Concepts in Human Movement Sciences
Stability
Lecture 1 – Stability
Should we train (core) stability?
How should we train (core stability?
How should we monitor progress?
Stability: The body’s ability to maintain or resume an equilibrium position of the trunk after
perturbation. (Zazulaket al., 2007)
“Ability to deal with perturbations”
➢ Precision
➢ Balance
➢ Ballistic movement: energy in the leg should not affect the trunk
Perturbation/disturbance: force, moment, or torque that cause an unintended change of current
equilibrium (the planned state)
Injury: Lateral trunk and knee abduction motion are important components of the ACL injury
mechanism
➢ Lack of control over trunk movement may cause large moments around lower extremity
joints
➢ Perturbation and sensing movement
➢ Muscle strength and muscle endurance
Stability of/control over trunk movement is important for performance and for preventing injury
Definitions of stability are often lacking or unclear
Factors considered important for stability are different between disciplines
Lecture 2 – Stability a mechanical perspective
Core stability: important for injury prevention in athletes
Stability: whether a system can be controlled or not
Pendulum: simple model of body segments
A normal pendulum is stable when hanging downwards
3 assumptions:
- Posture / movement can be described in terms of joint angles
- Body segments are rigid and cylinder or beam like
- Joint contact surface is small
,Inverted pendulum (IP): inherently unstable, must be actively balanced in order to remain upright
- First requirement = equilibrium
o
o
- Second requirement = stable equilibrium:
o dM/dβ < 0
How to stabilize an inverted pendulum: Add a spring
,Properties of the pendulum:
m*g*h: effect mass/ center of mass height
properties of the stabilizing spring:
-dMs /dβ: rotational or bending stiffness (Kb)
Stiffness (K)
Hooke’s law: F = -Kdl (F = cu)
More springs: lumped effect of multiple springs
➢ All the springs in 1 parameter
, Stiffness
- Amplitude decreases
- Frequency increases
Damping: dissipation of kinetic energy
- Smaller initial displacement
- Frequency the same
- Damping coefficient ‘b’
Performance = the ability of a stable system to limit effects of perturbations of a given magnitude
Robustness = Describes the biggest perturbation a system can tolerate
- Stiffness and damping improve the robustness: larger perturbations can be handled, larger
margin of safety to becoming unstable
Summary
- Control of joint angle and angular movement requires corrections based on (errors in) state
variables, segment orientation and angular velocity
- Gain of orientation feedback (stiffness: K) has to be sufficient to prevent instability
- Effects of perturbations are attenuated with increased gain of orientation (stiffness: K) and
velocity (damping: b) feedback
- Quality of control of a stable system can be measured in terms of performance and
robustness
Stability
Lecture 1 – Stability
Should we train (core) stability?
How should we train (core stability?
How should we monitor progress?
Stability: The body’s ability to maintain or resume an equilibrium position of the trunk after
perturbation. (Zazulaket al., 2007)
“Ability to deal with perturbations”
➢ Precision
➢ Balance
➢ Ballistic movement: energy in the leg should not affect the trunk
Perturbation/disturbance: force, moment, or torque that cause an unintended change of current
equilibrium (the planned state)
Injury: Lateral trunk and knee abduction motion are important components of the ACL injury
mechanism
➢ Lack of control over trunk movement may cause large moments around lower extremity
joints
➢ Perturbation and sensing movement
➢ Muscle strength and muscle endurance
Stability of/control over trunk movement is important for performance and for preventing injury
Definitions of stability are often lacking or unclear
Factors considered important for stability are different between disciplines
Lecture 2 – Stability a mechanical perspective
Core stability: important for injury prevention in athletes
Stability: whether a system can be controlled or not
Pendulum: simple model of body segments
A normal pendulum is stable when hanging downwards
3 assumptions:
- Posture / movement can be described in terms of joint angles
- Body segments are rigid and cylinder or beam like
- Joint contact surface is small
,Inverted pendulum (IP): inherently unstable, must be actively balanced in order to remain upright
- First requirement = equilibrium
o
o
- Second requirement = stable equilibrium:
o dM/dβ < 0
How to stabilize an inverted pendulum: Add a spring
,Properties of the pendulum:
m*g*h: effect mass/ center of mass height
properties of the stabilizing spring:
-dMs /dβ: rotational or bending stiffness (Kb)
Stiffness (K)
Hooke’s law: F = -Kdl (F = cu)
More springs: lumped effect of multiple springs
➢ All the springs in 1 parameter
, Stiffness
- Amplitude decreases
- Frequency increases
Damping: dissipation of kinetic energy
- Smaller initial displacement
- Frequency the same
- Damping coefficient ‘b’
Performance = the ability of a stable system to limit effects of perturbations of a given magnitude
Robustness = Describes the biggest perturbation a system can tolerate
- Stiffness and damping improve the robustness: larger perturbations can be handled, larger
margin of safety to becoming unstable
Summary
- Control of joint angle and angular movement requires corrections based on (errors in) state
variables, segment orientation and angular velocity
- Gain of orientation feedback (stiffness: K) has to be sufficient to prevent instability
- Effects of perturbations are attenuated with increased gain of orientation (stiffness: K) and
velocity (damping: b) feedback
- Quality of control of a stable system can be measured in terms of performance and
robustness
