Temperature and measurement changes over time for F-Scan sensors

Temperature and Measurement Changes over Time for F-Scan Sensors Andrew G. Herbert-Copley *,*** Emily H. Sinitski *, *** * Ottawa Hospital Research Institute Ottawa, Canada dherb090@uottawa.ca

Edward D. Lemaire *,**

Natalie Baddour

** Faculty of Medicine University of Ottawa Ottawa, Canada

***Mechanical Engineering University of Ottawa Ottawa, Canada

Abstract—Plantar pressure measurement is an important tool for understanding foot and gait biomechanics. F-Scan is a popular device for measuring in-shoe plantar pressures; however, the validity of the F-Scan force measurements has been questioned.

Therefore, a study was performed to analyze changes in plantar pressure and temperature over time. One participant was fitted with two F-Scan sensors before step calibration. Single leg standing trials were captured for each limb while the subject stood on a force plate, then the subject performed multiple trials of level ground walking. Sensor temperatures were measured immediately after each set of walking trials. This procedure was repeated every 10 minutes for 140 minutes. Total force values decreased over time, with the largest decrease in total force occurring in the first 60 minutes. Sensor temperature increased during the first hour and then leveled off. Centre of pressure trajectories were similar over 140 minutes, indicating that cell pressures change similarly over time. This study showed that FScan is appropriate for evaluating pressure profiles and centre of pressure shape but additional considerations are required when using total force as an outcome measure.

Keywords—plantar pressure; measurement; temperature; centre of pressure; force sensing resistor

I.

INTRODUCTION

The F-Scan VersaTek System (Tekscan Inc, Boston, MA) is often used to measure plantar pressure within the shoe, in both clinical and research settings. An advantage of this portable system is the ability to obtain in-shoe pressure measurements outside a laboratory setting. The F-Scan sensors are thin, flexible, and can be easily attached to a person’s insole. These sensors employ a grid of force sensing resistors (FSR) and changes in the applied load are related to changes in resistance [1]. This system has been used in plantar pressure assessments [2], orthotic development [3], and centre of pressure analyses [4].

Previous research has shown that F-Scan sensors are sensitive to temperature, loading rate, and surface density [1].

Hard surfaces resulted in significant pressure measurement errors while soft surfaces, such as shoe insoles, resulted in uniform pressure measurement. Sensor output is also more sensitive to temperature changes greater than 30°C [1]. The inshoe temperature may exceed this threshold over a lengthy testing session of continuous walking.

Another F-Scan sensor issue is measurement drift over time. Lord and Hosein reported significant differences in total This study was partially funded by the Natural Sciences and Engineering Research Council of Canada.

978-1-4673-5197-3/13/$31.00 ©2013 IEEE

force measurements over a 25 minute protocol with six walking trials [5]. Another study reported a 7% decrease in F-Scan measurements after seven walking trials [6]. Some studies suggested that changes in total force measurements might be due to changes in temperature [7,8].

Previously published works that validated F-Scan performance were limited to highly-controlled testing environments over short periods. An understanding of how these sensors perform during longer data collection periods is important for both research and clinical applications. The purpose of this research is to evaluate F-Scan sensor forces and in-shoe temperature over a typical, long testing duration.

II.

METHODS

A. Data Collection Two F-Scan 3000E sensors were trimmed and secured to a participant’s shoe insoles (male, 22 years, 92.2 kg). Plantar pressures were sampled at 100 Hz. The participant wore the F-Scan sensors for 10 minutes before calibration to warm the sensors based on recommendations in the literature [9]. The “step calibration” method was used to calibrate each sensor.

This is a supported F-Scan sensor calibration method [10]. For step calibration, the subject unloads the sensor by standing on the opposite limb, and subsequently applies their full body weight to the sensor by standing on one leg while evenly distributing their weight throughout the foot. An 8-camera motion-capture system was used to collect the subject’s motion at 100 Hz (Vicon, Oxford, UK). Each foot was tracked using four markers and a single marker was secured to the C7 vertebrae to track trunk motion. Two force plates, AMTI (Advanced Mechanical Technology Inc., Watertown, MA) and Bertec (Bertec Corporation, Columbus, OH), were used to sample ground reaction forces (GRF) at 1000 Hz. An eight second single leg standing trial was captured for each limb while the subject stood on the AMTI force plate.

After the standing trials, the subject walked at a self-selected pace along an eight metre level walkway, and six trials were collected. Immediately after the six walking trials, one shoe was removed and the in-shoe temperature of the F-Scan sensor was measured using an Extech Instruments IR 250 infrared sensor (Extech Instruments Corp., Waltham, MA). Five temperature measurements from different locations on the insole were collected within five seconds of removing the shoe.

Temperature measurements were repeated for the second shoe.

The shoe order was changed every trial so that half of the trials measured temperature of the left shoe first.

This testing protocol was repeated every ten minutes for a total of 15 trials (140 minutes). Between trials, the subject was instructed to stay standing and maintain a low level of activity by walking around the room.

B. Data Processing Total force and centre of pressure (COP) data were exported from the F-Scan Research v6.51 software. Total force is the sum of all the forces in each sensor per frame of data, and therefore is not a true vertical GRF. Only trials that had horizontal GRF less than 5 N were used for analysis, thereby minimizing the difference between total force and vertical GRF. The force plate and marker data were reconstructed using Vicon Nexus and analyzed using Visual3D (C-Motion Inc., Germantown, MD).

C. Data Analysis Total force measurements for the standing trials were compared to vertical GRF to examine F-Scan measurement drift. For each standing trial, 350 frames of vertical GRF data and 350 frames of F-Scan data were averaged for each limb.

The average sensor temperature was calculated for each trial.

This average temperature was correlated with total force to examine the relationship between insole temperature and force measurement. The Pearson correlation coefficients were calculated using MATLAB (MathWorks, Natick, MA).

Five F-Scan COP cycles for the right foot were exported for each walking trial. Five right COP trajectories, calculated from force plate data, were also exported for each trial using Visual3D. Individual COP trajectories were time normalized to 100% of stance and an average COP trajectory was calculated for each trial. Pearson correlation coefficients were calculated between trials for both medial-lateral (ML) and anteriorposterior (AP) COP components, to determine if the COP trajectory shape changed over time.

III.

RESULTS

The total force consistently decreased over time (Fig. 1).

The average vertical GRF was consistent throughout each standing trial, averaging 924.6 ± 0.93 N for both left and right limbs. At the first measurement, the F-Scan total force was 97.6% of the vertical GRF on the left limb and 93.9% of the vertical GRF on the right limb. The total force measurement from both sensors decreased after 140 minutes to 63.6% on the left and 56.3% on the right. The changes in total force between trials were largest in the first 60 minutes of testing and were smallest during the last 40 minutes (100-140 minutes). Time and total force were highly correlated for both left (r=-0.95; p<0.001) and right (r=-0.92; p < 0.001) sensors.

The F-Scan sensors demonstrated larger temperature changes in the first 50 minutes of testing and the temperature was more consistent for the remaining trials (Fig. 2). For the first trial, the left sensor temperature was 33.9˚C and the right sensor was 32.7˚C. Temperature increased to 35.7˚C on the left and 35.5˚C on the right after 50 minutes.

The total force decreased as sensor temperature increased, until approximately 35˚C was reached, where both the temperature

Figure 1. Force plate vertical ground reaction force and F-Scan total force measurements for single limb standing trials over time.

Figure 2. Average F-Scan sensors temperature over time.

and force leveled off. Temperature and total force were correlated for the left (r=0.71; p<0.01) and right (r=0.86; p<0.001) sensors.

Correlations between walking trials for F-Scan COP in the AP direction were high (r>0.90; p<0.001). The correlations between walking trials were not as strong for COP in the ML direction, with an average correlation of 0.947, maximum correlation of 0.995, and minimum correlation of 0.717 (p<0.001). 62.9% of the correlations were above 0.95 and 82.9% of the correlations were above 0.90.

Correlations between trials for the force plate COP in the AP direction were also strong (r>0.96; p<0.001). The correlations in the force plate COP between trials in the ML direction were lower with an average Pearson correction coefficient of 0.88, maximum of 0.99, and minimum of 0.62 (p<0.001).

IV.

DISCUSSION

As shown in previous studies, F-Scan total force output was smaller than GRF data while standing [11]. The F-Scan sensor follows the insole contour; therefore individual sensing elements are at an angle to the floor. Since compressive force measurement is normal to the sensor, the shear component of force will not be measured on these cells. Hence, total force should be less than vertical GRF. This experiment examined the change in insole temperature and F-Scan total force over a 140 minute testing period. A 2˚C increase in temperature and a 200 N decrease in total force were found in the first hour of testing. An earlier study recorded two standing trials, 3 minutes apart, prior to data

collection to ensure the total force had stabilized [5]; however, they reported a significant decrease in total force of 133 N after 25 minutes. These outcomes are comparable to the results in the current experiment, with an average decrease of 145 N being observed after 20 minutes and a 174 N change after 30 minutes.

The temperature increased 2˚C between Trial 1 and 6 (0-50 minutes), coinciding with the largest decreases in total force. The temperature leveled off during trials 7-15 (60-140 minutes) to an average of 35.6 ± 0.3˚C (Fig. 2).

Luo et al. reported that F-Scan sensors are sensitive to temperatures above 30˚C and that sensors should only be used for a short duration if the temperature is above that level [1].

The sensor temperature for all trials exceeded the 30˚C recommended operating temperature.

The literature recommended that individuals wear the FScan sensors for 5-10 minutes before calibrating and testing to allow the sensor to reach a stable temperature [9]. In the current experiment, the sensors were worn for 10 minutes prior to calibration, where the temperature increased 10˚C, from 23˚C to 33˚C. Sensor temperature continued to increase in the first hour of testing. After the first hour, both total force and temperature demonstrated small changes. Wearing the sensors for 5 to 10 minutes may not be long enough to stabilize F-Scan measurements. Ideally, participants should wear the pressure sensors and perform low-energy level exercise, such as walking, for approximately one hour before calibration. When this time delay is not practical, a 30 minute accommodation period would account for approximately 70% of the change in temperature.

The in-sole temperature was likely higher than what was observed in this study due to the temperature measurement method used. Although, the temperature readings were recorded within the first 5 seconds after shoe removal, there was likely an immediate temperature drop when the foot was removed from the shoe. Equipping a thermocouple inside each shoe would allow continuous temperature readings and would potentially improve temperature measurement accuracy.

The F-Scan COP trajectory curves were similar over 140 minutes of testing. Although correlations in the ML direction were lower than AP correlations, lower correlations were also observed between COP trajectories from the force plate trials.

These lower correlations likely represent natural stride variability for the subject. This is consistent with the research of Han et al. who reported that intrasession COP paths for walking have lower Pearson correlation coefficients and intraclass correlation coefficients (ICC) in the ML direction compared to AP direction [12].

V.

CONCLUSION

F-Scan is a popular tool for measuring in-shoe plantar pressure. This portable device allows for ubiquitous biomechanical measurement and the sensors can be trimmed to fit any insole shape. This study has shown that these sensors are appropriate for evaluating pressure profiles and COP shape; however, errors can be expected for total force measurements over time. This is important for clinical and research applications where plantar pressures are compared between activities, especially if the testing period is long. Calibrating periodically could improve total force accuracy in these situations. Additionally, longer accommodation time should reduce the temperature effects over the longer data collection duration.

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