Chapter 14: The Center of Gravity and Stability

Chapter 14:The Center of Gravity and Stability KINESIOLOGY Scientific Basis of Human Motion, 10th edition Luttgens & Hamilton Presentation Created by TK Koesterer, Ph.D., ATC Humboldt State University

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 • Female’s CG is ~ 55% of standing height • 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

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 • When a slight disturbance will drop the objects CG to a lower point Fig 14.5b

Neutral Equilibrium • When a object’s CG is neither raised nor lowered when it is disturbed Humans spend most of their time adjusting their positions to the type of equilibrium best suited to the task Fig 14.5c

Factors Affecting Stability • The ability to maintain one’s balance under unfavorable circumstance is recognized as one of the basic motor skills • The following factors affecting the stability of a performer’s equilibrium state • should make analysis easier • may suggest means for improvement

Size of the base of Support • CG must remain of the base of support in order to maintain 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

Height of the Center of Gravity • Height of CG changes with body position • As CG moves closer to base of support more angular displacement can occur before it goes beyond the base of support a > b > c with respect to lateral stability x x Fig 14.8 x

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

Friction • Friction is related to the size of the base of support • It has greater influence when body is in motion or being acted on by an external force • Inadequate friction makes it more difficult to maintain equilibrium

Segmental Alignment • The human body consists of a series of segments placed one above the other, the problem of retaining equilibrium is a multiple one • When segments are aligned in a single vertical line, there is less likely hood of strain to joints and muscles • When one segment gets out of line, another segment must compensate for it

Visual and Psychological Factors • The effect of crossing a swirling river on a foot bridge is a detriment to one’s equilibrium • Even if the supporting surface is adequate • The sense of balance may be disturbed, extraordinary stimuli • Compensation: fix eyes on a stationary spot above or beyond the “danger area” • Seems to facilitate neuromuscular control by reducing the disturbing stimuli

Physiological Factors • Semicircular canals can affect equilibrium • Colds, viruses, and other problems can affect the inner ear may also interfere with balance • Any disturbance of the general physical condition is likely to affect the sense of balance

Principles of Stability: I. Other things being equal, the lower the CG, the greater will be the body’s stability II. Greater stability is obtained if the base of support is widened in the direction of the line of force III. For maximum stability the line of gravity should intersect the base of support at a point that will allow the greatest range of movement within the area of the base in the direction of the forces causing motion

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

Principles of Stability: VI. Other things being equal, the greater the friction between the supporting surface and the parts of the body in contact with it, the more stable the body will be VII. Other things being equal, a person has better balance in locomotion under difficult circumstances when the vision is focused on stationary objects rather than on disturbing stimuli

Principles of Stability: VIII.There is a positive relationship between one’s physical and emotional state and the ability to maintain balance under difficult circumstances IX. Regaining equilibrium is based on the same principles as maintaining it.

Mobility • Mobility & stability have an inverse relationship • A critical point is the the change form a position of stability to a state of mobility & vice versa • To initiate a step, line of gravity must be shifted forward of the base of support, swing then moves forward to reestablish a base of support • Often is sport, it is necessary to alter stability intentionally to become mobile

Mobility • Often is sport, it is necessary to alter stability intentionally to become mobile • Ability to start, stop, or change direction quickly depends on manipulating the stability of the body Fig 14.14a

Mobility • Both speed and direction of desired mobility are used to determine the nature of the change in stability required to initiate motion • To enhance the speed of a start, the line of gravity should be as close as possible to the edge of the base of support • A quick stop requires a large base of support, lower CG, and move the line of gravity away form the leading edge of the base of support

FINDING THE CENTER OF GRAVITY IN THE HUMAN BODYReaction Board Method • 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

Chapter 14: The Center of Gravity and Stability