Equilibrium + Posture 2 - Balance

Question 1

What does quiet standing (balance) involve?

Answer 1

Keeping the centre of mass (COM) within the base of support (BOS). If COM moves outside the BOS, a fall or compensatory step will happen

Question 2

What is the inverted pendulum model of human standing??

Answer 2

During a normal stance, most motion will occur at the ankle joint. Torque will be provided by continuously active calf muscles

Question 3

What role does the centre of pressure (CoP) play??

Answer 3

The CoP actively oscillates forward and back to maintain the centre of the mass (COM) within the limits of stability.
Sway can be quantified by COM motion.

Question 4

What type of process is standing?

Answer 4

A sensorimotor control process

Question 5

Why do we sway??

Answer 5

1) Sensory estimation (an imperfect process with sensor noise)
2) Motor output (also an imperfect process)
3) External / internal perturbations (e.g., Breathing, wind, being pushed etc)

Question 6

What information keeps us upright?

Answer 6

Vision, proprioception, efference copy of motor command + prediction, vestibular apparatus, touch

Question 7

What are the 4 sensory systems?

Answer 7

Vision, vestibular otoliths, vestibular SC canals, somatosensation

Question 8

What is the frequency range and an example of a situation of the vision system?

Answer 8

< 0.1 Hz (just very slow movements)

Example being standing in a room with no movement

Question 9

What is the approx frequency range and an example of a situation of the vestibular otoliths sensory system?

Answer 9

< 0.5Hz (static gravity to moderate head tilt)

Example would be slow acceleration of a car

Question 10

What is the approx frequency range of vestibular SC canals and an example of the situation?

Answer 10

0.5 to 1.0 Hz ( rotational motion of the head)

Head and eye movement control during walking / rapid head turns

Question 11

Somatosensation approx frequency range with example of behaviour?

Answer 11

> 0.1 Hz (joint position, muscle length +tension)

Control of head position in relation to torso

Question 12

What controls for slow/low frequency control of sway?

Answer 12

Visual control

Question 13

What controls for fast/high frequency control of sway?

Answer 13

Vestibular input

Question 14

What did Paulus et al (1984) find when looking at sway?

Answer 14

Found reducing visual acuity increases sway
(Removing visual info increases sway)

Also found that blocking parts of the visual field increases sway -> hence peripheral vision just as important as foveal

Question 15

The quality of visual information is important, what does it mean??

Answer 15

Closer objects are more useful for controlling sway, as there is greater displacement on the retina

Distant visual information is effectively useless for balance - as there is no retinal motion

Question 16

What is motion parallax??

Answer 16

Where objects appear to move relative to each other upon swaying, depending upon focus point

Question 17

What did Guerraz et al (2000) find when looking at sway reduction?

Answer 17

They found the largest sway reduction was observed when foreground and background scenery are present together and furthest apart.

Question 18

What is visual vection?

Answer 18

A false sense of motion induced by a moving scene which has consequences for control of standing posture

Question 19

Which way do subjects sway when exposed to moving scenes?

Answer 19

Sway in the direction of the visual scene movement

Question 20

Why do subjects sway in direction of scene movement?

Answer 20

The brain interprets forward scene motion as backward body motion, therefore produces a compensatory forward response

Question 21

Discuss habitation of a visual response?

Answer 21

Reduced sway occurs upon repeated exposure to a moving room - visual responses can be easily ignored.

Question 22

Discuss the effects of predictability on visual response

Answer 22

In an unexpected condition, where experimenter triggers motion unexpectedly- a big sway response occurs

When subject triggers motion, virtually no response occurs.

The visual response is readily modified by expectation/prior experience

Question 23

Describe the basic characteristics of a visual perturbation response?

Answer 23

Initial sway in direction of visual field movement begins ~1 after onset. Corrective sway occurs after 2-3 seconds in the opposite direction

Responses habituate quickly

Question 24

What does a visual perturbation response respond best to??

Answer 24

Responds best to slow, low frequency movement (<0.1Hz, <0.5 deg/s)

Question 25

Visual field motion can be ambiguous, what does this mean??

Answer 25

The brain must interpret each visual stimulus and distinguish between object and self-motion. This involves expectation and prior knowledge

Question 26

The standing body is considered to be an inverted pendulum. How does the body stop itself toppling forwards?

Answer 26

The calf generates torque around to resist this tendency.

Question 27

How is the balance system tested?

Answer 27

Very small imperceptible torque perturbations are used to test the balance system. (Fitzpatrick et al)

Question 28

How is stiffness determined when looking at the balance system?

Answer 28

Resistance to waist pull, where changes in stiffness represent changes in muscle reflexes.

Question 29

Discuss the effects of sensory removal on postural response.

Answer 29

Standing is possible with only proprioceptive input, however better with all senses available. Balance reflexes can also be tuned by intention

Question 30

What is the equation for ankle stiffness (intrinsic stiffness)?

Answer 30

Angle / torque

Angle on X axis, torque on Y.

Question 31

Discuss the actual relationship of ankle and load stiffness.

Answer 31

The stiffness of the ankle is too low to permit passive standing (i.e. Standing without active modulation of EMG)

The limiting factor in stiffness is the tendon and soft tissues of the foot

Stiffness will not be changed by intention or sensation