Motor Control III Flashcards
postural control
- definition
- purposes
controlling the body’s position in space with dual purpose of
- orientation
- stability (postural equilibrium)
postural control: orientation
-what is it
ability to maintain an appropriate relationship between body segments and between the body and environment
postural control: stability
- what is it
- commonly referred to as…
ability to maintain one’s center of mass within the limits of the base of support
commonly referred to as “balance”
stability and orientation requirements vary with…
task and environment
primary goals of nervous system in postural control
control position and motion of the body’s COM
body’s rotation about the COM
COM
- define
- location in humans
- what is center of gravity
point that represents the average position of the body’s total mass
located 20 mm anterior to S2 in erect stance; instantaneous location depends on body position
COG
-vertical projection of COM onto support surface
BOS definition
area of the body that is in contact with the support surface
limits of stability (LOS)
-what is it
maximum range COM can be moves without changing BOS
center of pressure (COP)
-what is it
center of distribution of total force applied to support surface
for body to be in static equilibrium…
COP and COM must align over BOS
how is COM restored to desired position
torque created by action of trunk and limb muscles
COM and COP illustration
COM = sheep COP = sheepdog
current view of motor control of posture
complex motor skill derived from interaction of multiple sensorimotor processes and systems
systems framework for postural control
-components
individual -motor, sensory, cognitive postural task -steady state, proactive, reactive environment -support surface, sensory context, cognitive load
individual systems/constraints
motor -musculoskeletal --strength, ROM, alignment -neuromuscular --postural tone, amplitude, timing sensory -visual, vestibular, somatosensory cognitive -attention, adaptation, anticipation, confidence
task constraints
-functional tasks require what 3 types of postural control
steady state
reactive
proactive
steady state control
- what does it do
- when
control COM relative to BOS in predictable, non-changing conditions
reactive control
-what is it
recover a stable position after an unexpected pertubation
proactive control
-what is it
activation of muscles in advance of potentially destabilizing voluntary movements to avoid instability
feedback control (closed-loop)
- used in response to…
- example
used in response to external disturbances to equilibrium
during gait and in response to disruptions of gait (trip.slip)
feed-forward control (open-loop)
-what is it
pre-planned synergistic movements in anticipation of a voluntary movement
central command for a voluntary limb movement is associated with…
a simultaneous feed-forward command anticipating an expected postural pertubation
voluntary movement can perturb postural equilibrium, but knowledge of these potential pertubation is built into…
the voluntary motor commands and used to offset their adverse effects ahead of the event by feed-forward motor action
anticipatory actions must be _____, but eventually they _____
must be learned, but eventually they operate automay
tically in an open loop manner, being triggered by specific intended movements
steady state (static) balance -factors
musculoskeletal alignment
muscle tone
-intrinsic stiffness
-background muscle tone - exists normally due to neural contributions (e.g. stretch reflex)
postural tone - activation of antigravity muscles
-examples: cutaneous, vestibular inputs
movement strategies (motor)
reactive balance control
-most knowledge generated from…
studies using controlled balance disruptions (pertubations)
reactive balance control: automatic postural response
- defined as…
- this is called
defined as synergistic activation of a group of muscle in a characteristic sequence to maintain equilibrium
- latencies of muscle responses 80-120 ms (some as long as 160-200 ms)
- reactive postural adjustment (RPA)
short latency responses
-describe
nonfunctional
at spinal cord
could be a stretch reflex
medium latency responses
-describe
functional
in midbrain/brainstem
late-phase and latency responses
-describe
stepping, reaching
at cortex
movement patterns used to recover balance in sagittal plane
-these strategies function along a…
ankle strategy
hip strategy
stepping or suspensory strategy
these strageties function along a continuum
-in slow movements, utilize primarily closed-loop control
-rapid movements require greater open-loop control
RPA’s
- adapt to…
- set of muscles activated depends on…
adapt to changes in the requirement for support set of muscles activated depends on -biomechanical support -task exposure -sensory conditions
how to RPA’s change with biomechanical conditions?
with translation of wide surface, sequential activation of GAS/HS/PSP (ankle strategy)
with narrow bear, primary activation of proximal muscles (quad/abdomen (hip strategy))
ROM strength impairment will shift demand to other muscles/joints
automatic postural responses: postural strategy modulates according to…
changing environmental support
repeated exposure to stimulus
RPA’s and sensory conditions
-describe
RPA’s modulate according to sensory conditions
- availability of sensory inputs
- -muscle latencies to visual cues –> 200ms
- -muscle latencies to somatosensory cues –> 80-100ms
RPA’s modulate according to sensory conditions: sensory weighting hypothesis
- describe
- example
each sense provides unique contribution
the “gain,” or relative weight given to the sensory input by the brain, varies according to its accuracy as a reference point
example
-if touch becomes less reliable (peripheral neuropathy), visual inputs are weighted more heavily
RPA’s adjust to situational predictability or…
learning
RPA’s and learning: central set
- defined as
- organization of postural responses depends on…
- with experience
defined as a state of the nervous system influenced by the context of the task
organization 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
with experience, increased spatial organization/decreased EMG amplitude
central set functional application to ACL injury
muscles become more organized and efficient at preventing shear forces
anticipatory postural control
- sometimes referred to as…
- what are they
- nervous system has…
anticipatory postural adjustments (APAs)
synergistic postural responses executed in advance of primary body movement
nervous system has advance knowledge
anticipatory postural control
- when do postural adjustments occur?
- classic study found…
before action
study
-postural adjustments occur before activity
-raise arm to shoulder level
-80m sbefore deltoid contracts, lower back and leg extensors contract
-purpose: to stabilize the body to prevent shoulder movement from destabilizing the body (open-loop)
anticipatory postural control
-postural adjustments accompany voluntary movements only…
when needed
clinical applications for APAs
behavior context and speed of focal movement affect APAs
- with fast movements, APAs are earlier and more reliable
- greater movement resistance increases likelihood of APAs
- external support reduces APAs
- degreed of practice influences speed and accuracy of response
