CoP curves in foot pressure scanners
If you are a user of leg pressure scanning devices, you have probably seen dotted COP curves in the output pressure contours of your devices.In this post, we will learn about the concept of CoP and discuss its clinical applications.
What is the COP parameter?
COP parameter stands for Center of Pressure. In fact, the COP represents a point at each moment of a person’s stride. This point is the position of the effect of all the vertical forces exchanged between the foot and the ground. So that we can equate all the forces and torques coming from the ground to the body at that point.

What is the COP curve?
As mentioned, COP represents a point at any moment. Therefore, it is natural that if we measure the COP at any moment in time, by placing the COP points together, we will see a dotted curve, like the one shown in the figure below. The number of points in each COP curve depends on the stepping time as well as the data acquisition frequency of the device. The higher the data collection frequency of the foot pressure analysis platform, the more it will be possible to analyze fast foot movements.

Compression of points on the COP curve
According to the working frequency of the foot pressure analysis platform, the COP position is measured and displayed at regular intervals. For example, if the device works with a frequency of 100 images per second, in this case, the COP position is measured once every hundredth of a second. In this way, the large distances of the points in the COP curve indicate the fast movement of the step in each phase, and the compression of the points indicates the longer time the body remains in that movement phase.
Does COP only depend on the position or shape of the foot?
No, in answer to this question, it should be said that the position of COP is a complex part of the movement system, including the skeletal-muscular system and the body’s nervous system, which ultimately shows its effect in the distribution of foot pressure. Therefore, the analysis of this trajectory contains valuable diagnostic points about the state of the nervous system as well as the musculoskeletal system of the body.
What is the diagnostic function of COP?
Identifying the diagnostic functions of COP is one of the current research topics in neuroscience, physical medicine, and orthotics and prosthetics. Among the important functions of the pathological evaluation of COP, one can diagnose abnormalities and disorders of the nervous system in chronic neurological diseases, motor rehabilitation, identification of movement disorders in the foot (such as pronation and supination), balance measurements (Fall Prevention) and… cited. In the following articles, we will discuss the diagnostic functions of COP and its analysis and interpretation methods.
Pronation or inverted foot
The term foot pronation, which is sometimes used as the Persian equivalent of internal rotation, includes a natural movement of the foot, which after the heel strike phase is completed, prevents the transfer of shock stresses to other parts of the body’s musculoskeletal system. provides in the available scientific articles and sources, the internal rotation movement of the foot is divided into three joint movements in three planes, including subtalar eversion, ankle dorsiflexion and forefoot abduction.
In the stance phase, which begins with heel strike, the outer edge of the heel comes into contact with the ground. This contact creates a locked state in the movement mechanism of the body. In the continuation of this phase and before reaching the foot load, the body is either forced to accept the shock from the ground surface, or by creating an internal turning state, as seen in the figure below, while absorbing the shock caused by this contact. Makes it possible to continue moving smoothly and without shock. This movement is the natural pronation or internal rotation of the foot, which makes the distribution of force on the surface of the foot to remain normal and uniform, along with a smooth and shock-free movement.
Flat or inverted foot?
One of the interesting points in the published articles about foot disorders is the common mistake of equating flat feet with inverted feet. In the same way, it can be seen in some of the materials published on the Internet that sometimes under-pronation in the foot is equated with the Dicey arch foot disorder. This is despite the fact that, according to valid scientific articles, problems such as flat feet or Dicey’s arch are classified in the category of structural problems, and pronation or supine state are classified in the category of functional problems of the foot. Although the separation of these two categories of biomechanical forms does not mean that they are unrelated. In the following articles, we will discuss the differences between these two disorders in more detail and diagnose them using foot scanners and pressure analysis platforms.
CPEI index in foot pronation study
One of the important parameters that can be extracted from the COP curve is the CPEI index. This index, as shown in the figure below, indicates the amount of lateral deviation of the COP curve in the front third of the leg, compared to the line connecting the beginning and end points of the COP curve. To calculate the CPEI, the aggregate frame related to the entire stance phase in the data extracted from a dynamic foot scanner is analyzed and divided into three parts according to the figure. The construction line is drawn from the first starting point of the COP curve to its end point according to the image below. The AD line separates the front third from the middle third of the foot print surface. also known as foot width. The distance between the intersection of the AD line and the Construction line, i.e point B, as well as the intersection of the AD line and the COP curve, i.e point C, are calculated. In this way, the value of the CPEI parameter is obtained from the result of dividing CPEI=BC/AD.

Foot Scanner: Essential for Diagnosing Pronation Disorder
In a leg suffering from over-pronation disorder, the convexity of the COP trajectory is reduced, causing the curve to incline toward a straight line. In this case, the value of the CPEI parameter decreases and may even become negative. Conversely, in a foot affected by under-pronation, the convexity of the COP curve increases abnormally, leading to a rise in the CPEI. Our foot scanner provides valuable insights into these conditions. By analyzing the paths of the COP trajectory and the diagnostic parameters related to foot function, our device helps identify deviations effectively. Studies on CPEI values have presented pathological borderline values, which are crucial for diagnosing these deviations. The results demonstrate a significant relationship between deviations from anatomical norms and the occurrence of low back pain.

In the following content, more details about the points in the output contours of foot scanner devices, differentiation of structural and functional disorders of the foot, diagnostic indicators related to the foot, the effect of each factor in the design of suitable insoles or sandal therapy, software settings and we will also pay attention to the expectations of a proper foot scanner. It is hoped that with the publication of these articles by Aramed Academy, the field of improving public knowledge in the field of diagnosis and treatment of biomechanical foot disorders in technical orthopedic centers and other specialized centers active in the field of foot health will be provided.