706_Week 14 Flashcards
Postural control -
controlling the body’s position in space with dual purpose of: orientation and stability (postural equilibrium)
In postural control, orientation =
the ability to maintain an appropriate relationship between body segments and between the body and EN
In postural control, stability (postural equilibrium) =
- ability to maintain one’s center of mass (COM) within the limits of the base of support (BOS)
- commonly referred to as “balance”
Long Andy Tangent about posture, ask a friend
Long Andy Tangent about posture, ask a friend
In postural control, describe the contributing factors:
- stability and orientation requirements vary with the task and the EN
- orientation as primary goal (stability is sacrificed)
- stability as primary goal
Is the body inherently stable?
No, the body is inherently unstable
What are the primary goals of the nervous system in postural control?
- control position and motion of the body’s center of mass (COM)
- body’s rotation about the COM
Center of mass (COM) =
point that represents the average position of the body’s total mass.
• in humans, located in abdomen approximately 20mm anterior to S2 is erect stance (anatomical position), instantaneous location depends on body position (may move outside of body with hip/trunk flexion)
• force of gravity acts on all body segments, net effect on body acts through center of mass
Center of gravity (COG) =
vertical projection of COM onto support surface
Base of support (BOS) =
area of the body that is in contact with the support surface
Limits of stability (LOS) =
maximum range of COM can be moved without changing BOS
Center of pressure (COP)=
center of distribution of total force applied to support surface.
•for body to be in static equilibrium, COP and COM must align over BOS
• action of trunk and limb mm create torque to restore COM to desired position
sheep =
sheepdog =
sheep = COM sheepdog = COP
Historically, where is postural control thought to arise from?
brainstem and spinal circuitry
Historically, motor control of posture, assumed that postural control consisted of a set of reflexes that triggered equilibrium responses based on visual/ vestibular/ somatosensory triggers. Now viewed as:
complex motor skill derived form interaction of multiple sensorimotor processes and systems.
What does the systems framework for postural control include?
Individual (motor, sensory, cognitive), postural task (steady state proactive, reactive), and EN (support surface sensory context cognitive load)
What are motor constraints of postural control?
- musculoskeletal (strength, ROM, alignment)
2. neuromuscular (postural tone, amplitude, timing)
What are sensory constraints of postural control?
visual, vestibular and somatosensory
What are cognitive constraints of postural control?
attention, adaptation, anticipation and confidence
What 3 types of postural control do functional tasks require?
- steady state - control COM relative to BOS in predictable, non-changing conditions
- reactive - recover a stable position after an unexpected perturbation
- proactive - activation of mm in advance of potentially destabilizing voluntary movements to avoid instability.
In postural control, feedback control (closed-loop) is used in response to what?
- in response to external disturbances to equilibrium
* during gait and in response to disruptions of gait (trip/slip_
In postural control, feed-forward control (open-loop) is used in response for what?
pre-planned synergistic movements in anticipation of a voluntary movement
faster =
more preplanning
slower =
more time to plan, adjust/ react
In a feedback task constraint loop the flow is:
central command –> limb movement –> postural disturbance –> (feedback for unexpected postural disturbance —–>) postural adjustment
In a feed-forward task constraint loop the flow is:
central command –> (feed-forward for expected postural disturbance —–>) postural adjustment
For motor systems in postural control, steady state (“static”) balance =
- musculoskeletal alignment
- muscle tone (intrinsic stiffness, background muscle tone - exists normally due to neural contributions (e.g. stretch reflex)
- postural tone - activation of antigravity mm (cutaneous, vestibular inputs)
- movement strategies (motor)
For motor systems in postural control, reactive balance control =
- most knowledge generated from studies using controlled balance disruptions (perturbations)
- AUTOMATIC POSTURAL RESPONSE
Automatic postural response =
synergistic activation of a group of muscles in a characteristic sequence to maintain equilibrium
• latencies of muscle response 80-120 ms (some as long as 160-200 ms)
• reactive postural adjustment (RPA)
What are movement strategies?
movement patterns used to recover balance in sagittal plane
Name 3 movement strategies:
- ankle strategy
- hip strategy
- stepping or suspensory strategy
Movement strategies function along a continuum:
- In slow movements, utilize primarily closed-loop control
- rapid movements greater open-loop control
For automatic postural responses, RPA’s adapt to changes in the requirement for support. Set of muscles activated depends on what 3 factors?
- biomechanical support
- task exposure
- sensory conditions
What do RPA’s change with?
biomechanical conditions
• with translation of wide surface, sequential activation of GAS/HS/PSP (ankle strategy)
• with narrow beam, primary activation of proximal mm. ie quad/ abdom (hip strategy)
•ROM/ strength impairment will shift demand to other muscles /joints
In automatic postural response, postural strategy modulates according to:
• changing EN support
and
• repeated exposure to stimulus
(w/ exposure begin to show modulations)
In automatic postural responses, what do RPA’s modulate according to sensor conditions. Availability of sensory inputs such that:
- muscle latencies to visual cues –> 200ms
- muscle latencies to somatosensory cues –> 80-100 ms
(e.g. UE force
In the sensory weighting hypothesis (RPA’s modulate according to sensory conditions):
• each sense provides unique contribution
•the “gain”, or relative weight given to the sensory input by the brain, varies according to the accuracy as a reference point
(-if touch becomes less reliable _ peripheral neuropathy_ visual inputs are weighted more heavily)
(nashner: sensory organization in balance)
In automatic postural responses, RPA’s adjust according to situational predictability or LEARnING. An example of this is:
Central set:
• defined as a state of the nervous system influenced by the context of a task
• organized of postural responses depends on expectations of stability and / or perturbations
(in expected perturbations, postural responses are highly organized in a feed-forward control model, i.e. selected in advance / standing on a bus example)
• with experience, increased spatial organization/ decreased EMG amplitude (functional application: ACL injury)
Anticipator postural control (APA) =
- synergistic postural responses executed IN ADVANCE of primary body movement
- nervous system has advance knowledge
- relate this to potential influence of motor programs involved in postural control
In anticipatory postural control, postural adjustments occur _________ action.
BEFORE
In anticipatory postural control it has been found that postural adjustments accompany what?
voluntary movements only when needed
What are the clinical applications of APA’s?
Behavioral context and speed of focal movement affect APA’s
• with fast movements, APA’s are earlier and more reliable
• greater movement resistance increases likelihood of APA’s
• external support reduces APA’s
• degree of practice
(more research is needed in conditions of pathology)
