W4 - Balance II Flashcards
How do we analyse nonlinear systems?
- The derivative equations of x,y,z
- Cannot look at singular bits of the system as changes the way it works(alternative theories)
- State space needed to express the system
- Tries to isolate parts of the system, trouble is a small change at beginning can lead to big changes later on
Describe the appropriateness of Lorenz System Dynamics for postural coordination:
postural coordination patterns are considered as attractors, and transitions between patterns are thought to be governed by principles of self-organization.
Why is it not appropriate to analyse a singular part of a nonlinear system?
alternative equation analogy
- needs to be represented as a state space and not a derivative equation, as it disregards how the whole system evolves
- Change the way the whole system evolves
- Change how components in the system are used (vision vs no-vision)
- An alternative method may be required to assess the system in its entirety
- Misrepresents how we use vision in the first place, so need for alternative method
Chaos theory = non-linear dynamical systems
What are some alternative approaches to nonlinear signal analysis?
- Non-linear signal analysis
– Entropy
– Lyapunov Exponent (how things change over time) - Newell’s Constraints Model
- Uncontrolled manifold hypothesis
What are alternative approaches?
- Variability is not simply unwanted noise
- Variability is integral to the system
- Movement is integral to perception
- Sway magnitude is less important(less important)
- Pattern of sway is more important
Spectral analyses used to measure the phase angle between trunk and legs estimated from cross-spectral density
Describe entropy:
Provide some examples of signals with high/low entropy
- A lack of order or predictability
- Information entropy relates to the loss of information due to this reduced order
- Loss of info leads to loss of predictability (easy to predict the future)
– Periodic signal = low entropy (entropy of ~0)
– Complex signal = medium-high entropy (because has constant small changes)
– Random signal = high entropy
What is the Lyapunov Exponent?
- A measure of the local stability of the system in its state space
- Low stability leads to exponential divergence in the system trajectories
- Periodic signals have high stability with zero divergence of trajectories (circle on a graph)
- Complex signals have some instability with trajectories diverging as time progresses(butterfly graph) = chaotic system/random
Describe the Harbourne and Stergiou study(2003):
- Examined the development of sitting postural control in infants
As the child progressed from sitting with support(stage 1)to independent sitting(stage 3) - Lyapunov Exponent reduced throughout (became locally stable) = stable in terms of state space
- Approximate entropy reduced originally (more ordered) then increased slightly as they began to explore
As the child becomes more confident their entropy increases as they start to explore movement more
Explain Newell’s constraints model:
Driven by task, organism and environmental constraints, movement is the product of interactions between constraints
* Search of the perceptual motor landscape, as we change the landscape changes
* Stabilisation and refinement of functional movement patterns
* Optimisation of control by exploiting environmental and task information
What type of coordination is used for the:
- Ankle & Hip strategy
BARDY et al. (2002, 2007)
* Examined postural responses to tracking a moving target
* Target frequency was increased / decreased gradually (0.15 – 0.75 Hz)
* Ankle strategy = In-phase coordination
* Hip strategy = Anti-phase coordination
What were the findings of the study by Bardy et al., 2002/7?
- Strategy choice is dependent on task constraints (frequency)
- Required frequency may dictate which strategy is preferred
- Different transition points shows hysteresis (change of gait), dependent on speed in strategy selection
- A region of bi-stability (being inconsistent and uncoordinated in the area (uncomfortable)) indicates multistability, when either strategy is acceptable
Describe the UCM:
and draw it
- The CNS does not eliminate redundant degrees of freedom but uses this abundance to enhance performance
- Variance in elements that are not crucial to performance (𝑽𝑨𝑹𝑼𝑪𝑴) is allowed to increase (becomes uncontrolled)
- Variance in elements that are crucial to performance (𝑽𝑨𝑹𝑶𝑹𝑻𝑯) is reduced - to increase consistency
Provide an example of how the UCM relates to postural tasks:
- e.g.: keeping the COM within the base of support by using several joints that link the COM with the support area. An infinite number of joint configurations can lead to the same outcome
- uses muscle forces/activation as elemental variables to keep the body along the UCM and to reduce ORTH deviations.
What is a primitive?
- sets of force fields that are generated by muscle synergies causing reciprocal command activations = desired actions.
- The notion assuming that the motor repertoire is based on a limited set of elementary actions (primitives) that can be scaled and combined to match specific tasks. Primitives have been associated with force fields and combinations of muscle activation patterns.
What are the 2 studies which have shown simple single-joint actions leading to reciprocal commands over time in response to external perturbations?
- (Latash et al., 1999) - subjects performed a quick action in one of the joints of the elbow-wrist system without a special instruction about the behavior of the other joint
- (Domen et al., 1999)
- These observations suggest that there was indeed a synergy between the reciprocal pair changes at the two joints stabilizing the equilibrium of the postural joint at its initial position –> net torque at that joint = 0 at all times.
