Center of Gravity (SUPL) PDF

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. Discuss the fac...

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. Discuss the factors that affect the stability and energy cost of the erect posture(position of the body). 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. 14-1 CENTER OF GRAVITY (C OF G) 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.  Torques must be balanced. Fig 14.1 14-2  CENTER OF GRAVITY The location of the C of G remains fixed as long as the body does not change shape. If an object’s shape or position changes, the location of the C of G changes. Fig 14.3 14-3  CENTER OF GRAVITY As one changes the relationship of the body segments to each other, the C of G may even be located outside the body. Fig 14.4 14-4 PLACEMENT OF THE CENTER OF GRAVITY IN HUMANS  The location of the C of G of a human in standing position varies with body build, age, and sex.  Female’s CG is ~ 55% of standing height  Male’s CG is ~ 57% of standing height  In quiet standing, the C of G can be considered almost directly over center of pressure.  Center of pressure - point at which the force vector for ground reaction force is applied. 14-6 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. 14-7  STABLE EQUILIBRIUM  Occurs when an object is placed in such a fashion that an effort to disturb it would require its C of G to be raised. Fig 14.5a 14-8 UNSTABLE EQUILIBRIUM  When a slight disturbance will drop the objects’ C of G to a lower point. Fig 14.5b 14-9 NEUTRAL EQUILIBRIUM  When a object’s C of G is neither raised nor lowered when it is disturbed. Humans spend most of their time adjusting body positions for the type of equilibrium best suited to the task. Fig 14.5c 14-10 FACTORS AFFECTING STABILITY  The ability to maintain one’s balance under unfavorable circumstances is recognized as one of the basic motor skills.  Understanding the factors affecting the stability of a performer’s equilibrium state:  should make analysis of a balance problem easier.  may suggest means for skill improvement. 14-11 FACTORS AFFECTING STABILITY  Three primary factors: 1. Size of the base of support. 2. Relation of the line of gravity to the base of support. 3. Height of the center of gravity 14-12 SIZE OF THE BASE OF SUPPORT  C of G must remain within the base of support in order Standing to maintain equilibrium. Walking  Easier with larger base of support. Headstand Fig 14.6 14-13  SHAPE OF THE BASE OF SUPPORT Fig 14.6c Fig 14.6b Resistance to Resistance to AP forces lateral forces 14-14 HEIGHT OF THE CENTER OF GRAVITY  Height of C of G changes with body position.  As C of G moves closer to base of support more angular displacement can occur before it goes beyond the base of support. a>b>c with respect x x to stability x Fig 14.8 a b c 14-15  RELATIONSHIP OF THE LINE OF GRAVITY TO THE BASE OF SUPPORT  To maintain equilibrium, line of gravity must remain within the base of support  Notice the hyper-extended the trunk to maintain the line of gravity within the base of support in Fig 14.10 Fig 14.9 Fig 14.10 14-16 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. 14-17