Stability

Question 1

Pertubation

Answer 1

Force, moment or torque that causes an unintended change of current equilibrium

Question 2

Stiffness(K)

Answer 2

Strength of the feedback loop after a pertubation. Oscillations will occur.

Question 3

Damping

Answer 3

Resistance against velocity by the absorption of energy. Stops the oscillations and brings the Pendulum back to equilibrium.

Question 4

Performance

Answer 4

The faster the inverted pendulum can return to its initial position (static equilibrium)

Question 5

Robustness

Answer 5

Tolerance to pertubations. High robustness means that a pendulum can handle larger pertubations

Question 6

Kinematic state

Answer 6

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.

Question 7

Co-contraction

Answer 7

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).

Question 8

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

Answer 8

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.

Question 9

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?

Answer 9

Doubling moment arms has a larger effect. The effective stiffness increases linearly with an increase in spring stiffness. moment arms increases quadratically

Question 10

Joint structures and active muscles

Answer 10

Act as parallel springs and dampers. Are responsible for stability.
Cocontraction

Question 11

Muscle spindles

Answer 11

Are the source for the stretch reflexes.

Question 12

Three control systems when it comes to stability

Answer 12

Passive system (ligaments)
Active system (muscles)
Nervous system (reflex feedback: stretch reflex)

Question 13

Time delay

Answer 13

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

Question 14

Increasing reflex stiffness

Answer 14

Compensates, but only to a certain extend. Displacement will be smaller, but can still cause displacement in the same direction as the pertubation.

Question 15

Reduced cocontraction

Answer 15

Decrease stiffness and damping of the muscles. –> Faster displacement, larger amplitude, lower frequency of the oscillations and a slower attenuation of the oscillations

Question 16

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.

Answer 16

• 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.

Question 17

Observability

Answer 17

Availability of sensory information and quality of this information.

Question 18

Time delay depends on:

Answer 18

• 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

Question 19

Gain depends on:

Answer 19

• Receptor gain (sensitivity)
• Excitability motor neuron
• Muscle strenght

Question 20

Core stability

Answer 20

• Joint stiffness
• Muscle co-contraction
• Sensory function and processing
• Feedback delay
• Feedback gain

Question 21

Inertia

Answer 21

Given property of the system (mass and dimension) can be estimated on anthropometric measurements and data

Question 22

3 types of pertubations according to system identification

Answer 22

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.

Question 23

Increased intrinsic stiffness

Answer 23

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

Question 24

Admittance

Answer 24

Inverse of stiffness (less stiffness and more laxity) –> In a relax state admittance is high.

Question 25

Gait variability

Answer 25

Reflect pertubations due to neuromuscular and environmental noice

Question 26

Formula of stability

Answer 26

• 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

Question 27

Threshold

Answer 27

No signals are provided below the threshold. Nothing will change. Above the threshold the output will change.