Mastering the dance of center of mass. Who wouldn’t sign up for a class about that?
With a thick handle and off-set center of mass, the design of the kettlebell is unique and carries with it unique benefits and also some challenges. Traditional dumbbells and barbells tend to center the weight with your hand, but a kettlebell’s center of mass is about six to eight inches from the handle, and that changes depending on what exercise you are performing.
The purpose of this article is to describe this dance between the user and the center of mass of the kettlebell and how it can be used to your advantage and when to understand the challenges presented.
Understanding Center of Mass in Kettlebell Training
First let’s begin by defining center of mass:
- The center of mass is the location where all of the mass of the system could be considered to be located.
- For a solid body, it is often possible to replace the entire mass of the body with a point mass equal to that of the body’s mass. This point mass is located at the center of mass.
- For homogenous solid bodies that have a symmetrical shape, the center of mass is at the center of body’s symmetry, its geometrical center.
- The center of mass is the point about which a solid will freely rotate if it is not constrained.
- For a solid body, the center of mass is also the balance point. The body could be suspended from its center of mass and it would not rotate, i.e. not be out of balance.
- The center of mass of a solid body does not have to lie within the body. The center of mass of a hula-hoop is at its center where there is no hoop, just hula.
So for the kettlebell, the center of mass could be considered to be at the geometric center, but for the human body the center of mass is bit more “active” and changing depending on gravity and our movement.
As you see in the picture above, the center of mass of a person elevates by raising the arms and the center of mass falls outside the body during a toe-touch movement. The dance of center of mass means coordinating the center of mass of the kettlebell and the center of mass of the individual, which is ever changing depending on movement.
Center of Mass and the Kettlebell Sumo Deadlift
In the simplest terms, the kettlebell sumo deadlift allows you to place your center of mass over the kettlebell’s center of mass. A huge advantage when you consider how efficiently this allows you to load this pattern.
Performing a hip hinge, and the posterior weight shift with the trunk angle, means the individual’s center of mass will be changing and then re-centering once the kettlebell is grasped and lifted. This combined center of mass should be centered in the base of support. This should also explain why some deadlifts and swings will look very different depending on body structure and the impact it has on center of mass.
Center of Mass and the Kettlebell Swing
The kettlebell swing begins to make things interesting. Now the dance becomes a dynamic series of adjustments and balancing/counterbalancing as the force production greatly enhances or challenges this dance. Below is a picture of force plate data from my swing from Brandon Hetzler’s Science of the Swing:
Here you can see the float of the kettlebell at the top of the swing, the large eccentric load at the bottom of the downswing, and the force production to once again pop the kettlebell to the float.
At the bottom of the downswing, there are not only the forces of the eccentric hip hinge and weight of the kettlebell pulling my center of mass outside my body into a “face plant,” but also trying to pull me off my feet backward. All of this is balanced out as I drive into the ground to move back from deceleration into force production. Lots going on.
Didn’t realize all of that did you? And while driving to the top for the float, the kettlebell and the center of mass is being projected straight away from me – meaning I have to maintain my center of gravity and not get pulled forward by the kettlebell.
Center of Mass and the Clean and Snatch
We take advantage of this displaced center of mass during the clean and snatch, where the hip action of the swing is used to propel the kettlebell, but the arc of the swing is “tamed” and the centrifugal force and rotation center of mass of the kettlebell are directed to efficiently land the kettlebell at the rack or overhead position. And again on the downswing of both exercises, the arc must be tamed and the rotation and centrifugal force of the kettlebell must be dealt with to efficiently move into the downswing.
All of this means the person swinging, cleaning, or snatching the kettlebell must produce tension and relaxation at the right times, including the grip. During the ballistic drills, the dance of center of mass requires a unique set of skills versus when using a dumbbell.
Center of Mass and the Grinds
Take a moment to look at the pictures above. Note the center of mass of the kettlebell in relation to my right shoulder in the overhead position in the picture on the left versus the center of mass of the kettlebell over my left elbow in the same picture. Then, look at the center of mass of the kettlebell in the bottom-up position in relation to my right forearm and elbow in the other picture. I believe the kettlebells in the leftpicture are 32kg and the kettlebell in the picture on the right is a 24kg.
Notice how the overhead kettlebell center of mass basically lines up over my shoulder, keeping the center of mass of the kettlebell over the center of the joint supporting it. This also means that if the kettlebell is large enough, the arm supporting it may not look vertical but the center of mass of the kettlebell will line up over the shoulder/base of support.
During the press, the movement from the rack position shown on the left arm to the overhead position means you will move through the vertical position shown in the bottom-up picture. With kettlebells upwards of 32kg for men and 16-20kg for most women, the off-set center of mass will guide the path of the press and be advantageous for the mechanics of the press. Beyond 40kg and 20kg, the ability to hit a vertical forearm is challenged and individuals can end up pressing from a slightly internally rotated shoulder position. A volume of pressing from this position could cause some irritation at the shoulder.
During the get-up, the individual “moves around” this center of mass through the different positions and transitions. Again, if the kettlebell is large enough, there may not be a vertical arm at certain stages of the get-up, but the center of mass of the kettlebell will line up with the base of support. Otherwise, the off-set center of mass can produce a negative stress on the shoulder.
Understanding Center: The Dance of Mass
Also keep in mind that the larger the kettlebell and the more displaced the center of mass, the more it will impact the stability to keep the alignment of the center of mass with the supporting structures. Limb length also plays a role. Longer limbs mean more displacement of center of mass and shorter limbs can mean more of a mechanical advantage. Read more on this in Brandon Hetzler’s article A Long Way to Press.
So, there you have it. A quick rundown of the dance of center of mass in kettlebell training. This dance means kettlebell training is more “alive” than some other forms of resistance training and can be a benefit or a challenge depending on the exercise.