The center of gravity is a theoretical place in your body where your mass is considered to concentrate.
What do I mean by that? Well, here on earth, weight and mass are pretty much the same thing.
You can think of mass as how much resistance your body has when moving - or the force of inertia as it applies to your weight. (In outer space, you become weightless, but your mass stays the same. This is because in outer space the force of gravity does not act on your body.)
Center of Mass - Theory and ApplicationAnother way of looking at this is that the center of gravity (mass) is the point at which the body's mass is equally balanced. This changes depending on one's position (arms up/down, leaning, etc). Dancers, gymnasts and tight-rope walkers are examples of how the human body compensates for changes in the center of gravity to maintain balance.
Usually the center of gravity is located in front of your sacrum bone, at about the second sacral level. (The sacrum is made up of five bones fused together vertically.) So when you are on planet earth your weight, or mass, is thought to be concentrated at this point in front of your sacrum. The downward pull of gravity (line of gravity) passes through this point, as well.
To understand the difference between theory and practical application of this concept, let’s compare the human body to a baseball for a minute. From a point in the exact center, the baseball is of equal weight and shape all the way around, is it not? So, with any movement of the ball, this center point moves right along with it. Easy.
But when we consider center of gravity in the human body, things get more complicated. As we mentioned before, because the body has moving parts (arms, legs, head, various areas of the trunk), every time you do, well, anything, the shape of your overall form changes. And if you carry something like a suitcase or grocery bag or if you wear a backpack, this adds weight, which changes the center of gravity, too.
So, we can say that the center of gravity is a constantly changing point in the body that represents where the weight (mass) of the rest of your body is equally balanced in every direction. This point can and does change based on what you’re carrying and how you’re carrying it, as well as the position you take and the movements you make.
My personal take on it this: In an ideal situation I like to think of the center of gravity as the place from which you can operate your whole body as a unit, while gracefully coordinating the movement of appendages as you go.