Validity of the Pedar Mobile system for vertical force measurement during a seven-hour period
Introduction
To avoid complications, instruction on partial weight bearing is often given during the rehabilitation of orthopedic patients with various pathologies of the lower extremity (Chow and Cheng, 2000; Endicott et al., 1974; Gapsis et al., 1982; Huiskes, 1998; Perren and Matter, 1996; Phillips et al., 1991; Siebert, 1994; Tveit and Kärrholm, 2001; Weaver, 1975; Wirtz et al., 1998). It is evident that the ground reaction forces under the foot during weight bearing (i.e. when walking and standing) generate forces and moments in other structures in the lower extremity, such as the hip (Bergmann et al., 1993; Davy et al., 1988). In daily clinical practice, because the forces in the hip cannot be directly measured, the ground reaction force under the foot is used as a load measure, often expressed in percentage body weight. Patients are generally instructed to perform partial weight bearing during a period of 6–8 weeks. To evaluate the effectiveness of this instruction and to quantify the loading of the lower limb during the day, objective measurement is needed of the actual amount of loading (vertical ground reaction force) and other aspects of loading (i.e. step duration, vertical force impulse) during weight bearing, both in and outside the clinic, and during a long-term period.
Portable insole pressure devices can measure the actual amount of load bearing during daily activities and over a long-term period (hours) (Hurkmans et al., 2003). However, the validity of vertical force measurements performed by insole systems (especially during long-term periods) may be influenced by temperature or humidity in the shoe, and by loading of the sensors during an entire day (Cavanagh et al., 1992). Moreover, insole sensors measure the “normal” force, which is not necessarily similar to the vertical ground reaction force (Kalpen and Seitz, 1994; Kernozek et al., 1996; McPoil et al., 1995). Only a few studies have used an insole pressure system for long-term measurements (Abu-Faraj et al., 1997; Perren and Matter, 1996; Siebert, 1994; Tveit and Kärrholm, 2001). Perren and Matter concluded that their insole system (based on a hydraulic principle) was not technically reliable enough for routine use in the clinic. Discrete insole pressure systems, developed by Tveit and Kärrholm (2001) and Abu-Faraj et al. (1997), have some disadvantages compared to matrix insole devices: the transducer may act as a foreign body in the shoe, and inaccuracies may occur due to imprecise positioning of the sensors (Abu-Faraj et al., 1997; Cavanagh et al., 1992; Lord, 1981). No reports were found on the validity of these insole systems to measure the vertical ground reaction force during long-term measurements.
Arndt (2003) performed long-term measurements using Pedar pressure matrix insoles (Novel GmbH, Munich, Germany) and found a 17% sensor creep after 3 h. To correct for this creep, Arndt presented a correction method in which short standing trials were used to reset the signal based on the assumption that the measured body weight does not change during the trial. In that study, no data were presented regarding the validity of the Pedar system after the correction method was used. However, for long-term partial weight bearing measurements, we believe that Arndt's correction method is not optimal because the patient uses a walker or crutches meaning that the total body weight cannot be measured. In the present study, we introduce a drift correction algorithm to correct for the possible offset drift during walking in the Pedar mobile system.
The aim of this study was to investigate the validity of the Pedar Mobile system to measure vertical force during a long-term period. The main research questions were: What is the amount and type of drift when using the Pedar system for 7 h? How accurate is the Pedar system in measuring vertical force over a long-term period when corrected for possible offset drift?
Section snippets
Subjects
Five healthy subjects (3 females and 2 males) with an age range of 21–35 years (mean 26 years) and weight range of 60–89 kg (mean 69 kg), participated in the study. None of the subjects had a history of musculoskeletal trauma or disease of foot or ankle. An overview of the subjects’ characteristics is presented in Table 1.
Material
The Pedar Mobile system (a portable device with matrix insoles containing 99 capacitance sensors) was used to measure vertical force during a long-term period. Three custom-made
Amount and type of drift
The amount of drift found over 7 h for the dynamic and static measurements is presented in Table 2. The data generally showed minor drift for the first 3 h and an increase in drift after hour 4. The individual drift data for the dynamic measurements showed a relatively small drift for the first 4 h for subjects 1, 2, and 3, while drift increased from hour 4 to hour 7 (Fig. 2); these latter subjects were the three females with insoles W. The two male subjects (4 and 5), wearing insoles X, showed a
Discussion
This study investigated the amount and type of drift when the Pedar Mobile system was active for 7 h, as well as the validity of the Pedar Mobile system to measure vertical force over a long-term period when corrected for offset drift.
The Pedar data showed a drift of up to 14% when the system was active for a period of 7 h. During the dynamic measurements, the pair of Pedar insoles used by the three female subjects generally showed less drift for the first 3 h than the pair of insoles used by the
Acknowledgements
The authors thank Femke van Dijk and Ronald van Beusekom for their help with the data collection.
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