Postural Control - Lecture notes 10 PDF

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Erik Wilkstrom; lecture summary for exam 2...

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Postural Control I.

Postural Control a. Ability to maintain upright position against gravity b. Ability to maintain total body COM within base of support i. Geographical space ii. Limited area allows for frequent excursion outside of stability iii. Forced to modulate BOS so that we don’t fall when walking, because COM is outside of BOS c. Stability limit i. Point at which someone changes their BOS to achieve stability ii. Dynamic based on task, individual, and environment d. Static postural control i. Ability to maintain the COM within a stable BOS e. Semi-dynamic postural control i. Ability to maintain COM within moveable base of support f. Dynamic postural control i. Ability to maintain COM within a changing BOS

II.

Postural Control Strategies a. Motor control of quiet stance, which is characterized by small amounts of spontaneous postural sway i. Body alignment and muscle stiffness are important factors, but not enough due to sway; need movement strategies to maintain stability b. Two main models to study strategies that we can move back and forth between i. Inverted pendulum ii. Multilink pendulum 1. Ankle strategy 2. Hip strategy c. Movement strategies during perturbed stance i. Strategies are used in feedback and feedforward 1. Feedback: response to external perturbation 2. Feedforward: preparing for a voluntary movement ii. ML stability (left to right) 1. Primarily at hip and trunk 2. Ankle helps with narrow stance width (tandem) 3. Descending response if perturbed in quiet stance: head, hips, ankle a. Head moves opposite of ankle and hip movements iii. AP stability (front to back) 1. Ankle strategy used after small perturbation and when there’s a firm support surface a. Distal to proximal response: ankle, thigh, trunk b. If lean forward due to perturbation, posterior muscles will fire to straighten back up 2. Hip strategy used after large and fast perturbation or if there’s a small/compliant support surface iv. Evidence of synergies 1. All strategies require synergies of multiple muscles (not just ankle and hip) 2. We adapt motor strategies based on our standing surface

Postural Control a. When standing on narrow beam, takes 5-15 trials to switch from ankle to hip strategy. Took about 6 trials to switch back to ankle after returning to normal surface. Used complex variations of strategies during transitions 3. Learned responses III.

Sensory Contributions to Postural Control a. Visual i. Major source of info ii. Difficulty discerning between object and self-motion b. Somatosensory i. Provides info relative to horizontal surface; not helpful when surface is not horizontal 1. This is why it helps to look at land while on sea c. Vestibular i. Position and movement of head, but an incomplete picture 1. Head nod with stable trunk and trunk flexion are same to CNS d. Static postural control i. Vision 1. Balance is worse with eyes closed ii. Somatosensation 1. Anesthesia and cooling studies 2. Vibrations studies 3. Fingertip studies e. Perturbed postural control i. Vision 1. Moving room experiments – sway with room ii. Somatosenation 1. Preferred over vision during rapid support surface displacements a. 80-100ms vs 200ms latency iii. Vestibular 1. Muscle responses are ~10x smaller to head perturbation 2. Rotational perturbations require vestibular system to control M3 responses f. All 3 sources of info must be integrated for optimal postural control g. Adapt sensory organization to changes in environment i. As accuracy of one system decreases, rely more heavily on the other

IV.

Central Contributions to Postural Control a. APA i. Arm raising experiment 1. Postural muscles fire ~50ms before prime mover 2. Synergies are modulated based on muscle usefulness for recovery a. Based on experience, expectations, and practice b. Role of cognition/ attention i. Attentional demands vary based on postural control task, the individual, and age ii. Processing theories 1. Capacity a. Have an attention capacity, when it is exceeded, performance suffers in one or both tasks 2. Bottle-neck

Postural Control a. Multiple tasks fight for priority to use neural processes to determine task demands V.

Postural Control Development a. Stabilization of head in babies allows for more mature movements and behaviors to emerge...