IM whole module flashcards W3 & W4

Question 1

define human balance

Answer 1

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

Question 2

Why can’t humans stand perfectly still?

Answer 2

• Passive instability
• Muscle Error
• Sensory Error
• Feedback Delays
• Control Strategies

Question 3

What does passive stability depend on?

Answer 3

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

Question 4

define stability

Answer 4

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

Question 5

State the three types of equilibrium

Answer 5

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

Question 6

Why might something be unstable?

Answer 6

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

Question 7

Why might muscle errors cause instability? (2 points)

Answer 7

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.

Question 8

What three sensory organs contribute to movement?

Answer 8

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

Question 9

problems with sensory input for balance

Answer 9

• 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

Question 10

Explain a study that looked at sensory thresholds

Answer 10

(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

Question 11

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

Answer 11

(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

Question 12

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

Answer 12

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

Question 13

what is electromechanical delay?

Answer 13

• 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

Question 14

what is sensory delay?

Answer 14

• 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

Question 15

what is neurological delay?

Answer 15

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

Question 16

List 5 control strategies for balance

Answer 16

• Ankle strategy
• Hip strategy
• Mixed strategy
• Arm swings
• Stepping

Question 17

what is ankle strategy?

Answer 17

• 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

Question 18

what is hip strategy?

Answer 18

• 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

Question 19

What does PID controller stand for?

Answer 19

PROPORTIONAL
INTEGRAL
DERIVATIVE

Question 20

What does each letter label represent in the control model?

Answer 20

Question 21

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

Answer 21

Question 22

Explain how a PID controller can relate to human movement

Answer 22

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.

Question 23

PID controller issues

Answer 23

• 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

Question 24

PID controller applied to humans standing example

Answer 24

(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%

Question 25

PID controller applied to handstands

Answer 25

(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

Question 26

Explain the Lorenz system

Answer 26

* 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

Question 27

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

Answer 27

* 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

Question 28

How does balance relate to complex nonlinear systems?

Answer 28

* 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

Question 29

Sway magnitude and alternative approaches to assessing balance

Answer 29

* 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

Question 30

Different approaches to measuring balance

Answer 30

* nonlinear signal analysis (patterns) -> entropy -> Lyapunov Exponent * Newell's Constraints Approach * Uncontrolled manifold hypothesis

Question 31

What is Entropy?

Answer 31

* 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)

Question 32

What is the Lyapunov Exponent? Give an example

Answer 32

* 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

Question 33

What does a low Lyapunov Exponent indicate?

Answer 33

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

Question 34

Lyapunov Exponent applied to humans

Answer 34

(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

Question 35

What is Newell's constraints approach?

Answer 35

* 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

Question 36

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

Answer 36

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

Question 37

(Bardy et al., 2002, 2007)

Answer 37

* 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

Question 38

What is the Uncontrolled Manifold Hypothesis?

Answer 38

* 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

Question 39

Research around untroncolled manifold hypothesis

Answer 39

(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