Since plantar measurement systems may be inaccurate, skill and expertise are vital to measuring foot pressure

Most foot function research has been based on pressure measurements, a technology that has evolved during the past two decades using in-shoe sensors to provide real-time quantification of the forces on the feet.¹ In fact, several techniques and systems for taking pressure measurements have been developed.

However, a recent study² has called into question the validity of these laboratory findings, as well as their clinical conclusions and recommendations. The limitations of this form of foot evaluation must be considered whenever literature concerning foot function is reviewed.

Scientific Tool or Toy?
Plantar pressure measurements can be useful when investigating and describing the biomechanics of foot function and dysfunction. Several companies have responded to this need, and a variety of commercial systems have been introduced. Unfortunately, this has led to many unfounded claims and questionable science. A recent investigation² has called some of these practices (and one sample system) into question.

Gait Analysis: Researchers have found large errors when testing a commonly used system for measuring foot pressures.² When the insole measuring system was compared to internal and external forceplates, dynamic (gaitlike) testing found major discrepancies with this system. Specifically, with repeated loading, the sensor became flattened and gave a greater output value, which would result in significant skewing of research data. Bending tests also showed a surprising change in response, causing researchers to be concerned that bony prominences—which cause increased local flexing and bending of the insole—would produce inaccuracies in the results of pressure testing.

Walking (dynamic) tests found the anterior- to-posterior shear forces to be much higher than during the static tests. This indicates that shear force is a control variable, which affects the accuracy of the system’s reported measurements. Researchers who tested the insole pressure measurement system found that it was "strongly dependent on inaccurate calibration, poor hysteresis, preconditioning, bending, and the shear and temperature effects on the output of the system."² They conclude that "this raises serious questions about the reliability of the system."² There are also doubts as to whether these types of measurements could be used in either research or clinical settings.

Toe-Off Pressures: A good example of the reasonable use of plantar pressure measurements is a study³ of the push-off or toe-off part of the stance phase of gait. In this investigation, scientists used an electronic measuring device to record the peak force at several points on the foot at toe-off. After testing 42 individuals with asymptomatic feet, they found that after the heel has risen off the ground, the forefoot carries the total load of the body. They determined that the highest pressures develop under the first toe, which is loaded very heavily. The first, second, and third metatarsal heads share the load. This study reinforces the need for support of the longitudinal and metatarsal arches, and also helps explain why so many patients with poor foot biomechanics develop forefoot symptoms.

Figure 1: Five red flags of poor foot function.

Figure 2: Screening for postural instability with the PSI card.

Expertise Required
As we learn more about the actual function on the foot, we find that the clinical evaluation becomes increasingly important. In most cases, we do not need special electronic equipment to understand the source of our patients’ problems.

We must look carefully for the five red flags of poor foot function—foot flare during gait, bowed Achilles tendon, excessive shoe wear, internal knee rotation, and low medial arches (Figure 1). We can also use screening procedures, such as the Postural Stability

Indicator™ (PSI) card (Figure 2), or gait and muscle testing to help us determine the specific needs of an individual patient.

Evaluating and treating individuals are important aspects of our profession. This requires skill and expertise—not dependence on electronic systems—to measure foot pressures.

About the Author
John K. Hyland, DC, DACBR, DABCO, CSCS, consults, advises, and trains on the concepts of spinal rehabilitation. He has also served as director of several chiropractic rehabilitation practices for the past 7 years and works with the US Figure Skating Association. He can be reached at 303-465-9158.

References
1. Lord M, Reynolds DP. Foot pressure measurement: a review of clinical findings. J Biomed Eng. 1986;8:283–294.
2. Nicolopoulos CS, Anderson EG, Solomonidis SE, Giannoudis PV. Evaluation of gait analysis FSCAN pressure system: clinical tool or toy? Foot Intl. 2000;10:124–130.
3. Hayafune N, Hayafune Y, Jacob HAC. Pressure and force distribution characteristics under the normal foot during the push-off phase of gait. Foot Intl. 1999;9:88–92.