3. Reaction force

Reaction force

Reaction force is a force acting on a person when the soles of the feet come into contact with the pad. Reaction force´s magnitude reflects the force of gravity (based on the weight of the evaluated segments) and the force caused by its own movement (involuntary or voluntary change of body position).

A force platform with built-in force sensors is used to measure the reaction force.

Force platforms

The force platform measures the magnitude and direction of the reaction force vectors during standing and walking.


There are two types of power platforms - static and mobile. Platform types differ in the number and properties of force sensors.

The effect of force > surface deformation > it is recorded by force sensors si an electrical signal is sent > it is processed and evaluated.

The signals from all force sensors are summed > the resulting reaction force is created.

The resulting reaction force is the result of all partial forces acting on the substrate.


Place of action of the resulting reaction force = Center of pressure (COP).


see chapter 3. Force platforms


COP

  • indicates the average of all pressures acting on the pad.
  • represents the beginning of the reaction force vector of the substrate.
  • The resulting reaction force can be decomposed into individual components perpendicular to each other - vertical (Fz), mediolateral (Fx), anteroposterior (Fy) and their moments (Mz, Mx, My).

The position of the force vector is determined on the basis of the position coordinates x, y, z, with respect to the defined reference point of the platform.



The output evaluated parameters for the platforms are the position of the COP and the size of the individual components of the reaction forces (vertical, mediolateral, anteroposterior).



  • The vertical component reaches the highest values ​​during the support phase with a maximum of 120% of the person's gravity.

  • The anteroposterior component (shear force) determines the load on the foot in the braking and acceleration phase of the foot's contact with the pad.

  • The mediolateral component is variable, its use for gait analysis is difficult.

The position of the COP indicates the projection of the center of gravity of the body onto the base (so-called center of gravity - COG).



Based on the COP trajectory, the size of titrations during standing or the course of stepping cycles during walking is evaluated (the symmetry of the lower limbs can also be assessed here).



The magnitude of the reaction force of the surface (vertical part) is usually assessed in connection with the analysis of walking (treadmile with a built-in force platform) and its modifications (dynamic computer posturography).