W3 - Balance Flashcards
Define Human balance:
- the active control of the physical shape of the limbs to provide varying degrees of passive stability and muscular actions to provide compensating active stability for passive inefficiencies
- Balance is considered to be a simple skill, but is actually quite complex underneath
- Humans are passively unstable, so there is a need for active control to maintain balance
What is meant by posture?
includes the body orientation e.g.: vertical posture, maintained over a certain period of time & resistance to external perturbations[local stability] –> such conditions effecting posture include Parkinson’s disease, impairing balance and body equilibrium
Provide 3 examples of postural control:
- A figure skater maintaining a beautiful arm configuration while sliding over the skating rink (arm orientation maintained
respect to the body but not with respect to the environment) - A musician playing the violin, moving the body and the violin (hand posture may be maintained with
respect to the violin, not necessarily to the body or the external space) - Holding the handles while riding a bicycle (posture is maintained with respect to a part of the bicycle, not the whole bicycle, e.g., during sharp turns, not the body, and not the environment).
What are the 2 views on postural sway (Nikolai Bernstein, 1896-1967)?
- A consequence of noise in the neuromotor system, a sign of imperfection in the human body(kept low by good neural control & to avoid losing balance. The necessary background component for any voluntary motor action
- A purposeful design of the neurophysiological system for postural control e.g.: Parkinson’s patients may loose balance due to little perturbations. Meaning healthy people have higher sway but more control
What are the reasons for postural sway?
- Passive instability
- Muscle errors
- Sensory errors
- Feedback delays - delays from neuron control between sensory and muscle systems
- Control strategies
What is the posture-movement paradox(Von Holst & Mittelstaedt, 1950)?
How does this relate to a postural equilibrium?
- a number of posture-stabilizing mechanisms generating forces against external perturbations applied during steady-state motor tasks at very short time delays (under 100 ms), shortest simple reaction time
- caused quick postural corrections, but not voluntary movements
- Where posture is a spatial characteristic e.g.: muscle length, joint angle, stabillised –> maintained by synergies
- similar to the equilibrium hypothesis, assuming that equilibrium states of the body in environment is controlled by muscle correction reflexes
What is stability?
goes hand-in-hand with sway
* The tendency of a body to remain or return to its initial position following the application of a force
* Stability can be passive or dynamic
* Maintain CoM within base of support
Provide examples of:
- stable equilibrium (triangle)
- unstable equilibrium (hammer)
- neutral equilibrium (circle)
- Stable - force has to rise(humans) e.g.: a triangle
- Unstable - energy e.g.: gravity keeps getting lower. Perturbations cause imbalance(humans) e.g.: Pendulum
- Neutral - energy does not change, it just keeps going does not fall e.g.: sphere
What are the factors that determine passive stability?
What makes things stable/unstable
* weight - harder to perturb it as has increased weight
* area of the base
* horizontal distance of the CoG to pivot point - CoM has to go further to reach pivot point
* height of the CoG above the base - does have to be tipped as far when higher up
- CoP is the vertical resultant forces acting on the body from the supporting surface
What is passive instability?
In standing, humans are passively unstable, based on:
– Small base of support
– High CoG above the base
– Like balancing a pencil
—> Resistance to current state, thought of in a passive state
Human standing is dynamically stable
– Needs active muscle contractions
Describe muscle errors:
Muscles cannot produce a perfectly consistent force:
– Variation / error
– Underestimate and overestimate force causes sway
Muscles produce force between attachments:
– Across multiple joints: variation between muscle forces
– In multiple planes / axes: needs to be counteracted by
Other muscles
what are sensory errors?
Our sensory system is not perfect
There are many different sensory organs:
– Vision 10%
– Vestibular apparatus 20%
– Somatosensory proprioception 70%
This leads to sensory conflict from different systems having different inputs
Some require movement to work, not an absolute state
Describe and draw the sensory threshold effect:
(Clark et al., 1995)
(Fitzpatrick & McCloskey, 1994)
Slow movements can only detect position sense, and only detect movement sense if moved enough along position sense
- Vestibular system is not used for quiet stance as does not detect small movements/perturbations
When can feedback delays occur?
Draw the graph by (Blenkinsop, Pain & Hiley, 2006)
INPUT –> delay –> PROCESS –> delay –> OUTPUT ¬v
¬> FEEDBACK DELAY <———–
What is sensory delay?
- Sensory delay is the amount of time it takes the sensory threshold to be reached, fast movement is smaller delay
- the delay of signals coming into the brain that cause muscle contraction etc
What are the different types of feedback delays?
Sensory Delay - Time taken to detect movement (sensory thresholds)
Neurological Delay (65-130 ms)
- Afferent signal transmission (fairly constant delay ~50 ms) - depends
How far away from the core it is coming from
* Decision time
- Efferent signal transmission (fairly constant delay ~50 ms)
Electromechancial Delay (13-55 ms): closer to 30ms = Time from muscle activation until force is produced - transmission at the synapses takes the most time, easier to increase force than decrease force
Name control strategies for human movement:
- Ankle strategy - useful for small slow perturbations (y-axis)
- Hip strategy - used for large or rapid perturbations or when the base of support is small (x-axis)
- Mixed strategy
- Arm swings - trying not to step after landing
- Stepping - comes sooner in day-to-day movement such as after ankle strategy
All of which we want inside the tolerance region, and can only avoid falling over within the region of reversibility
Define:
Rambling
Trembling
Rambling - measures the time series of the COP trajectory in the AP and ML directions
Trembling - the difference between the COP and rambling, body oscillations around a set point
Describe the ankle strategy:
- Fixed hip angle
- Controls COM via ankle torque
- Good when low surface friction
- Bad on narrow surfaces
- Our preferred strategy during quiet stance
Describe the hip strategy:
- Hip angle opposite to ankle
- Controls COM via horizontal force
- Good on narrow surfaces
- Bad when low surface friction
- Requires less effort
- Is generally quicker
What is the control model?
- This is an expansion of the information processing model : stimulus identification -> response selection -> response programming
- Involves a model with assumptions, & some simplification of the true system
- Any variation is seen as unwanted error
What are the different posture-stabilizing mechanisms?
- how long does each reflex take roughly?
- Preflexes - These act to external perturbations with a 0 time delay
- Tonic Reflexes - time delay of 50-70ms as there is a transmission of sensory signals to the CNS
- Long-loop reflexes - take 100-150ms
- Voluntary contractions take 150-200ms to respond to perturbations
- Visual and vestibular reflexes take 180-200ms
Preprogrammed reactions - are suboptimal and prepared by the CNS prior to a perturbation to be compensated by a voluntary contraction
What are the main 3 sections of a PID controller?
Proportional:
– Present state of the system (position/angle)
– Quick corrections, but leads to overshoot
Integral:
– Past state of the system (average over time)
– Corrects for drift, but is slow to correct. Pushing back to the reference state
Derivative:
– Future state of the system (current velocity)
– Prevents overshooting and is similar to dampening(things oscillate quickly then return to normal
What is the example PID by Peterka 2002
- Used numerous sensory perturbations
- Determined weights for sensory input:
– Vison = 10%
– Vestibular = 20%
– Proprioception = 70%
Shows how a controller and plant can work together, no need to know in depth
What is the example PID by Yeadon and Trewartha 2003?
handstand balance
* Fit experimental data to mechanical model
* Used repeat regressions with time offsets
* Determined feedback time delays of 160 ms to 240 ms in handstand
Name some PD/PID issues:
- Simulation output is too good
- Relies on excessive noise to reproduce typical postural sway
Assumes everything in human sway is error or noise - Struggles with large delays near 200ms, even more with 2/3 segments
- Can used intermittent control models
Where controller turns on and off to control force
Revisit the control model PID
slide 28 w3
