August 2019

Force and Pressure: Understanding Differences and Purposes

By Antonio Robustelli, MSc, CSCS

Whenever I talk about foot pressure-mapping technology during my courses or keynote lectures at international conferences, I am always surprised by the volume of misunderstanding that surrounds  force and pressure measurements and questions about differences between them. While many of the answers seem obvious to me,  I will try to clear up the topic in terms of both technology and biomechanics.

Force vs Pressure or Force + Pressure?

When talking about force and pressure values in biomechanics and sport science, it is important to note that they actually represent two complementary metrics and they can provide an in-depth insight into various aspects of performance as well as help in injury prevention and training individualization. In purely physics terms, force (F) represents a vector quantity, whereas pressure (p) represents a scalar quantity.

Force is one of the two most important kinetics quantities. As described by Knudson, it is “a straight-line push or pull, usually expressed in pounds (lbs) or Newtons (N).”1 Force, which is the effect of an interaction between two bodies, is a vector with both size and direction.

Pressure, on the other hand, is defined as an external force divided by the area over which the force acts; it is expressed in pascal (Pa), corresponding to one Newton per square meter (N/m2). Unlike force, pressure is a scalar quantity because it has no size and no direction: it is the ratio between the normal force acting on a given surface and the area over which this force is applied. Pressure is not directed in a specific direction, so changing the orientation of the surface over which force is acting will cause a change in the normal force (the force acting at right angles to the surfaces of objects that are in contact) but not in pressure, which remains the same. This is the main difference between force and pressure from a biomechanical (physics) point of view.

Let’s have a look at the differences from a purely practical and conceptual standpoint as well as clearing up how and why force and pressure are two complementary metrics providing different data output for analyzing performance.

Force and Pressure Measurement Technology

A force is defined as a push or a pull that makes an object change its state of motion or direction. Pressure is a force spread over a certain area

First, it is important to point out that pressure plates measure force indirectly, whereas force plates measure it directly. The technology behind force plates is relatively simple; it consists of a certain number of load cells measuring ground reaction forces and moments as a result of the force applied over the top surface of the plate.

Force plates can measure vertical forces only (single-axis platforms) or both vertical and shear forces (multi-axis platforms). Moreover, they can be unilateral or bilateral, depending if one needs to measure total forces being applied on the plate or forces for each limb separately.

Load cells measure force by monitoring variations in the electrical current when a force is being applied over them: the current flows through the sensors, which can be characterized by different types of transducers (strain gauge, piezoelectric, and piezoresistive, among others) that sense the variation in electrical current and this variation is converted into Newtons.

Pressure plates, on the other hand, directly measure pressure, surfaces, and time, where force is being indirectly calculated by summation of all the sensors’ pressure values.

Unlike force platforms, pressure mats have a higher number of sensors (resistive or capacitive), usually ranging from 3000 in basic models to 16000 in high-end products. Also, they have a lower average frequency of acquisition (from 100Hz up to 500Hz). From a technological point of view, pressure mats are able to measure pressure whether by measuring variation in resistivity of a thin film that puts the sensors in contact with each other (resistive sensors technology) or by sensing the variation in electrical capacity between the surfaces of two films that are able to vary their distance and return to the starting point (capacitive sensors technology).

Data Output Differences

The main advantage of using both types of data is that one can have a clearer picture of what is happening in terms of force application into the ground and force transmission over the foot. In addition to the measurement of the total vertical force resulting from force plate testing, pressure measurement can provide a force measure for each of the individually loaded sensors on the mat, thus allowing for the division of the foot into regions of interest (rearfoot, midfoot, forefoot).

Segmentation of the foot, together with Center of Pressure (CoP) progression, is probably one of the most appreciated features of pressure mats: being able to isolate ground reaction forces and obtain force-time curves for each foot region represents a paradigm shift in performance analysis, injury prevention and return to competition evaluations.

Using both force and pressure measurements can help in making better decisions while providing a bigger picture of key performance parameters.

Antonio Robustelli is a professional sports performance consultant and elite coach from Italy; his areas of expertise include injury prevention, sports technology, strength training programming, speed development, recovery monitoring, and return to play assessment. He has worked worldwide for 16 years with semi-professionals, professionals, and Olympic athletes as well as professional teams in various disciplines. Regularly invited as a Keynote Speaker for international conferences in Sports Science and Strength & Conditioning, he is currently a consultant for Federations, Governing Bodies, Olympians and for First Division football and basketball teams in Europe, Asia, and USA.  He is a member of the LER Editorial Advisory Board and can be reached at Antonio.robustelli@omni-athlete.com.

This article has been adapted from its original version of the same title, which can be found here https://www.omni-athlete.com/single-post/force-and-pressure-understanding-differences-and-purposes.

References

Knudson D. Fundamentals of Biomechanics.  New York: Springer US; 2007.

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