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Objectives 1. Define 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. Locate the center of gravity of an individual using either the reaction board or the segmental method

CENTER OF GRAVITY (CG) The “balance point” of the body The point where the weight of the body acts The point where all forces acting on the body equal zero Linear forces must be balanced Rotary forces must be balanced Fig 14.1

CENTER OF GRAVITY The location of the CG remains fixed as long as the body does not change shape If an object’s shape or position changes, the location of the CG changes Fig 14.3

CENTER OF GRAVITY As one changes the relationship of the body segments to each other, the CG may even be located outside the body Fig 14.4

Placement of the Center of Gravity in Humans The location of the CG of a human being in the normal standing position varies with body build, age, and sex

Placement of Center of Gravity Male’s CG is ~57% of standing height In quiet standing, the CG can be considered to be almost directly over the center of pressure Center of pressure is the point at which the force vector for ground reaction force is applied Female’s CG is ~ 55% of standing height

STABILITY AND EQUILIBRIUM All objects at rest are in equilibrium All forces acting on them are balanced The sum of all linear forces equals zero The sum of all torques equals zero However, all objects at rest are not equally stable

Stable Equilibrium Occurs when an object is placed in such a fashion that an effort to disturb it would require its CG to be raised Fig 14.5a

Unstable Equilibrium A slight disturbance will drop the objects CG to a lower point An unstable objects seeks stability by falling from a smaller to a larger base of support Fig 14.5b

Neutral Equilibrium Occurs when an object’s CG is neither raised nor lowered when it is disturbed Remains the same Inertial property Fig 14.5c

Size of the base of Support CG must remain within the base of support in order to maintain stable and in equilibrium Easier with larger base of support Standing Walking Headstand Fig 14.6

Shape of the base of Support Fig 14.6c Fig 14.6b Resistance to AP forces Resistance to lateral forces

Base of Support Both feet and hands a quadruped Babies crawl, roll and sprawl in their form of locomotion Children and adults biped Seniors quadruped with the help of a walker Age and fitness specific

Vertical Center of Gravity Height and location of Center of Gravity along a vertical line (y axis)

Height of the Center of Gravity a > b > c with respect to lateral stability x x Fig 14.8 x

Height of Center of Gravity (CG) Height of CG changes with body position along the vertical line As CG moves closer to base of support more angular displacement can occur before it goes beyond the base of support

Relationship of the Line of Gravity to the Base of Support To maintain equilibrium, line of gravity must remain within its base of support Fig 14.9 & 14.10

Mass of the Body Only a factor when motion or an external force is involved Amount of force needed to effect a change in motion is proportional to the mass being moved The greater the mass, the greater the stability

Principles of Stability: IV. Other things being equal, the greater the mass of a body, the greater will be its stability V. Other things being equal, the most stable position of a vertical segmented body is one in which the CG of each weight-bearing segments lies in a vertical line centered over the base of support

Mobility Fig 14.14a

Reaction Board Technique Relies on the fact that the sum of the moments acting on a body in equilibrium is zero The location of the gravitational line is found for each plane The CG is the intersection of the values for each of these three planes Limited to the body in a stationary position

Segmental Method A highly useful procedure find the CG for someone in action Technique uses; a photograph of the subject Involves finding the CG of each body segment The position of these gravity points are placed on x and y axes Knowledge of the ratio between the individual segment weight and total body weight

Center of Gravity Vertical Alignments