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3rd year sem 2 Motor Control > IM whole module flashcards W3 & W4 > Flashcards

IM whole module flashcards W3 & W4

1
Q

define human balance

A

the active control of the physical shape of the limbs involving muscle action to compensate for passive insufficiencies

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2
Q

Why can’t humans stand perfectly still?

A
  • Passive instability
  • Muscle Error
  • Sensory Error
  • Feedback Delays
  • Control Strategies
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3
Q

What does passive stability depend on?

A

Weight
Area of base
Horizontal distance of COG from pivot point
Height of COG from the base

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4
Q

define stability

A

The tendency of a body to remain or return to its initial position following the application of a force

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5
Q

State the three types of equilibrium

A

Stable equilibrium (Triangle- wide base of support)
Unstable equilibrium (Hammer)
Neutral Equilibrium (Circle)

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6
Q

Why might something be unstable?

A

Small surface area of base with COG much higher than the base of support

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7
Q

Why might muscle errors cause instability? (2 points)

A

Humans cannot produce a perfectly consistent force, they often overestimate or underestimate how much force they need to produce.

Muscles work in planes and axes; when one muscle produces a force, it affects muscles on the same plane.

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8
Q

What three sensory organs contribute to movement?

A

Vision (10%)
Vestibular Apparatus (20%)
Somatosensory Proprioception (70%)
- percentages when in normal stance, two feet, stable surface, eyes open

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9
Q

problems with sensory input for balance

A
  • Having lots of different sensory inputs leads to sensory conflict where one sensory system says you’re doing something and another says you’re doing something else
  • E.g. sea sickness, where our visual input and vestibular input give us different info
  • E.g. alcohol affects vestibular system and you get sensory conflict, you think you’re moving when you’re not and then feel sick
  • Some sensory inputs require some movement to work
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10
Q

Explain a study that looked at sensory thresholds

A

(Clark et al., 1985)
- They found that we have different sensors: sensors based on position and ones based on movement
- Sensors based on movement are based on how quickly it’s moving so if you’re moving really slowly they wont detect any movement
- But if it moves far enough the position sensors will pick up the movement
- This means we have a period of movement with no feedback, either because we’re moving too slow or haven’t moved far enough

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11
Q

Explain a study that looked at imbalance and the input of different systems

A

(Fitzpatrick & McCloskey, 1994)
- found that the somatosensory proprioception system was the best (most acute) at detecting speed and displacement
- vision not far behind
- vestibular system not as good
- but even the best system won’t pick up very slow or small movement

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12
Q

List the three delays in a “Displacement, Time, Velocity” graph in order with reference

A
  • sensory delay
  • neurological delay
  • electromechanical delay
    (Blenkinsop, Pain & Hiley, 2006)
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13
Q

what is electromechanical delay?

A
  • the time taken for the calcium to move around the system
  • and for the muscle to pick up any slack to produce enough tension to produce a force
  • it is the time taken from muscle activation to force production
  • 13-55 ms
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14
Q

what is sensory delay?

A
  • the time taken to detect movement based on sensory thresholds and to recognise that an action must be made
  • time of sensory delays varies based on movement
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15
Q

what is neurological delay?

A
  • the time taken to make a decision (Afferent and efferent signal transmission + decision in between)
  • 65-130 ms
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16
Q

List 5 control strategies for balance

A
  • Ankle strategy
  • Hip strategy
  • Mixed strategy
  • Arm swings
  • Stepping
17
Q

what is ankle strategy?

A
  • controlling the movement of our COM via ankle joint torque
  • fixed hip angle
  • good when low surface friction
  • bad on narrow surfaces
  • humans preferred strategy during quiet stance
18
Q

what is hip strategy?

A
  • hip angle opposite to ankle
  • controls COM via horizontal force
  • good on narrow surfaces
  • bad when low surface tension
  • requires more effort
  • quicker
  • suitable for larger perturbations
  • bigger response means error more likely
19
Q

What does PID controller stand for?

A

PROPORTIONAL
INTEGRAL
DERIVATIVE

20
Q

What does each letter label represent in the control model?

A
21
Q

What do the three K labels represent in the PID controller?

A
22
Q

Explain how a PID controller can relate to human movement

A

Proportional Component: The output is proportional to the error between the desired outcome and the actual outcome. For example, when trying to produce a particular outcome, the proportional component will highlight the difference between the intended angle and the actual angle.

Integral Component: This component integrates the cumulative arror over time. In humans, it is associated with the continuous adjustments made to correct any persistent errors in movements, and contribute to the corrective response over time.

Derivative Component: Measures the rate of change of the error and provides a control signal based on this rate. For a person performing a fast movement, this would sense the rapid changes in joint position and generate an appropriate response to maintain stability and control.

23
Q

PID controller issues

A
  • Simulation output is too good by itself so need to add noise
  • Relies on excessive noise to reproduce typical postural sway
  • Struggles with large delays
  • Can used intermittent control models
24
Q

PID controller applied to humans standing example

A

(Peterka, 2002)
- Used numerous sensory perturbations on a person
- Determined weights for sensory input:
- Vision = 10%
Vestibular = 20% - used perturbations to get these numbers so maybe not 20% vestibular if you are standing still
- Proprioception = 70%

25
Q

PID controller applied to handstands

A

(Yeadon & Trewartha, 2003)
- Balance in handstands
- Fit experimental data to mechanical model
- Used repeat regressions with time offsets
- Determined feedback time delays of 160 ms to 240 ms in handstand

26
Q

Explain the Lorenz system

A
  • based on 3 equations that describe the variables that change
  • shows how those 3 equations evolve with time in a 3D space
  • it is a strange attractor - never repeats itself
  • by changing one variable, it completely changes how the system evolves
27
Q

Why can you not always ignore noise? (related to Lorenz system)

A
  • if you ingore noise to understand a system you may be understanding 99% of it
  • but in getting rid of noise this could alter how the system evolves
  • it may start in a different place than you thought which would change the Lorenz pattern
28
Q

How does balance relate to complex nonlinear systems?

A
  • if you close your eyes, you change how we use different components
  • If you close your eyes you’re not going to use vision for 10% of balance, so where does that 10% go? We don’t know
  • The idea is that if you remove some sensory input, maybe we change some other things as well
  • Maybe it’s not as simple as we just use one sensory system a bit more or less than we did before, it could be that our balance outcome changes as well, which is actually what we find
  • If you stand on one leg and close your eyes you fall more quickly
29
Q

Sway magnitude and alternative approaches to assessing balance

A
  • non-alternative idea is that if centre of pressure moves around more you’ve got poorer balance
  • this is not always true - there’s research to show that gymnasts (who have very good balance) sway more than healthy controls
  • so sway magnitude does not tell us everything
  • so instead of looking at the magnitude of sway, alternative approaches will look at the pattern of sway and see that as more important
30
Q

Different approaches to measuring balance

A
  • nonlinear signal analysis (patterns)
    -> entropy
    -> Lyapunov Exponent
  • Newell’s Constraints Approach
  • Uncontrolled manifold hypothesis
31
Q

What is Entropy?

A
  • A lack of order or predictability.
  • Information entropy relates to the loss of information due to this reduced order
  • low entropy means there was a small loss of information (easy to predict)
32
Q

What is the Lyapunov Exponent? Give an example

A
  • A measure of the local stability of the system (How much the trajectory stays within the state space)
  • E.g. two points that seem close together in one instance will by far apart in another, LE is the value of the change in distance between two points
33
Q

What does a low Lyapunov Exponent indicate?

A

That the system is very stable and there is not much divergence

34
Q

Lyapunov Exponent applied to humans

A

(Harbourne & Stergiou, 2003)
* looked at centre of pressure motion in young infants, as they were learning to balance in a sitting position
* measured it by approximate entropy and the Lyapunov exponent
* as infants progressed to needing less support to sit Lyapunov exponent reduced (became more locally stable)
* Approximate entropy reduced originally from needing support to without support but then increased slightly as they began to explore
* the child originally reflects a periodic system, and then it moves to a nonlinear system as it begins to explore

35
Q

What is Newell’s constraints approach?

A
  • Movement is the product of interaction between constraints
  • Organism constraints (constrains within the person - e.g. strength to weight ratio, poor sensory input)
  • Environmental constraints - e.g. standing on ice, standing on a platform, standing on a moving bus
  • Task constraints - e.g. saying don’t move your feet, or could be other things such as how you control your head
36
Q

How does Newell’s constraints approach relate to motor learning?

A

motor learning is an ongoing dynamic process driven by constraints, involving a:
* Search of the perceptual motor landscape
* Stabilisation and refinement of functional movement patterns
* Optimisation of control by exploiting environmental and task information

37
Q

(Bardy et al., 2002, 2007)

A
  • examined people’s postural responses to tracking a target moving backwards and forwards
  • traget frequency increased/decreased gradually
  • looked at in-phase and antiphase coordination
  • in-phase means the leg and torso are moving together in-phase so that means the hips are still and you’re using ankle strategy
  • Anti-phase means it’s a hip strategy because your torso goes the opposite way to your legs

region of bistability
* region where both strategies are equally useful or equally acceptable
* shows hysteresis
-> staying with current strategy until we have to change

  • used ankle strategy for lower target frequency until had to change to hip strategy for higher target frequnecy and vice versa

Conclusions
* The control strategy of choice is dependent on the task constraints (target frequency)
* The target frequency/speed you have to move at is determining everything
* Required frequency may dictate which strategy is preferred
* Different transition points shows hysteresis in strategy selection - which is a hallmark of nonlinear dynamics
* A region of bi-stability indicates multi-stability, when either strategy is acceptable

38
Q

What is the Uncontrolled Manifold Hypothesis?

A
  • A theoretical framework in motor control that aims to explain how the central nervous system organises and coordinated multiple degrees of freedom during an action.
  • It suggests that the CNS will strive to find a solution to a motor task while allowing variability in non-task-relevant degrees of freedom (VARucm)
  • orthogonal direction is the things we are trying to control and reduce variance of
  • It’s not possible to have perfect motion, it’s not possible to stand perfectly still, there’s going to be some movement and variation, so let’s just deal with the bits that are important
39
Q

Research around untroncolled manifold hypothesis

A

(Hsu et al., 2007)
looked at 4 different joint configurations:
* Ankle joint torque (Single-inverted pendulum) variation at the ankle highly correlated with variation in the orthogonal with little variaiton in UCM
* Ankle+Hip Joint (Double-inverted pendulum): orthogonal (task-relevant) variation is reduced, variation in the UCM is increased
* Multi-joint coordination: UCM variation high and task-relevant variation (orthogonal) low
* Stiffening all joints: low variation in both

Conclusions:
* all major joints contribute to the stabilisation of posture, it’s not just an ankle strategy
* Muscles work together to stiffen other joints and prevent unwanted movement, but there is still some movement

3rd year sem 2 Motor Control (4 decks)

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