Vertical force and stance timings

´│░an> Orthotic Biomechanical Solutions 2010

A comparison of vertical force and stance timings using a Novel pedar and Fscan plantar pressure system with a Kistler force plate.

Measurement of hemiplegic gait can be problematic due to the characteristically irregular gait pattern of these patients. Ankle foot orthoses are used to influence the walking patterns but quantification is difficult. It was postulated that a plantar pressure system may be used to collect pressure distribution and vertical ground reaction force data to produce lower quality but possibly more representative data than a standard force plate. Force plates can provide accurate information from which all three planes of force can be calculated, but are limited to one step per trial. Plantar pressure measurement systems can measure vertical force but over a series of steps. To investigate this a series of tests were conducted to compare the accuracy of the Pedar (Novel) and F-scan (Tek scan) in-shoe plantar pressure measurement systems in collecting vertical force and timing data. Separate measurements were made with each of the in-shoe systems as a normal subject passed over a Kistler force plate whilst wearing an ankle foot orthosis. Twenty trials were completed using both systems
The F-scan system both under and over estimated vertical forces over the trials. Of the twenty trials four were removed due to problems with calibration and recording of data. The F-scan recorded a mean difference in first vertical peak of 75.5N with maximum error of 173N and a standard deviation of 45N The second peak recording showed F-scan to measure a difference of 128N with a maximum error of 203N and standard deviation of 35N Mean differences in stance timing at each step on the plate were 0.06 seconds with a std dev of 0.0 The mean stance time recorded was 0.76 s over the trials. See table 1.
Pedar recorded a mean of 163N difference from Kistler with a standard deviation of 36N and maximum error of 205N. The second peak mean difference was 203.45 Newton’s with a standard deviation of 39N and a maximum error of 266N. Pedar stance time recording were more accurate than Fscan with an average difference of .05 seconds, a standard deviation of 0.02s and maximum error of 0.08s. The Pedar trial also showed an average reduced stance time of 0.51 seconds indicating that the tests may have been conducted at a higher velocity.
The Kistler force plate recorded marked inter step peak force variation, with a standard deviation of 30.4 first peak and second peak variation 87N during the F-scan trial. During the Pedar trial the first peak had a standard dev of 94N and second peak 34N

image of pedar forces

In the study the F-scan did provide a more comparable force peak over the trials when compared with the Kistler system. However if the first three trials were added to the study this would have reduced the quality of the data considerably. The Pedar system more accurately recorded stance time duration. This may have been due to noise and shoe interface pressures masking heel and toe off events within the F-scan trials. The Pedar system has an offset facility to reduce interface pressure data and was used before each trial. There is also a 2 Ncm2 threshold level before pressure is recorded. This has the effect of reducing noise but may also reduce the magnitude of force slightly. This effect was not accounted for in this study. There was variation between the Kistler force peaks throughout all of the trials. This variation in peak forces would have an adverse effect upon any attempt to detect changes in ankle moment that relied upon one step as evidence of change. Further investigation will be required to determine if plantar pressure measurement systems can provide an alternative to force plate data in some applications.

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