In Motion

Under Pressure: A brief overview of Pedobarography

Posted by Oliver Watson

Oct 9, 2015 9:30:00 AM

Pedobarography AKA Plantar pressure measurement has been gaining in popularity of late.  Examples of pressure measurement have been showcased on TV, demonstrating its practical value in controlled conditions.

National Geographic featured pressure mapping technology during a "Fight Science" segment broadcast on the network, see below:

This is an interesting demonstration of pressure mapping tech being used in a controlled setting.  Before digging any deeper into how pressure mapping can be effectively utilized across research, performance analysis, and rehabilitation, let's look at the history of pressure measurement and how the tech works.

What is Plantar Pressure Measurement?

Plantar Pressure Measurement (PPM) is the collection of data about how a person's (or animal's, for that matter) foot transfers pressure onto the ground - whether it be a floor, a treadmill, or the earth.  In other words, as you walk, run, jump, or perform other movements on your feet, you are exerting pressures on the ground.  While PPM can be used to measure (less accurately) force data (we will cover pressure measurement devices and force measurement in a future post), users are most often interested in how pressure is distributed across, for example, a person's foot.  Pressure measurement in general does not only apply to feet, as there are many systems available that have been specifically designed for measuring pressures exerted by the hand, as well as on the lower body while sitting, for example (check out this page for an example).  There also exist systems which can be used for custom applications, but, for the purpose of this blog, we will focus on measuring foot (plantar) pressures.  

How does it work?

 

Plantar Pressure Measurement systems come in a wide range of shapes and sizes.  From walkways, to individual pressure plates, to insoles, there are several different varieties available, each with its own set of advantages and disadvantages. Pressure measurement systems typically contain one of 4 types of sensors: Capacitive, resistive, piezoelectric, and piezoresistive.    The sensors, in simple terms, measure the voltage passing through different types of conductors housed in the system.  When the sensors are deformed by an external force, say, by somebody applying pressure to them, the resultant voltage changes.  Extensive studies in the field has lead researchers and manufacturers to understand how the change in voltage applies to the amount of pressure exterted, which allows them to display the data to the user.  The manner by which these sensors measure the pass-through voltages is different, but the concept itself is the same, and is not that different from other types of measurement devices - think force plates, which measure the change in voltages as electricity passes through transducers and is manipulated by forces enacted on the surface of the plate.  

pressure measurement foot

Benefits of PPM

As mentioned previously, the main advantage of using pressure measurement is that it provides a clear representation of how a person distributes his or her forces and/or weight.  This is extremely important for several applications, primarily because inefficiencies in weight distribution can signify OR lead to future problems¹.  Having access to this information means that individuals' pressure distribution stats can be tracked for whatever purpose, whether it is research, athletics, or healthcare. Insole pressure measurement provides an even clearer picture, because it shows how the foot interacts with the inside of a show as the outer sole contacts the ground.  

In gait labs that include pressure measurement, the main advantage of a fixed pressure measurement system in conjunction with a multi-camera video capture system is objectivity.  Every time you conduct an analysis of a patient, assuming your camera angles are consistent (i.e. mounted on the wall or a mounting rail), you will have no problem comparing and contrasting different patients and different trials of the same patient in terms of objectivity.  This is important, because often times you will see a side by side comparison of an athlete or patient (before and after) with videos captured from completely different perspectives.  If you try to measure the amount of knee flexion and compare the values between these two different videos, your comparison will not be accurate, and anybody who understands the concepts of perspective consistency and objectivity will immediately discount the work you have done.  

Disadvantages/Issues

zebris pressure sensing walkway

By P0todd0p (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons

Stationary measurement systems, including pressure plates and walkways, are very accurate and can be found in clinics, research labs, running shops, and performance laboratories.  These systems also have a portable element to them, and some models can be taken to the location where you will be collecting data.  However, two distinct limitations are presented with these types of systems.

Lab-Coat Effect/Observer Effect

The "Lab-Coat Effect," is the concept that being observed changes your behavior.  It is also known as the "Observer Effect."  In terms of pressure measurement, this term is relevant because if have a measurement conducted in a laboratory of medical setting, there is a chance that your values could be skewed.  Professionals use these types of devices to get an idea of how you walk normally, and will make recommendations, or provide interventions, based on the data they collect.  However, if you are subconsciously changing the way you walk, then any recommendations made will not be in line with how you walk while you're in the real world.  There exist methods that can be employed to "trick" those being measured into walking normally, such as having the patient walk across a mat barefoot to retrieve a document from his or her bag while you take a reading of their pressures.  In a controlled setting, you run the risk of this subconscious bias on the patient side, and it is necessary to keep this concept in mind and employ processes and techniques that minimize observer effect.

Targeting

Targeting is another problem that is consistent with any type of measurement plate, including pressure plates and force plates.  The idea is simple, the analyst instructs you to walk over a pressure plate while he takes a measurement.  It seems easy enough, but the fact of the matter is that you will adjust your stride to contact the plate correctly, which can vary the results of the measurement. Unless you are using an extended pressure walkway which removes the need to target a small plate, it is necessary to keep this in mind when calculating results.  However, extended pressure measurement walkways can be significantly more expensive than single plates: more sensors=more $'s.  

Insole Systems?

Novel Pressure Mapping InsolesBy P0todd0p (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons

A pressure mapping solution that is used to minimize inconsistencies related to targeting and potentially the labcoat effect is the use of an insole pressure measurement.  These systems, usually consisting of a flat measurement surface that fits inside the shoe, and a control system that is strapped to a subject's ankle or waist, remove the need for targeting a specific space on the floor, and allow [somewhat] free movement in the analysis space, as well of measurement in the field.  The limitations of these types of systems usually comes down to the invasiveness of the receivers.  During recording, subjects are required to wear receivers on their waist or ankles.  Wires must be run from the receiver, down a persons legs, and then attached to the insoles.  This is done with good reason, because it allows for the sensors to be extremely thin - paper thin in some cases.  However, the feeling of having a device strapped to you, and the fact that these must be used in the presence of an operator (in most cases), only contributes to the observer effect mentioned above.  

So, what's the deal?

For years, the three types of systems mentioned above have been the standard in terms of plantar pressure measurement.  They have their advantages, and they have their disadvantages.  However, these systems are limited to measuring foot pressure in a (somewhat) controlled environment.  With all of the new, high tech, wearable devices being released these days, it's a shame nobody has developed a system that can measure foot pressure in a patient or athletes footwear over long periods of time, with minimal invasiveness and distraction.  Wait, what about OpenGo?

OpenGo was developed by Moticon GmbH as the first wire-free measurement insole that can be used to capture patient data over the course of days, weeks, or even months.  Comparing OpenGo to the other systems is a difficult task, because they are all designed for different purposes.  Nonetheless, OpenGo is truly an innovative pressure measurement system because it allows doctors, therapists, coaches, trainers, and researchers to view long term data that was previously only a reality in their dreams.  

For more information about OpenGo, check out the Moticon website, or contact us below!

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The zFlo blog is maintained by our team in hopes of providing some insight into the motion-analysis industry. We work with a large number of companies and have over 10 years of experience working with video and optical motion capture technology - this is where we do our best to explain some of the more important aspects of how this technology can be used.