Summary stability book and articles
Chapter 1-2.
Core stability is often used in different contexts and situations, which
makes some articles unable to compare to each other. You can look at
the independent variable of muscle strength, or neuromuscular control.
- Instability as a functional problem (use skill training to strengthen)
- Instability as a joint problem (use taping to stiffen joint)
- To control movement
- To control the whole body
A system is stable when it can successfully return to its planned
state after an infinitesimally small perturbation.
Potential energy is most in upright position stable. When moved,
height becomes less, so mgh becomes less, less optimal.
‘In terms of potential energy, the increase in elastic potential energy
due to perturbation d(beta) is larger than the decrease in
gravitational potential energy’
Ms = -K * d(beta).
Low K, unstable and with the smallest of perturbation falls over
Higher K, stable and displacement is dependent and bounded on
magnitude of perturbations
Even higher K, smaller displacement of pendulum and frequency of
oscillations are higher
Dampers are used to attenuate oscillations. They dissipate kinetic
energy and transform it to heat. In a real physical system, oscillations will
always disappear more or less gradually due to frictional losses of kinetic
energy. Dampers enhance this effect by absorbing more kinetic energy.
,You limit perturbations through stiffness and you recover from them
through dampness. performance of a system.
The robustness of a system is the property that describes the
maximum perturbation that a stable system can tolerate.
Artikel college 1: Spine stability: The six blind men and the elephant
Section 1-3
Prevention and rehabilitation efforts soon focused on retraining the
central nervous system (CNS) to increase muscle recruitment in order to
enhance spine stiffness to avoid low back pain (LBP).
But this is for static spine, so not too relevant
When dynamic, the environment or the type and outcome of certain
movements and situations need to be considered.
For a system to be considered stable, it’s trajectory needs to be limited
(we need to be able to assume the limits of the trajectory). If you can limit
the region in which the system lies by limiting the size of the perturbation,
then the system is stable. If the region is the same size no matter what
size of perturbation, then the system is unstable.
Robustness: How well can a system cope with
perturbations? Also, how well can the system
change its own stiffness without losing stability?
Performance: how closely and rapidly the
disturbed position of the system tends to the
undisturbed position.
, Chapter 3-4 stability book + lecture 2
Ligaments and joint capsules are passive tissues that act as springs.
They provide stiffness, but also provide damping due to their visco-
elastic behavior. Visco-elastic behavior refers to the fact that the force
in the structure depends not only on its length (elastic behavior), but also
on the rate of length change (viscous behavior).
Joint stiffness Joint laxity
ligaments and joint capsules show non-linear regressions, with low
stiffness at low lengths (1) and high stiffness at high lengths(*). This is
functional as it allows most movements without must resistance. But
ligament don’t provide enough to make a joint stable due to this fact
above. Also, due to constant loading (2), the length of ligaments
becomes longer over time, their effectiveness as a spring will decrease
over time.
* Ligaments and muscles are also non-linear because they can’t exert a
force below their rest-length; they can’t push.
Ligaments also provide damping.
Injuries or degeneration could cause passive structures to lose their
stiffness, which can result in a loss of stability around that joint. This can
be seen directly or indirectly, as in the case of feeling pain due to
overstretching of tissues etc.
Lecture 2a;
Question 1:
When the perturbation is constant (10 kg pulled to the ground),
there will be a displacement of the pendulum to the right, creating a new
equilibrium to that side with oscillations around that point.
, Question 2:
Second diagram: spring is stiffer frequency change + lower amplitude.
At the end the equilibrium point is lower, confirming that the spring was
stiffer.
Third diagram: Damping has increased. The damping already decreases
the amplitude of the first oscillation. 2 and 3 have the same final
displacement which shows that they had the same frequency.
Question 3:
At the first extreme (lowest or highest point) there is no velocity no
damping force or moment
so only gravitational force. Because the pendulum is inclined towards
the right, a gravitational moment is created (towards the right).
on the left a force is caused by the springs.
State variable: angle (orientation) / angular velocity
Lecture 2b:
When looking at a joint, you take all the muscles and ligaments together
in your equation, looking at the combined effects.
Mgh = Kb critically stable. There is no gravitational force moving the
ball further than the perturbation is doing.
If a muscle is more active it’s stiffer. At (0, 0) a muscle is inactive, and
the stiffness is also 0. So a muscle must be active in order to provide
stability. Except for very high muscle lengths, because the muscle then
has a passive force providing stiffness. In this case, the muscle is not
actively producing force, but due to its length it has a passive force.
Muscles can only provide stiffness over a limited range of movement due
to cross-bridge de-attachment (force-velocity relationship). With higher
