Week 12 Flashcards
Which motor function is frequently overlooked, sometimes thought to be “unimportant”?
standing balance
Which ability often goes unnoticed until dysfunction occurs?
standing balance
What is this? A complex task requiring vast, constant integration of sensory and motor information
standing balance
We need to be able to maintain out balance during three different contexts. What are they?
- during quiet standing
- during unexpected perturbations
- during self-initiated movements
What is postural control?
controlling body position in space to maintain stability and orientation
What is centre of mass
location where all the mass of the body is concentrated ~ 5/9 of height
What is centre of gravity?
vertical projection of COM to ground
What is centre of pressure?
point of application of vertical ground reaction force exerted on body
What is the base of support?
region bounded by body parts in contact with a support surface or surfaces (e.g. ground that exerts counterforce against the body’s applied force)
When trying ti stand perfectly still, there is some _____ ____
body sway
Why does sway happen?
- in part due to small physiological perturbations (e.g. HR, breathing, physiological tremor, muscle tone etc.)
- our bodies cannot continually maintain perfect equilibrium, but can be ‘stabilized’ → i.e. sway is inevitable but we can maximize stability so the sway doesn’t cause us to fall
amount of sway is affected by…..
body alignment
- better alignment = less sway
A “stable” posture exists if the….
vertical line of action of the force of gravity passes through whole body
When the body is aligned with line of gravity…
- minimize energy required to maintain balance
- maximize stability
Generally, a person has stable posture as long as…
the line of action of COG is within base of support
Postural stability depends on three biomechanical factors. What are they?
- postural stability is decreased if the base of support is reduced
- postural stability is decreased if the line of the COG is closer to the edge if the base of support
- postural stability is decreased if the heigh of the COM is increased
How is sway an exploratory mechanism?
standing balance required constant sensory input to know where we are positioned in space
How can the exploratory mechanism of sway be proven?
- in the absence of COM displacement (i.e. no sway), we “drive” our bodies to try to sway (increase in COP)
- allows us to gather the sensory information we need to balance
What is corticomuscular coherence?
how similar are the signals measured in your motor cortex to signals measured in your muscles
Corticomuscular coherence is an estimate of…
how much your cortex is driving that muscle activity
Is sway during quiet standing cortical or subcortical?
predominantly subcortically driven?
Various factors & pathologies affect posture. What are they?
- medication
- pain
- aging
- neurological disorders
Why study posture and balance?
- the maintenance of balance/equilibrium and body orientation in the standing position is essential to the performance of the activities of daily life
- understanding how posture is controlled might develop better diagnoses, rehabilitation and prevention tools
In order to know when and how to apply forced or to activate appropriate muscles, the CNS must have…
an accurate picture of where the body is in space and what forces are acting on it
the contribution of each sensory system is typically assessed by…
- removing or altering/distorting the sensory information available
- using patient populations where the sensory information is absent or distorted.
Typically assess the involvement in different settings/contexts. Two common methods:…
- quiet standing (COM or COP)
- external perturbations (reactive balance control)
Normally, peripheral inputs from three main sources are available to detect the body’s position and movement relative to space and environment. What are they?
- vision
- vestibular (otoliths, semi-circular canals)
- proprioception (muscle spindles, GTO, joint receptors, cutaneous receptors)
When there is no visual input, what happens to standing balance?
postural sway increases between 30-40%
How do we know that vision is involved in quiet standing?
- remove vision → greater sway, as indicated by grater movement of COP/COM
- alter optic flow to give the illusion that a person is swaying in one direction → they will move the COM in the opposite direction
How do we know that vestibular plays a role in standing balance?
COP total displacement increases with head tilt even when visual input remains constant
What happens to postural sway when there is reduced sensory feedback from legs & feet?
increased postural sway
Vibration to muscle used in postural control causes…
an illusion of change in body orientation or sway
Cooling feet or applying a topical anesthetic causes…
an increase in postural sway
loss of lower limb somatosensory inputs (hypothermia, ischaemic block) leads to…
increased sway
When we add proprioceptive feedback, such as through the finger, what happens to the displacement of COP/COM?
decreased movement of COP/COM
What evidence to we have to answer the question: will blind individuals always exhibit a deficit in postural control?
- blind participants had smaller COP-COM displacement compared to sighted with eyes closed
- if a deficit in postural control due to a loss of vision remained it should stay the same
Immediately after unilateral/bilateral loss of vestibular info, patients adopt…
abnormal alignment and increases in sway behaviour
Describe sway in vestibular loss patients
- patients rapidly adapt to loss of vestibular info
- after initial recovery period, postural sway with eyes open is often similar to that observed in controls
- if you get them to close their eyes or adopt a challenging posture, sway is greater
How are aging and standing balance related?
aging is associated with reduced muscle strength and sensory function (vision, vestibular, proprioception) → increased postural sway, response to perturbations slower with greater muscle activity
During quiet standing, we must maintain…
antigravity posture
During unexpected perturbations, we use…
reactive balance
During voluntary self-initiated movements, we use…
anticipatory control
How does the postural control system maintain a desired orientation and stability if there is an external perturbation (i.e. decelerating bus)?
- postural control system comes into play in response to a destabilizing external force or perturbation
- it utilizes feedback mechanisms where the CNS responds to info received during and after a movement and attempts to restore stability
Automatic, reactive postural responses usually involve a particular pattern of muscle activation known as….
What is muscle synergy?
particular muscles work together (in synergy) to achieve the task; in case of reactive balance, it occurs without voluntary control
- muscles involved in the synergy depend on the requirements of the task
What are the two main responses of reactive balance
- Ankle strategy - involves distal-to-proximal sequence of muscle activation
- Hip strategy - involved proximal-to-distal activation
- two strategies activate opposite muscles
- typically observe a mixed (ankle + hip) response
What are the reactive balance strategies?
- Ankle strategy: rotate about ankle only - body stays straight
- Hip strategy: rotate about hip only- legs and torso stay straight
- suspensory strategy : crouch down - mostly observed in young children
- step strategy: take a step due to COM moving outside limits of stability
Small balance perturbations usually result in use of an…
ankle strategy
Lare perturbations →
switch to hip strategy and finally stepping strategy
If support surface is changed to a narrow beam, then use…
hip strategy at a smaller perturbation magnitude
How is reactive balance context dependent?
switch will occur at different perturbation magnitudes depending on context
Describe standing balance of de-afferented patients
have a very hard time standing unsupported
- some cannot stand unsupported
- others can stand but ONLY with vision and very conscious control
How do cerebellar disorders impact standing balance?
- cerebellar patients have problems scaling response amplitudes (hypermetria)
- response is too large and too long, so they overshoot and must activate antagonist
What is hypermetria?
a condition of cerebellar dysfunction is which voluntary muscular movements tend to result in the movement of bodily parts (as the arm and hand) beyond the intended goal
