Chapter 14: The Center of Gravity and Stability

Outline

• Center of Gravity

  Definition of Center of Gravity

  Placement of Center of Gravity in Humans

• Stability and Equilibrium

  Factors Affecting Stability

  Principles of Stability

  Mobility

• Center of Gravity and Posture

  Postural Adaptation

  Dynamic Posture

  Principles Applied to Posture

• Finding the Center of Gravity in the Human Body

  Reaction Board Method

  Segmental Method

• Laboratory Experiences

Objectives

At the conclusion of this chapter, the student should be able to:

1. Define the term center of gravity, and explain the basis for its location in the human body.

2. Estimate the location of the center of gravity of individuals in any position.

3. State the principles of equilibrium, and explain and demonstrate applications of each.

4. Discuss the factors that affect the stability and energy cost of the erect posture.

5. Explain the effects that the postural adaptations have on static and dynamic postures.

6. Explain the value of both anticipatory and compensatory postural adjustments.

7. Locate the center of gravity of an individual using either the reaction board or the segmental method.

Center of Gravity

Definition of Center of Gravity

The center of gravity of a body is sometimes described as its balance point or that point about which a body would balance without a tendency to rotate. For this reason, the center of gravity is often identified as the point where all the weight of the body or object is concentrated. More accurately, it is the point where the weight of the body may be said to act.

The ability to locate the center of gravity of a body is based on the knowledge of what it takes for a system to be balanced, or in equilibrium. Two conditions must be met:

1. All the linear forces acting on the body must be balanced.

2. All the rotary forces (torques) must be balanced.

Another way of expressing these necessary conditions for equilibrium is to say that the sum of all the forces acting on the body must equal zero. If there is a downward-directed linear force, there must be an equal upward force so that the vector sum of these forces equals zero. If there is a negative clockwise torque, it must be canceled out by a positive counterclockwise torque of equal magnitude (Figure 14.1).

In this illustration it is represented as the intersection of the x-, y-, and z-axes. It may be located by application of the principle of torques.

A simple experiment to locate the center of gravity, or balance point, consists of suspending an irregularly shaped object by a ...