E7 Stability and Balance

Topic

Stability and balance shape the physical mechanisms that maintain the body in static and dynamic equilibrium, integrating three elements: the position of the center of mass, the base of support, and ground reaction forces.

The first element is the center of mass: the point where the body's entire weight is considered to be concentrated, and whose position constantly changes as the body moves or shifts posture.

The second element is the base of support: the area defined by the points where the body contacts the ground—such as the soles of both feet while standing, or an area that varies in size depending on the number of contact points at any given moment.

The third element consists of ground reaction forces: the force the ground exerts back on the body at each contact point within the base of support; their exact magnitude and location determine the direction in which the body tends to lean or correct itself at any given instant.

Based on the center of mass and the base of support, stability and balance are quantified using the "limit of stability" margin: the distance between the projection of the center of mass onto the ground and the nearest edge of the base of support. The larger this margin, the further the center of mass can shift before its projection moves outside the base of support and the body loses its balance; the smaller the margin, the less displacement the body can tolerate before falling.

In addition to this limit of stability margin, stability and balance involve the dynamics of postural sway: the center of mass never remains perfectly still but constantly oscillates within the base of support as the body makes small adjustments to keep it away from the edge and within the limit of stability margin.

The closer this postural sway comes to the edge of the limit of stability margin, the closer the body is to losing stability and balance; maintaining this sway within a wide margin—far from the edge of the base of support—is what allows the body to remain upright or in motion without falling.

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