The relation between GRF and CHS in golf!
Before looking at the relationships between Ground Reaction Forces and Club Head Speed in the golf swing, here is a reminder of why we need force, and how we can generate it.
Why do we need force?
In Physics, the equation of force is Force = mass x acceleration!
When you want to make an object to get some change of its velocity (= acceleration), you must apply a force to it to make its speed go up or go down. If we consider the mass of the object is constant (like a golf club), the only way to make this object moving faster is to apply it more force.
Conversely, if you cannot apply more force, the object will only move faster if you reduce its mass.
As we are not varying so much our body mass (or weight), and we play regularly with the same set of clubs having their own constant mass, the only possibility to increase speed of our motion and / or our golf club is to produce more force, and transmit it to our body segments and to the club.
Mechanically, if you increase the force, you can increase the speed! But only if the additional force is applied to the club and not lost or wasted anywhere else!!!
How can we generate some force and motion ?
The first answer is: the body can produce forces through our muscles!!!
Muscles are connected to bones though tendons, around the joints, and by shortening a muscle, you change the angulation of the joint and you create a motion of the distal body segment around the joint. On the other side of the joint, the proximal body segment is normally remaining stable or in fixed position (unless if this segment is also in motion with same principle…).
How do you stay stable and also move?
To hold a fixed or stable position, any body segments will be linked to one or more other body segments on which they can stabilize themselves. And if you find the end of this chain, you will always finish to the body part which is in contact with the ground (or an element in contact with the ground itself) and exchange with it equal and opposite forces, the 3rd law of motion described by Isaac Newton. The stability always comes from the ground!
Without the ground, you will not have stability, and you will be very limited in the max force you can produce. In fact, to get our muscles working all together and produce powerful and complex motion, you first need an external support to push on… If you don’t have something to push against, you will be very limited in the max force you can produce, and complex motions are not possible.
Imagine you were sitting with your legs and arms off the ground, how much force do you think you are able to create by kicking with one foot on a soccer ball? And how precise will be the control of trajectory? And yet you are in contact with the ground…
Notice also you can not displace your body in space without a ground, or a stable surface to push against… except with a flying machine!
To displace yourself, you apply to the ground forces, that can be represented by a vector (point of application, direction and magnitude), and according the 3rd law of Newton, the ground sends you ground reaction force in the opposite direction. Depending on the direction and the amplitude of the vector, or the amount of force, you will move more up or down, left or right, front or back. These forces help us to move our entire body, or on parts of our body, or even an object we carry for example.
In addition, we can also create motion or displacement not only along the 3 axes (Antero posterior, lateral and vertical), but also around these 3 axes.
How can our body create rotation?
To generate rotation, we can use 2 main principles of physics: the friction between us and the ground, and the relation between Ground Reaction forces and our Center of Mass.
- The friction is a resisting force of 2 elements that tends to slide against each other. An example a car driving on the road, the mud or the ice: sliding surfaces reduce the friction the traction and control of the vehicle.
It’s the same if you try to make a golf swing on the ice, as the friction is nearly null, the feet will not be able to resist to the rotation of the upper body and the lower body will turn in the opposite direction of upper body rotation (conservation of angular momentum principle).
On the contrary, if you stand on a normal ground surface, with spikes under your shoes, the lower body will be able to resist to the rotation of the upper body to create the golf swing motion.
If your body is in contact with the ground with at least 2 contact points, separated by a gap, and the vertical projection of body center of mass remains in the supporting base formed by the contact points, and if you apply horizontal forces in opposite direction through the contact points, then you can generate some vertical torque, this means you create rotational force to make the body or segments turning horizontally.
During a golf swing, by applying opposite horizontal forces mainly in the anterior-posterior axis under each foot, the player can create some rotation in the horizontal plane to generate club head displacement and speed.
- The alignment between global GRF vector and CoM: if you apply a linear force beside the Center of Mass of a system, it will tend to create the rotation of the system.
If the global GRF vector is pointing on the left of the CoM, the system tends to rotate to the the right in the frontal plane.
If the global GRF vector is pointing on the front of the CoM, the system tends to rotate toward the back in the sagittal plane.
What about us on earth?
When we stand on the ground, we are in contact with the earth, which is relatively heavier than us, and it proposes an inertia we can’t fight with! So, without the ground, we would not be able to displace ourselves in space or create torques with our body. We need to use the ground to push against and displace our body in space, and to use the friction force between our feet and the ground to generate some vertical torques. Our interaction with the ground is the origin of the motion, and the efficiency of a motion is link with the control of stability and the variations of forces (point of application, amount & direction) we apply.
How can GRF generate Club Head Speed?
In the golf swing, to generate control and speed of the club head at impact, we produce a complex pattern of motion primary based on lower body action. To generate an efficient club head displacement and velocity, we observed on 3D forces plates the kinetic sequence: all player generates a max peak of lateral force first, then a max peak of rotational force (also called vertical torque) and then the max peak of vertical force from the top swing to shaft parallel to the ground before impact.
This sequence, lateral / rotational / vertical, is the resultant of the forces applied by both legs, but we can observe several different ways to generate a good kinetic sequence with different types of strategies of individual forces applied under each foot.
The golfer can manage his/her stability throughout the swing, and generate more forces and torques by using the Ground Reaction Forces amount and direction (to play with the Center of Mass) to deliver more force to the body that could be applied to the club, and therefore increase club head speed!
Jean-Paul FERNANDEZ – Sport Scientist
April 2020