Table
6).
Table 6
Inter-session reliability of average pressure (kPa).
Total | 1137.57 (196.13) | 1176.79 (215.74) | 0.84 (0.68 - 0.92) | 7.4 | 235.35 | -264.77 - 205.93 |
Heel | 1343.51 (215.74) | 1333.70 (245.16) | 0.82 (0.65 - 0.91) | 7.3 | 264.77 | -264.77 - 274.58 |
Midfoot | 500.13 (127.48) | 529.55 (147.09) | 0.44 (0.10 - 0.69) | 20.0 | 284.39 | -313.81 - 254.97 |
MPJ345 | 1255.25 (196.13) | 1265.05 (196.13) | 0.75 (0.55 - 0.88) | 7.8 | 274.58 | -284.39 - 264.77 |
MPJ2 | 1755.39 (254.97) | 1745.58 (264.77) | 0.78 (0.58 - 0.59) | 6.9 | 333.42 | -333.42 - 343.23 |
MPJ1 | 1117.95 (245.16) | 1117.95 (245.16) | 0.88 (0.76 - 0.94) | 7.7 | 235.35 | -245.16 - 235.35 |
Hallux | 1049.31 (245.16) | 1059.11 (245.16) | 0.75 (0.55 - 0.88) | 11.5 | 333.42 | -353.03 - 313.81 |
Lesser toes | 578.59 (147.09) | 578.59 (147.09) | 0.81 (0.64 - 0.90) | 11.2 | 176.51 | -176.51 - 186.32 |
|
Median of three trials
|
Total | 2716.44 (254.97) | 2706.63 (225.55) | 0.88 (0.76 - 0.94) | 8.3 | 245.16 | -333.42 - 196.13 |
Heel | 2432.04 (372.65) | 2441.85 (382.45) | 0.78 (0.59 - 0.89) | 7.5 | 274.58 | -304.00 - 245.16 |
Midfoot | 843.37 (392.26) | 882.59 (402.07) | 0.75 (0.53 - 0.87) | 21.5 | 294.19 | -304.00 - 294.19 |
MPJ345 | 2167.26 (343.23) | 2196.68 (372.65) | 0.69 (0.44 - 0.84) | 11.4 | 382.45 | -431.49 - 392.26 |
MPJ2 | 2647.79 (353.03) | 2598.76 (333.42) | 0.69 (0.44 - 0.84) | 8.6 | 382.45 | -460.91 - 402.07 |
MPJ1 | 1637.71 (451.10) | 1657.32 (480.52) | 0.77 (0.57 - 0.88) | 8.5 | 254.97 | -235.35 - 284.39 |
Hallux | 1814.23 (382.45) | 1716.16 (392.26) | 0.77 (0.56 - 0.88) | 12.9 | 372.65 | -392.26 - 362.84 |
Lesser toes | 970.85 (274.58) | 931.63 (313.81) | 0.71 (0.47v0.85) | 12.6 | 196.13 | -225.55 - 176.51 |
When using the mean of three measurements, CVs ranged from 6 to 20%, the MDC ranged from 176.51 to 333.42 kPa and the 95% LoAs ranged from -353.03 to 343.23 kPa. When using the median of three measurements, CVs ranged from 7 to 21%, the MDC ranged from 196.13 to 382.45 kPa and the 95% LoAs ranged from -460.91 to 402.07 kPa.
There were no systematic differences in mean values as evidenced by paired t-tests for any of the regions assessed.
Discussion
Information elicited from the analysis of plantar pressures and forces during walking can be an integral component in the formulation of patient intervention plans [
13]. Therefore, it is necessary to ensure that measurement systems, such as the TekScan MatScan
® which are commonly employed in the research and clinical setting, can accurately capture and reproduce plantar pressure measures of dynamic foot function on different occasions.
Intra-session repeatability was assessed for the three variables of interest by calculating CVs between three trials captured in a single session. The intra-session CVs for the seven analysed regions ranged from 3 to 22% for maximum force, 4 to 32% for peak pressure and 6 to 27% for average pressure. The midfoot and lesser toe regions displayed the greatest percentage differences for all three variables, which is consistent with previous reports using the Novel EMED
® plantar pressure platform [
19] and indicates that these regions of the foot may be subject to inherent variability during gait. However, the CV for the total foot region for all three variables was relatively low (maximum force: 3%; peak pressure: 4%; average pressure: 6%). It can therefore be concluded that while the total force and pressure under the foot is relatively stable between repeated trials within the same session, there is greater variability within different regions of the foot.
Relative reliability was generally very high, with most ICC values greater than 0.70. Maximum force was shown to be the most reliable variable compared to peak pressure and average pressure. The two different calculation methods (the mean and median values of three trials) displayed moderate to good reliability for the variable maximum force throughout all seven regions (ICCs ranging from 0.71 to 0.97), whereas peak pressure and average pressure values were somewhat lower displaying poor to moderate reliability (ICCs ranging from 0.51 to 0.87, and 0.44 to 0.84, respectively). With some exceptions, taking the median of three trials, as opposed to taking the average of three trials generally resulted in slightly higher ICC values for all three variables. This may possibly be attributed to median values not being influenced by outliers, thus yielding a more reliable outcome. Therefore, the authors recommend the use of the median value in place of the mean value in future studies using the TekScan MatScan® system.
Assessment for systematic differences between sessions indicated that maximum force in the lesser toes region exhibited a significant mean difference between sessions for both average and median calculations (p = 0.01 and p = 0.03, respectively). However, Cohen's d calculations indicated only a relatively small effect (d = 0.23 and a small percentage decrease of 10% for the mean, and d = 0.19 and a small percentage increase of 10% for the median). The remainder of the seven regions across all three variables did not display any systematic differences in mean or median values when captured one week apart.
Findings from this study assessing the TekScan MatScan
® system are in agreement with those reported by Gurney et al. [
19] who assessed the reliability of the Novel EMED-at
® plantar pressure platform. Gurney et al. [
19] conducted a between-day study protocol (5 separate days) assessing the reliability of nine asymptomatic participants for 10 regions of the foot for the variables of peak pressure, maximum force, impulse and contact time. The study concluded that areas of relatively high loading, such as the forefoot, showed higher reliability (ICC >0.90) than areas of lesser loading, such as the medial midfoot, which displayed lower reliability (ICC < 0.80). The Novel EMED-at
® platform is similar to the TekScan MatScan
® system, but has a slightly higher resolution of 2 sensors/cm
2 in comparison to 1.4 sensors/cm
2 and a slightly greater sampling frequency of 50 Hz in comparison to 40 Hz.
There are several limitations of this study that need to be considered when interpreting the findings. First, healthy young participants were recruited, so the reliability of these measurements cannot necessarily be generalised to other clinical populations. Confounding variables such as pain in symptomatic populations may have a significant impact upon the reproducibility of plantar measurements taken one week apart. Second, unlike the Novel EMED
® system, which uses automated software to apply the masks during data analysis, the TekScan MatScan
® requires a mask to be manually constructed and applied to the plantar pressure outputs for each individual participant. The standardised mask (Figure
1) could be altered in accordance to foot size and positioned with reference to the three foot regions (rearfoot, midfoot and forefoot) and anatomical landmarks (metatarsophalangeal joints, hallux and lesser digits). Although the mask template for each participant is saved and reapplied to subsequent trials, there is some potential for error resulting from different positioning of the foot between trials, thereby necessitating adjustment of the mask template upon application. This may affect the reliability of measurements [
41]. Third, the relatively low sampling frequency (40 Hz) of the TekScan MatScan
® system makes this apparatus suitable for assessing walking trials only. Due to the low sampling rate it has the potential to inaccurately capture true peak data from more vigorous activities such as running. Fourth, although the relatively small size of the TekScan MatScan
® makes it portable and convenient it is unable to record consecutive steps and is limited to capturing only one plantar pressure recording, of either the left or right foot during each trial. Fifth, the discovery of the median value being reported to be more reliable than that of the mean could suggest that the sensor capabilities of this system are limited. Therefore, the performance characteristics of the plantar pressure mat sensors may be undesirable and should be interpreted with caution [
42]. Sixth, previous work has shown that while the two-step gait initiation protocol we used provides similar forefoot peak pressure values to those obtained with the midgait protocol, rearfoot loading is reduced [
43]. As such, rearfoot loading parameters need to be interpreted with some caution when using the two step protocol. Finally, the system under review is predominantly used to assess barefoot walking. Therefore it may be more suitable to implement an in-shoe pressure measurement system to assess plantar pressures associated with interventions such as insoles or therapeutic footwear.
Future investigations should now explore differences in plantar pressures and forces in a variety of other foot pathologies with consideration of the reliability values obtained in this study. The authors now intend to use the TekScan MatScan
® measurement apparatus to assess changes in plantar pressures and forces in people with hallux limitus/rigidus following treatment [
44].
Competing interests
GVZ has no competing interests to declare. HBM is Editor-in-Chief and SEM is Assistant Editor of the Journal of Foot and Ankle Research. It is journal policy that editors are removed from the peer review and editorial decision-making processes for papers they have authored or co-authored.
Authors' contributions
GVZ, HBM and SEM all conceived and designed the study. GVZ collected and analysed the data. GVZ drafted the manuscript with the assistance of both HBM and SEM. All three authors approved the final manuscript.