L8 Postural Control and Stability

postural control

• act of maintaining, achieving, or restoring a state of balance during any activity

• a complex interaction of: higher level planning, sensory processes (perceptual systems), motor processes (action systems)

• has 3 distinct aspects: orientation, equilibrium, an anticipatory postural adjustment (APA)

perceptual system

• complex networks in nervous system that are responsible for receiving, interpreting and making sense of sensory information

• includes visual, somatosensory (proprioceptive, cutaneous, and joint receptors), vestibular system

anticipatory control

• higher level planning from FRONTAL and MOTOR CORTEX

• brain evaluates surroundings before movment

• plans movement sequence to avoid obstacles

brain stem coordination

• regulates postural tone: maintains muscle activation for stability

• inhibits excessive flexion: ponto-medullary reticular formation prevents over-flexion, ensuring upright posture

• balances flexion and extension: coordinates trunk muscle activity for postural stability

  • ____ ____ ____ plays a huge role in maintaining upright posture by modulating msucle tine and preventing excessive forward bending

cerebellum

• fune tunes motor output to maintain balance

• adjusts postural muscle activity based on environmental and task demands

• works with the vestibular system to enhance stability and coordination

• ______ acts as the brains “stability controller” continuously refining movements to maintain posture and balance

basal ganglia

• contribute to the automatic adjustments and refinement of postural responses

• help regulate muscle tone and influence the gain of postural reflexes, ensuring appropriate muscle stiffness and responsiveness

• ______ _______ work with cerebellum, brainstem, and cortical areas to achieve stable and adaptable posture

1 million

there are over __ _____ neurons in the optic nerve, compared to 30,000 neurons in the auditory nerve

• vision provides key sensory input for motor control, helping to giode movements

rods, cones

2 types of photoreceptors:

• ____: night vision (sensitive to low light and motion detection)

• ____: day vision (detects color and provide high spatial resolution)

**Function: contribute to depth perception, object recognition, and spatial awareness

peripheral vision

plays a vital role in detecting motion and changes in the environment, which helps maintain balance by providing contextual awareness

direct vision (foveal)

responsible for focusing on objects, used for precise movements (such as reaching or tracking a target during sports)

vestibulo-ocular reflex (VOR)

• eye movements result from stimulation of the vestibular system

• ____________ ______ helps keep eyes steady while the head is moving, ensuring a stable visual field

• compensates for rotation of the head

vestibular system

• located in the inner ear and is crucial for sensing head position and movement in space

  • includes semicircular canals and otolithic organs

semicircular canals

• 3 fluid filled tubes arranged in perpendicular planes

• senses angular acceleration

How it works: when the head rotates, fluid within canals shifts, causing bending of tiny hair cells → hair cells signal to brain, providing info about direction and speed of head rotation

angular, linear

______ acceleration: rotational head movement

_____ acceleration: head movement in a straight line

otolithic organs

Utricle and saccule: fluid filled sacs containing hair cells embedded in a gelatinous membrane

• senses linear acceleration and gravity

How they function: tiny crystals called _____ _____ rest on the gelatinous membrane, bending hair cells as the head moves → this provides information about head tilt and linear acceleration, which is important for maintaining balance and spatial orientation

posterior, superior, lateral

1. _____ semicircle canal = right/left side bending

2. _____ semicircle canal = flexion/extension

3. _____ semicircle canal = right/left rotation

yaw, roll, pitch

1. ____ : rotation around z-axis

2. ____ : rotation around x-axis

3. ____ : rotation around y-axis

vestibular

______ system and postural stabiloity:

• Balance: adjust posture and msucle tone to counteract changes in body position and prevent falls

  • dysfunction can lead to dizziness and imbalance

• Eye-head coordination

  • dysfunction can cause nystagmus (involuntaru eye movements) and difficulty tracking moving objects

• postural control: provides info about head position which is important for maintaining upright posture + adjusts msucle tone in neck, trunk, and limbs to nmaintain balance

• Spatial orientation of head

  • dysfunction can lead to disorientation

postural orientation

• Definition: the ability to actively and intentionally position the body to achieve a specific goal

• role: ensures tha the body’s alignment in space is suitable for task, allowing for efficeint movmeent

  • ex. sprinter gets into crouching position at start line to prepare for race

  • ex. adjusting body positionwhile reaching for high shelf

feedforward

type of control strategy where actions are planned and executed in anticipation of a movement, before any sensory feedback is received

• postural orientation and anticipatory postural adjustments are __________ mechanisms

  • orientation is planned and adjusted before movement

  • APA has the nervous system predict destabilizing effects and makes pre-emptive adjustments to maintain stability (adjustments are based on learned patterns and past experiences)

postural equilibrium

• definition: the ability to maintain or restore balance while standing still or performing dynamic tasks; involves controlling the body’s center of mass (CoM) relative to it’s base of support (BoS)

• role: fundamental aspect of stability, ensuring external and internal forces do not cause loss of balance

  • Ex. static and dynamic balance

static balance

standing on one foot or maintaining balance while standing on unstable surface

dynamic balance

walking, running. or recovering from stumble, where CoM is constantly shifting but must remain controlled

closed loop

• equilibrium depends of real-time sesnory feedback from proprioceptors, vestibular system, and vision to make continuous adjustments that keep body balanced

• this is part of a _______ ____ control system

anticipatory postural adjustments (APA)

• definition: pre-emptive alterations that occur before volunatry movement to counteract the destabilizing effects of that movement

• Role: essential for maintaining stability when body is preparing for a movement that could shift its CoM during locomotions (like reaching, lifting, change of direction

  • ex. lifting heavy object → body engages muscles in legs and core the stabilize before bending down and lifting object

body alignment

posture orientaion includes maintaining proper ____ _______ during exercises

1 ft^2

equilibrium means keeping projection of CoM (weight vector) within the BoS

• typical base of support is ____

pre-programmed responses

automatic responses (not reflexive) that controls posture

• takes longer than a reflex but faster than voluntary RT

• involve more complex neural circuits

• not hard wires, but planned and can be reshaped

inverted pendulum

think of your body as residing within a cone, with point at ankles

• if your sway angle travels outside of cone you will fall

  • anterior/posterior: 12.5 degrees

  • medial/lateral: 16 degrees

postural sway

• continuous small movements or oscillations of the body CoM over its BoS during still standing

noise

postural sway is the result of _____

• maintaining balance is hard and requires energy

• some error or sound within system causes involuntary sway

  • allowing sway might be a tradeoff between energy expenditure and absolute stability

intentional, CNS

sway is _____ by the ____

• we follow a moving reference point as we sway

• continuously explore the limits of stability

increase

factors that ____ postural sway:

• age, vestibular impairment, alcohol, disease (parkinson’s or stroke), sleepiness, anesthesia (local), vibrations beneath feet, lower extremuty injury

center of mass (COM)

point that is the middle of total body mass

center of gravity (COG)

the vertical projection of center of mass

center of pressure (COP)

• the center of distribution of the total force applied to the supporting surface

• a force product of the nervous system to control the movement of CoM

• ______ ___ _________ moves around the COM

ankle, hip, stepping

movement strategies during perturbed/unsettling standing include _____ ____ and ________ strategies

ankle strategy

• restore COM to position of stability in situation where pertubations to equilibrium is small and support surface is firm

• generating torques to counteract instability resulting in small shifts in the COM

forward instability

the muscles activated in at the ankle response to ______ _______ are gastrocnemius (calf) → hamstrings → paraspinalis (muscle in back)

**distal to proximal activation helps to pull body backward to become stable

backward instability

muscle activity at the ankle in response to _____ ______ are tibialis anterior (shin) → quadriceps → and abdonimis

*distal to proximal activation helps to pull body forward to become stable

hip strategy

• controls motion of COM by producing LARGE and RAPID motion to counteract instability

• the support surface is smaller than feet (narrow beam)

abdominals, quadriceps

muscle activity in response forward instability at the hip: ______ → ______

**proximal to distal activation helps to rotate the upper body backward, bringing the COM back over the BOS

paraspinals, hamstrings

muscle activity in response to backward instability at the hip: ______ → ______

**proximal to distal activation helps to rotate the upper body forward

stepping strategy

• when the ankle and hip are insufficient to recover balance, a _____ ______ is used to realign BOS under COM