Center of Gravity and Midline Rhythm, Stay Centered in Your Swing

In 2014 I co-wrote a 3D biomechanics analysis program based on the 8 Athletic Movement Principles with the desire to quantify the 8 Athletic Movement Principles. This was not a full-time endeavor, but it was very enjoyable and provided me the luxury of designing a program on a system that I would not typically gain access to with a partner who was very well educated in technology and physics. The program took us roughly a year to produce and our work was validated by biomechanists.

For the purposes of quantifying each Athletic Movement Principle, we designed a virtual point to track the position of the Midline which we call the Midline Axis Point, a point that represents the position of the Midline throughout the swing. This point was on the back of a player’s body half way between C7, which is the most prominent bone at the base of your neck; and the top of your sacrum close to the lowest point of your lower back. We also created a virtual point to approximate the Center of Gravity or Center of Mass. This point is the average of all the Centers of Gravity of each body segment, it is very close to the belly button for most people.

During any data capture the player and club were marked with sensors, and the player was swinging on 4 force plates. The movement and force data were captured by the force plates. Ten infrared cameras captured 3D motion at a rate of 500 frames per second.

Observing the Midline and Center of Gravity Rhythm
We can observe the Midline and Center of Gravity Rhythm when an athlete is applying force from the ground in a hitting or throwing motion in closed chain, or when they are in contact with the ground. In sports like golf and hitting in baseball, where athletes are essentially swinging from a fixed axis, the extremities rotate around the Center of Mass to create Centripetal Force. Power is generated in an arc and the spine is a force multiplier that adds acceleration to the extremities with parametric acceleration. In this way, the management of the Midline and head relative to the Center of Mass provides an additional force in a Rhythmic Sequence. The fulcrum for the axis of the shoulder girdle in a 2-handed swinging motion like the golf swing and the hammer throw is the middle of the shoulder girdle closely approximated by C7.

3D video of the golf swing that tracks the COM or Center of Mass in the frontal plane (along the line of the feet) is of interest to players who want to improve their leg action and control their Center of Gravity because it shows that the Center of Mass does not move from the time of the Moment of Maximum Ground Force- when the arms are close to hip height in the downswing until well after impact. This is quite different from the traditional idea of shifting your weight through impact. In observing the data, the golf more is much more like cracking a whip than a hit while shifting.

As much as possible we like to measure discrete events as they are analogous to musical notes when we talk about Sequencing, Rhythm and Tempo. The Moment of Maximum Ground Force has been useful in this regard. It occurs after the Transition into the downswing when the player is gravity assisted swinging the club in Double Support. It is the moment when the force into the ground is the greatest and the acceleration of the clubhead is the greatest. It occurs just after the player has lowered his Center of Gravity for control and pushes into the ground to move the clubhead around his or her axis in a wide arc at the ball. This may open a debate on the Athletic Movement Principle vernacular of pushing and pulling so we define the use of the 2 terms here:

Pushing: the maintenance of an implement in front of Midline during a movement

Pulling: dragging an implement behind Midline or leading with the body in a movement

After the Moment of Maximum Ground Force which is analogous to a burst or firing of jets to get a rocket off the ground, the player must be able to relax to some degree as the arms decelerate in order to square up the Center of Mass of the clubhead with the golf ball close to Midline. This is the “cracking of the whip.”

The moment of “cracking the whip” lasts from the Moment of Maximum Ground Force until just past impact. The fact that the Center of Mass does not move appreciably during “the cracking of the whip” may be confusing and feel remarkably different to those players who have sought to consciously shift their weight as they turn. Rather than shift weight, the more athletic movement is to use the Penultimate Leg and Double Support to set up the rotation and push the club in an arc, letting the force generated swinging the club move the body through to the finish. This will result in greater control of the Center of Mass and the golf club which translates into a cleaner, consistent strike.

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