Force and Pressure: Understanding Differences and Purposes

May 15, 2019

Whenever i talk about foot pressure mapping technology during my courses or keynote lectures at international conferences, i always got surprised by the deficits in knowledge and understanding of the differences between force and pressure measurements.

I often receive questions that actually sounds a little bit obvious to me but i will try to clean 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 represents 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 individualisation. 

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 and it is "a straight-line push or pull, usually expressed in pounds (lbs) or Newtons (N)" (Knudson, 2007). Force, which is the effect of an interaction between two bodies, it's a vector and it presents a size and a direction. 

Pressure, on the other hand, it's defined as an external force divided by the area over which the force acts; it is espressed in pascal (Pa), which is corresponding to one newton per square metre (N/m2).

Unlike force, pressure is a scalar quantity because it has no size and no direction: it's 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 and even by changing the orientation of the surface over which force is acting, this 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 the how's and why force and pressure are two complementary metrics giving differents data output for analysing performance.

 

Force and Pressure Measurement Technology

Firstly 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 as 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 (in this case they are called single-axis platforms) or both vertical and shear forces (in this case they are called 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 characterised by different types of transducers (strain gauge, piezoelectric and piezoresistive among others), they senses the variation in electrical current and finally this variation is being converted in Newtons (N).

On the other hand, pressure plates directly measure pressure, surfaces and time whereas force is being indirectly calculated by summation of all the sensors pressure values.

Respect than 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 wheter 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 Between Force and Pressure Plates

The main advantage of using both dataset is that one can have a more clear 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 resulted from force plates testing, pressure measurement can provide a force measure for each of the individual loaded sensor on the mat thus allowing to divide the foot in regions of interest (rearfoot, midfoot, forefoot).

Segmentation of the foot, together with CoP (center of pressure) 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 the mastermind behind the Omniathlete Performance Concept. He is a professional sports performance consultant and elite coach from Italy: his area of expertise includes injury prevention, sports technology, strength training programming, speed development, recovery monitoring and return to play assessment. He works worldwide since 16 years with semi-professionals, professionals and Olympic athletes as well as professional teams in various disciplines. Regularly invited as a Keynote Speaker to hold lectures during international conferences in Sports Science and Strength & Conditioning, he is also an Editorial Advisory Board member of the Lower Extremity Review, the most authoritative magazine for lower extremity biomechanics, sports medicine and rehabilitation. Currently he is consultant for Federations, Governing Bodies, Olympians and for First Division football and basketball teams in Europe, Asia and USA. 

 

 

 

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