Balance and Posture Flashcards
1
Q
what is balance
A
- quiet standing involves keeping the centre of mass (COM) within the base of support (BOS)
- if the COM moves outside the BOS, a fall of compensatory step will happen
2
Q
what is the inverted pendulum model of human standing
A
- during normal quiet stance, most motion occurs at ankle joint
- torque provided by continuously active calf muscles
- can be represented as a point of mass, tending to fall forwards, with active torque stabilisation
3
Q
what is centre of mass and pressure
A
- centre of pressure (COP) actively oscillates forward / back to maintain the centre of mass (COM) within the limits of stability
- sway can be quantified by COM motion
4
Q
why do we sway
A
- standing is a sensorimotor control process
- imperfections in this process arise from
- sensory estimation: an imperfect process with sensor noise
- motor output: also an imperfect process
- external / internal perturbations e.g. breathing, wind, being pushed etc
5
Q
what information keeps us upright
A
- efference copy of motor command and prediction
- vestibular apparatus
- touch
- proprioception
- vision
6
Q
what is the comparison between sensory units
A
- visual control for slow / low frequency control of sway
- vestibular input for fast / high frequency control
7
Q
how does removing visual information increase sway
A
- reducing visual acuity increases sway
- blocking parts of the visual field increases sway
- peripheral vision just as important as foveal
8
Q
why is the quality of visual information important
A
- closer objects are more useful for controlling sway
- greater image displacement on retina
- distant visual information effectively useless for balance - no retinal motion
9
Q
what is motion parallax
A
- objects appear to move relative to each other, depending upon focus point
- this is also true during the small motion caused by sway
- largest sway reduction observed when foreground + background scenery are present together and furthest apart
10
Q
what is perturbating visual input
A
- visual vection
- a false sense of motion induced by a moving scene
- e.g. watching an adjacent train depart the station, you may get the impression that your train has started to move
- subjects sway in the direction of visual scene movement
- the brain interprets forward scene motion as backward body motion
- therefore produces a compensatory forward response
11
Q
what is habituation of visual response
A
- reduced sway upon repeated exposure to moving room
- visual responses easily ignored
12
Q
what is the effect of predictability on visual response
A
- experimenter triggers motion unexpectedly - big sway response
- subject triggers motion - virtually no response
- visual response readily modified by expectation / prior experience
13
Q
what are the characteristics of visual perturbation response
A
- initial sway in the direction of visual field movement, begins ~1s after onset
- corrective sway 2-3s by sway in opposite direction
- responds best to slow, low frequency movement (<0.1Hz, <5 deg/s)
- responses habituate quickly i.e. modifiable by expectation and cognitive influence
14
Q
how do you test the balance system
A
- standing body can be considered as a simple inverted pendulum
- the body tends to topple forwards. calf muscle activity generates torque around the ankle to resist this tendency
- very small imperceptible torque perturbations used to test the balance system
- resistance to waist pull measured to determine stiffness
- changes in stiffness represent changes in muscle reflexes
15
Q
what is the effect of sensory removal on postural responses
A
- standing is possible with only proprioceptive input
- however, balance reflexes are more effective with all senses available
- balance reflexes can also be ‘tuned’ by intention - i.e. trying to stand still
