ch 14: stability and mobility

stability and equilibrium

- all objects at rest are in equilibrium - all forces acting on them are balanced >linear forces = zero >torques = zero - all objects at rest are not equally stable

mass of the body

- 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

COG varies on...

- build - height - weight - sex

friction

- friction is related to the size of the base of support larger BOS = more friction - greatest influence when body is in motion - inadequate friction makes it more difficult to maintain equilibrium ex: trying to regain balance on ice/slippery surface

a quick stop requires:

- large BOS (base of support) - lower COG - posterior movement of the line of gravity (bringing it towards center)

dynamic posture

- movement disrupts posture: >all action has a reaction (newton) - reaction transmitted from support surface - relationship between COG, center of pressure, and momentum is key

mobility

- often in 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 - speed and direction are used to determine the change in stability required to initiate motion - line of gravity closer to the leading edge can increase start speed

center of gravity and posture

- posture implies shape or configuration of the body - the multi-segmented human body does not have a single posture - dynamic posture is of greater concern that static posture - all posture is somewhat dynamic, including postural sway during quiet standing

shape of the base of support (BOS)

- resistance ot AP forces (anterior posterior) - resistance to ML forces (medial lateral)

postural adjustments will take place:

- some are anticipatory (feed forward) - others are proprioceptive (feedback)

center of gravity (COG)

- 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

relationship of the line of gravity to the base of support (BOS)

- to maintain equilibrium, line of gravity must remain within the BOS (base of support) - line of gravity is a vertical line from COG to the ground

neutral equilibrium

- when COG is neither raised nor lowered when it is disturbed - we adjust our body position to find the most suitable equilibrium for a task ex: when we walk, we fall then regain stability ex: a ball rolling, the balls center of gravity is consistent and the ball is just rolling

principles of stability (simplified)

1. broad/wide BOS increases stability 2. the closer the line of gravity is to the center of the BOS, the more stable 3. lowering the height of the COG increases stability

to initiate a step (controlled falling)

1. line of gravity moves to front of BOS (base of support) 2. swing leg moves forward to re-establish BOS

size of the base of support (BOS)

COG must remain within the base of support in order to maintain equilibrium easier with larger base of support

female's COG

about 55% of standing height

male's COG

about 57% of standing height

COG and relation of body segments to each other

as one changes the relationship of the body segments to each other, the COG may even be located outside the body ex: high jumper

3 primary factors of balance/stability

balance (maintaining stability) is a basic motor skill 3 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 COG

neurological factors of stability

external visual cues may affect stability: - no vision = less input to control stability - roller coaster = optic flow vestibular input can be affected by illness/injury proprioceptive cues may be altered after injury

2 primary strategies on a continuum (postural adaptation)

hip strategy: - sudden hip flexion/extension to maintain balance in sagittal plane - rapid, high magnitude perturbations*** ankle strategy: - angle DF/PF to shift COG anterior or posterior - slow, low-to-high magnitude perturbations***

in quiet standing, the COG can be considered almost ____ over center of pressure

in quiet standing, the COG can be considered almost directly over center of pressure

relationship between mobility and stability

mobility and stability have an inverse relationship a critical point is the change from a position of stability to a state of mobility and vise versa

dynamic adaptation (postural adaptation)

must interpret threats to stability - acceleration - change in base (ex: one foot) - change in direction

stable equilibrium

occurs when an object is placed in such a fashion that an effort to disturb it would require its COG to be raised

center of pressure

point at which the force vector for ground reaction force is applied

unstable equilibrium

when a slight disturbance will drop the objects COG to a lower point

height of the COG

- COG height changes with body position - more movement capable when the COG is closest to the BOS (base of support) a > b > c with respect to stability *the diagonal lines is the range of motion/movement allowed *the straight lines are drawn up from the base of support

center of gravity and shape

- COG remains fixed as long as the body does not change shape - if an object's shape or position changes, COG will also move

segmental alignment

- The human body consists of a series of segments placed one above the other > compounds the problem of maintaining equilibrium - when one segment moves, it affects the line of gravity and another segment must compensate for it