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Posture (2)

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Assignment Of Advance
Motor Learning
Topice:
Posture Control
Submitted To:
Dr. Umair Aehmad
Submitted By:
Anbreena Rasool
UIPT, University of Lahore
POSTURE
Posture can be defined as a combination of the relative position of the body segments or of the whole
body with respect to reference.
REFERENCE FRAME.
Postural control may be refer to keeping the position of a body segment with reference to an external
reference frame.
Example to the environmental or to an external object moving in the environment (Consider holding
the handle while riding a bicycle).
POSTURAL SEGMENT. Postal control may also refer to postural segments with respect to body
itself.
Some Examples.
A. Ballet dancer or a figure skater maintain a beautiful arm configuration while moving over the
floor or the ice rink.
B. A waiter carrying a loaded tray.
C. A carpenter holding a nail with the hand.
PROBLEM ASSOCIATED WITH STANDING:1- Humane vertical posture is inherently unstable because of the relatively high center of mass
location.
2- Sometimes a standing is compared to an inverted pendulum. This is a kinematically
redundant problem.
3- The joints between the COM and the support surface. The main joints are the ankle, knee, and
hip.
Center Of Pressure:
Postural studies typically considered a number of mechanical variables that can be measure a
force platform. One of the most commonly used variables in postural studies is coordinate of the
center of pressure (COP).The COP is the point of application of the resultant vertical force
acting on the body from the supporting surface.
COP is a two dimensional concept again. It can measure by a pressure sensing mat or a force plate
(just a sophisticated “bathroom scale”!). COP is a point on the same plane as the BOS, same as the
COG.
Picture shown here, foot’s image with many peaks.
Higher peaks mean larger forces. Forces are distributed
Over the foot. However, we can calculate a representative
point, which correspond to the average location of all the
forces. The green trajectory shown in the picture is the COP
trajectory over time when a subject stand quietly on a force plate.
POSTRUAL SWAY.
An ability to keep balance when posture is perturbed. When a person tries to stand quietly
and not to move, the centre of mass projection demonstrate small, seemingly chaotic deviation
from some average coordination, Called postural sway. Here center of mass displacement is
smaller on the ordered of couple milliliter.
RAMBLING AND TRAMBLING DURING STANDING.
Major two processes are quantified during quite standing using so called rambling trembling
decomposition of postural sway.Based on idea of the body oscillation about a moving coordination
(RUMBLING). Because of its initial properties it moves from it state oscillation about it trambling. It
can be defined as horizontal force selected direction is (zero). Example a typical scheme for
postal control based on a single joint inverted pendulum approximation.
SWAY AND POSTURAL STABILITY.
The relation between postural sway and postural stability is not trivial. Mechanical stability is
typically defined as an abilioty to keep a state following a perturbation that take a system
away from the state.
Example. A metal road placed on one of its flat ends on the ground may show zero sway but it is
very unstable. Because even a slight push will make it fall to the ground. Here, postural stability an
ability to keep balance when posture is perturbed. Here elder person shows increased sway and
worse postural stability.
Postgure Movement Paradox And The Equillibarium Point Hypothesis.
In an early section, the posture movement paradox has been mentioned, with respect to
vertical posture, two body configuration were considered vertical and leaning forward. Only one
hypothesis in the area of motor control that handles the postural movement paradox naturally.
This is the equilibrium point (Reference configuration) Hypothesis. According to the hypothesis
neural central result is setting a referent configuration of the body. If an actual body
configuration considered with referent one, no muscle activation is observed with the referent
movement.
Example Sway or sit to stand, is associated with the time shift to the referent configuration that
result in muscle activation changes leads to the new body postures.
Vertical posture is protected against potentially destabilizing factors (perturbations) by several
lines of defense. Posture-stabilizing mechanisms differ in their relative timing with respect to a
perturbation and efficacy.
Postural Stability:
Balance is the ability to keep the vertical projection of the center of mass (COM), within the
limits of base of support
∑
COM is a point in 3-dimensional space, usually around L2 in standing
∑
COG is the vertical projection of the COM on a 2-dimensional plane, usually the ground
To maintain balance in a standing position, one needs to keep the body’s COG within the
BOS or stability limits. COG is located on the same plane (i.e. ground as in standing) as the BOS.
Balance also means that the body’s COM is kept within the stability cone, which is a three
dimensional concept (COM is located at L2 at standing and stability cone is the
maroon colored cone shown in the picture on the right).
Stability limits refer to the boundaries within which the body can maintain
stability without changing the base of support.
ANTICIPATORY POSTURAL ADJUSTMENT.
If a person is performing an such a expected to compromising posture changing in the steady
state activation level of postural muscles occur in anticipation of the action. Anticipatory postural
adjustments (APAs) are commonly seen about 100 ms prior to activation of muscles that initiate
the intended, focal action. Examples of such actions include fast arm movements performed by a
standing person, pushing against an object, lifting, catching, and dropping objects, etc
APAs is that they represent consequences of neural commands generated in anticipation of a
predictable perturbation (in a feed-forward manner) based on the estimated effects of the
perturbation on posture. The purpose of APAs has been assumed to generate forces and
moments of force that would counteract the perturbing forces and moments of force and, in the
best-case scenario, cancel them out.
Anticipatory Postural Adjustment
1- Based on prediction.
2- Always suboptimal.
3- Muscle tendon tendency = 0
4- Always against the local perturbation.
Clinical Implications:
∑
APAs increase with ↑ movement magnitudes and speed. APAs more frequently present
with faster movements and heavier loads
∑
Practice can affect the timing of APAs, e.g. dancers activate APAs much earlier in a leglifting task than untrained individuals
∑
APAs are reduced when a support is given ‡Your patients will never improve balance if
they practice balance tasks while holding on to bar.
REFLEX:
∑
Reflexes are typically negative feedback loops, which means that they always act to reduce
the effects of a factor that caused them.
∑
Monosynaptic reflexes are rarely produced by postural perturbations because they require a
very quick muscle stretch. This does not mean that monosynaptic projections of Ia-afferents on
alpha-motoneurons are not functioning or unimportant.
∑
Polysynaptic reflexes, in particular the tonic stretch reflex, represent major posture-stabilizing
mechanisms. They are assumed to be the main mechanism of ensuring an equilibrium state
given a referent configuration of the body and the external force field.
MOVEMENT STRATEGIES TO RECOVER ANTEROPOSTERIOR STABILITY:
1. Ankle Strategy
a. Distal to proximal muscle activation pattern
b. Body sways at ankles with hips and knees in relatively
extended positions
c. Utilized in response to small perturbations on firm surface.
2. Hip Strategy
a. Proximal muscles activated first
b. Produces large, rapid motion at hip joints
c. Utilized when standing on narrow BOS (e.g. beam), on soft surface, or during larger,
faster perturbations
3. Stepping or Reaching Strategy
a. When subjects were not asked to keep feet in place, they more frequently step or reach,
instead of utilizing ankle or hip strategies to restore balance (McIlroy & Maki, 1993)
b. Stepping or reaching are natural responses, not the last resort to restore balance!
c. Older adults more frequently step than young adults (Mille, 2003)
Clinical Implications of Movement Strategies for Postural Control
∑
Both quiet stance and recovery of stability in response to perturbations use common postural
synergies.
∑
Training in one context, e.g. quite stance, may transfer to improve stability in the other context,
e.g. recovery of perturbed stance
∑
Do not limit training to the activation of a specific synergy, e.g. ankle or hip strategy
SENSORY CONTRIBUTIONS TO BALANCE:
ÿ CNS processes information from sensory receptors throughout the body to determine the body’s
position in space
ó Vision (especially peripheral vision)
ó Somatosensation (proprioception, cutaneous, joint receptors)
ó Vestibular system
ÿ Each sense provides a different frame of reference, i.e. “map”
Somatosensory Contributions
¢ Somatosensory contributions typically dominate postural control in response to transient or fast
surface perturbations (in this type of situation, visual and vestibular inputs do not help)
¢ Lightly touching a stable surface reduces sway significantly. The somatosensory inputs from the touch,
rather than the contact force through touching a surface (Jeka, 1994; Lackner, 1999)
Vestibular Contributions
¢ Typically vestibular system contributes less than somatosensory system
¢ For example, CNS cannot tell whether it is just head bending forward or the whole body is leaning
forward
¢ Vestibular system provides a frame of reference relative to the gravity
Cognitive Contributions to Balance
¢ Dual-task paradigms
ó ↓ performance in either task because of limited capacity in information processing to handle
both tasks simultaneously
ó Different postural and secondary tasks affect postural control differently
ó Older or balance-impaired individuals increase postural sway with ↑ diÔ¨Éculty of secondary
cognitive task.
References:
1.
2.
Fundamentals of Motor Control by Mark L. Latash
Motor Control by Anne Shumway Cook
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