Posture and Balance Flashcards
what are three elements of postural equilibrium?
- Feedforward (anticipatory) control of balance
- Feedback (reactive) control of balance
- Choosing from multiple solutions
describe postural equilibrium
standing balance is a dynamic task. Center of pressure and center of mass keep shifting.
describe feedforward balance control
Standing lever example. Ankle muscles get activated before the lever is pulled to keep the body in equilibrium (unless the body is otherwise stabilized).
optional: voluntary movements are accompanied by anticipatory postural adjustments that help us maintain balance before we perform a voluntary movement such as a reach. These anticipatory postural adjustments are therefore preplanned motor actions that help counteract any disturbance to balance before the initiation of a voluntary movement. Preplanned movements are also referred to as feedforward motor actions.
For example, when a subject is asked to pull on a handle, there are large changes in ankle torque more than 500 ms before any force is measured at the handle. Increased ankle torque causes the body to lean forward to counteract the backward movement of the body that will be caused by pulling on the handle. Although we may only be aware of the motor intention to pull the handle, the activity of leg muscles is closely modulated depending how much force the subjects intends to exert with the arm. Thus, the postural leg muscles activity anticipates the upcoming voluntary arm motor task. If the body is stabilized by bracing it to a support, then no anticipatory postural activity in the legs is measured prior to the pull.
Therefore, the anticipatory postural adjustment predicts the impending postural destabilization that may accompany a voluntary movement. These anticipatory postural adjustments are considered “automatic”, as they are not necessarily under conscious control, and are finely tuned to the details of the intended task.
describe feedback balance control
Task level (move COM) > Execution (Activate muscles) > Execution (Muscles stretched, foot pressure uneven) > Task (COM is tilted to right/left) > start over.
Sensory receptors + muscles important
explain the integration of sensory information for postural equilibrium. Role of three senses?
- NEED MULTIPLE SENSES.
- Somatosensory receptors necessary for timing and patterning of postural responses
- Vestibular visual signals play modulate roles
- Flexible reliance on senses
describe how stretch reflex is not enough to maintain posture
If platform is flat and translates back, calf gets activated (due to STRETCH response), which is fine.
If platform rotates upwards, stretch response (spinal mediated) activates calves initially but that is bad so postural response (brainstem) corrects it by activating TA.
If platform rotates down, TA gets activated as a stretch response but calf gets activated later as a postural response.
optional:
The muscle activated by the spinal stretch reflex cannot always activate the proper muscles. That means that the information from muscle spindles alone cannot tell us how to respond to a perturbation. The muscle spindle encodes muscle stretch information and therefore can tell us how we have moved in relation to the ground. But if the ground moves relative to the vertical, as defined by gravity, then muscle spindle information will not be appropriate for balance control.
example of how visual and vestibular info alone is insufficient to maintain posture
Visual: if picture is spinning, you start leaning in the direction of rotation
Vestibular: if we turn, vestibular system picks it up and defines a new “vertical”
describe how vestibular info is bad
- vestibular canal info is poor for low-freq and low-amplitude perturbations
- vestibular otolith info cannot distinguish gravity and linear acceleration - cannot distinguish between tilt to the right and linear acceleration to the left
examples of how a single sense is insufficient to maintain postural equilibrium
- Stretch reflexes (tilting platform)
- Vestibular canal (tilting platform)
- Vestibular otoliths (can’t tell linear accel vs head tilt)
- Visual (cannot tell self-motion from object motion)
what is special about postural perturbation?
common goal (maintaining balance, i.e. maintain CoM over BoS), different execution strategies( ankle, hip, etc)
what is the timetable of postural responses to perturbation?
- Perturbation
- Passive response + sensory feedback
- Spinal response (monosynaptic stretch reflex, only in stretched muscles) (50 ms) (short-latency)
- Brainstem response activates different muscles depending on situation (100 ms) (long-latency)
- Voluntary response, cortex, (> 250 ms)
no one-to-one mapping between task-level and execution level variables
In detail:
Spinal reflex response (50 ms):
Activates only in stretched muscles;
Due to monosynaptic connection between muscle spindle afferent and motoneuron;
Cannot be eliminated voluntarily, but can be modulated;
Brainstem automatic postural response (100 ms):
Activates different muscles, depending upon direction of falling, postural configuration, strategy, and adaptation;
Due to polysynaptic neural pathway;
Cannot be eliminated voluntarily, but can be modulated;
Voluntary postural response (>250 ms):
Requires cortical inputs and can alter the response strategy
describe ankle vs hip strategy (execution-level control of muscles for posture)
Ankle strategy: using back, leg, calf muscles to maintain equilibirum
Hip strategy: Swinging hips to maintain posture (like when you are on a beam that slides back).
Experiment shows that after doing many trials on the beam you habituate and automatically do the hip response even when you’re back on the floor. After several trials, you’re back to ankle strategy
describe responses of Parkinson’s patients to pertubation
Basal ganglia deficit -> cannot select appropriate muscles to activate based on context (leg muscles active when seated)
describe response of cerebellar patients to perturbation
Over-respond to perturbations, oscillate. Do not adapt to blocks of the same perturbations. b/c they cannot use prior info to modulate response
muscle synergy concept (task-level control of muscles)
important! check the slides
complex muscle activation patterns are formed by combining a few muscle synergies
functional muscle synergies translate task-level goals into execution-level commands:
Modular building blocks for motor control;
Preferred patterns of multi-muscle co-activation;
Produce consistent task-level function;
Flexibly combined to produce a continuum of muscle activation patterns;
Differ in number and pattern across subjects;
