Stability Flashcards
Pertubation
Force, moment or torque that causes an unintended change of current equilibrium
Stiffness(K)
Strength of the feedback loop after a pertubation. Oscillations will occur.
Damping
Resistance against velocity by the absorption of energy. Stops the oscillations and brings the Pendulum back to equilibrium.
Performance
The faster the inverted pendulum can return to its initial position (static equilibrium)
Robustness
Tolerance to pertubations. High robustness means that a pendulum can handle larger pertubations
Kinematic state
Orientation of the segment, velocity of movement. If you want to control the kinematic state of the system (the posture and velocity) you need stiffness and damping.
Co-contraction
Necessary for equilibrium. An equal amount of muscles of muscles (on both sides of the joint) are active to cancel out each other’s moments by muscle contraction. Agonists and antagonists contract to gain equilibrium (sum of moments = 0).
Increasing the stiffness of the springs in the model can be used to simulate a specific strategy used by subjects tested to control their trunk posture as well as possible. Which strategy and explain why it can be modelled in this way
Co-contraction of muscles on the left and right side of the trunk can be simulated by increasing spring stifness in the model. Mechanical stiffness of muscles increases with muscle activation, therefore increased levels of co-contraction will result in increased stiffness.
To assess the importance of the anatomy, the moment arms of the springs are doubled, and the effect is compared to doubling of the stiffness of the muscles. Which of the two changes of model properties has the largest effect on the kinematic response that the model predicts?
Doubling moment arms has a larger effect. The effective stiffness increases linearly with an increase in spring stiffness. moment arms increases quadratically
Joint structures and active muscles
Act as parallel springs and dampers. Are responsible for stability.
Cocontraction
Muscle spindles
Are the source for the stretch reflexes.
Three control systems when it comes to stability
Passive system (ligaments)
Active system (muscles)
Nervous system (reflex feedback: stretch reflex)
Time delay
Tau. There is a delay in the nervous system. Reflex stiffness and damping. Delays reduce performance and may cause instability. Performance gets worse if reflex stiffness and damping stay the same but delay increases. The less delay the better the performance
Increasing reflex stiffness
Compensates, but only to a certain extend. Displacement will be smaller, but can still cause displacement in the same direction as the pertubation.
Reduced cocontraction
Decrease stiffness and damping of the muscles. –> Faster displacement, larger amplitude, lower frequency of the oscillations and a slower attenuation of the oscillations
A group of low-back pain patients and a group of matched healthy controls are tested with
the setup described above. EMG signals of muscles on both sides of the trunk are measured
during the test. It is found that the changes in muscle activity after the perturbation occur
somewhat later in patients with low-back pain than in healthy controls (40 vs 30 ms). On this
basis, it is concluded that reflex delays are longer in patients than in healthy individuals.
- The short delays indicate that these are based on muscle spindle feedback. These delays are mainly determined by nerve conduction. Physiologically an effect of pain on nerve conduction velocity is not plausible
- Patients with pain have been shown to have higher levels of co contraction. This means higher stiffness and damping, so the movement after the pertubation will be slower. The displacement may remain below the thresholds of mechanoreceptors for a longer time. Which would delay responses.
- Lower backpain –> Higher co-contraction –> higher intrinsic stiffness.
Observability
Availability of sensory information and quality of this information.
Time delay depends on:
- Threshold receptor (how much length change have to be before muscle spindles sense the change)
- Distance to spinal cord
- Number of synapses
- Elektomechanical delay
- rate of force rice
Gain depends on:
- Receptor gain (sensitivity)
- Excitability motor neuron
- Muscle strenght
Core stability
- Joint stiffness
- Muscle co-contraction
- Sensory function and processing
- Feedback delay
- Feedback gain
Inertia
Given property of the system (mass and dimension) can be estimated on anthropometric measurements and data
3 types of pertubations according to system identification
Impulse: Sudden change
Step: pertubation leading to permanent change in equilibrium
(Pseudo)random: Continuous pertubations which are preferably constant with a known frequency –> Most informative. May affect systems behavior.
Increased intrinsic stiffness
Caused by pain (lower backpain –> More co-contraction). May cause apparent delay in the reflex. But, this doesn’t mean it’s a slower reflex. It can mean that the input just arrives later
Admittance
Inverse of stiffness (less stiffness and more laxity) –> In a relax state admittance is high.
Gait variability
Reflect pertubations due to neuromuscular and environmental noice
Formula of stability
- The moment of the spring is linear depended on the spring force
- The spring force is linear depended on spring moment arm
- mgh –> the potential energy –> the negative stiffness
- a^2k –> the total bendig stiffness of the joint arm. Square of the moment arm times the stiffness of the spring
Threshold
No signals are provided below the threshold. Nothing will change. Above the threshold the output will change.
