What is the Moment and the Torque on The S2M Dual 3D Force plates?

Published by Jean-Paul Fernandez on October 30, 2018

I think some explanations are needed to not confuse everybody about moment and torque with force plates, so from a kinetic point of view!

As an introduction, I would specify that the Torque is to the rotational movement the equivalent of what the Force is for a linear movement.

The Moment is calculated around the Center of Mass of an object. It represents the quantity of rotation this object is turning around its center of mass. If you apply a force right away on the center of mass of an object, it moves linearly in the direction the force has been applied, and don’t turn. If you apply a force besides the CoM of an object, it will start a rotation around its center of mass. More you apply a force far away from the center of mass of an object, more this object will increase its rotation around the center of mass. Its rotational motion is called the Moment, and it has been generated by a force called Torque (linear force applied away from the center of mass of an object, and the distance between point of application and CoM position is called the lever).

In mechanics, we are looking at this quantity of rotation around the 3 axis x,y and z, and expressed the 3 moment Mx, My and Mz.

On a force plate, the Center of Mass is located right in the middle of the force plate, this is why moment of force plates are always measured from the center of the plate. The force plate is in contact with the ground and is generally fixed, so it can’t turn, around any of the 3 axes. But we can apply some force on the force plate that could intend to make this plate turning around its center of mass, and this is what the moment of the force plate is showing as numbers / graphs Mx, My and Mz.

To sum up, the Moment of the force plate represent how much the force plate will rotate around its center of mass if the plate was not fixed and influence by no other force (gravity, Ground reaction force, friction or resistance, external forces etc…). As a consequence, I do not create any moment if I apply force right on the center of mass, but I create big moment of the plate if I’m on the edges… even if I produced the same force or motion in both cases.

Let’s talk now about the torque.

The Torque tends to make an object / subject or system rotating around an axis. The calculation of torque is the vectoral product of forces applied to a plan. We are standing on the ground, and the ground is a horizontal plan. So, when I’m creating force to the ground, if I calculate the torque, I’m doing the vector product of Fx & Fy (horizontal forces) at the point of application of the force (the Center of Pressure), and it gives me the vertical Torque Tz, that means the quantity of rotation around the vertical axis (perpendicular to the plan of ground).

 

Like you have the ground pushing you up to the sky when you apply a vertical force with your muscles toward the ground (3rd law of newton), the vertical Torque indicates to you how much rotational force the ground is given back to you when you apply some rotation on it… and its point of application is where the Ground reaction forces are applied, so the Center of Pressure. With our S2M Dual 3D force plate, you can calculate 3 vertical torques:

  • Individual vertical torque generates by each foot => how much each foot / leg screwing or unscrewing into the ground and get it back from the ground as vertical torque
  • Global vertical torque => how much the system “Body + Club “is generating some rotation to the ground (with shear force produced in the horizontal plan by each foot working in opposition) and get it back as vertical torque

 

So, the vertical Torque represent the amount of rotational force you applied around each Center of pressure, and as the 3rd of newton describes, the principle of action reaction, the “Ground reaction Force of Rotation” give back a certain amount of rotation that tends to create rotation around the vertical axis.

To sum up, the vertical torque indicates how much rotational force you create around vertical axis from the ground under each foot (around left CoP and around right Cop) and for the system Golfer + club (around Global Cop). Like the global GRF is not applied at that physical point, the vertical Torque is the same, but both will generate some effect on the whole Body + club system.

To remember:

Moment (Mx + My + Mz) => represents how much the plate could rotate if it was not fixed, and calculated around the Center of Mass of the object
Vertical Torque (Tz) => represents the Ground Reaction quantity of Rotation around vertical axis at a point of application of force, in our case Center of Pressure. The torque is creating an amount of “force of rotation” applied to the whole golfer + club system!

The next step is to analyze how the global Ground Reaction Force Vector vs the position of the Center of Mass of the golfer + Club system in dynamics, and how Ground Reaction Forces will create some displacement and some moment around the Center of Mass (torque generated by the point of application of GRF global vector vs Position of CoM in space) … But we are leaving Kinetics measurement done by Force plates and entering in Kinematics measurement to get proper measurement of Center of Mass position, body segments position, club position …