center of mass and base of support

Try standing on one foot, then on two with feet wide apart — the wide stance feels far steadier. Now lean until you tip over: you fall the moment your weight passes beyond your feet. Those two simple experiences are governed by two ideas that decide whether you stay upright: where your weight is concentrated, and how big the patch you are standing on is.

The center of mass (often called the center of gravity) is the single point where the body's whole weight can be thought of as concentrated; in a person standing normally it sits roughly in front of the lower spine, a little below the navel. The base of support is the area on the ground bounded by your points of contact — for two feet, the patch of floor between and around them; adding a cane or walker enlarges it. The rule of stability is this: you stay balanced as long as the downward line from your center of mass falls inside your base of support. You become unstable, and must step or grab something, the moment that line drifts toward or past the edge of the base. A wide base, a low center of mass, and keeping your weight centred all make you harder to topple.

Almost everything in balance and mobility rehabilitation is an application of these two ideas. A walking frame or a wider stance enlarges the base of support and buys stability; a person who leans or carries a load shifts their center of mass and may need to widen their base to compensate. It also reframes falls honestly: a fall is not simply 'weak legs' but the center of mass escaping the base of support faster than the person can move their feet to catch it — which is why balance training works on quick stepping and weight-shifting, not only on raw strength.