Movement Science Week 8 - Postural Control and Balance Emerging Technologies of Movement Analysis (Transcripts)

Postural Control

• Balance depends on individual, task, and environment interaction
• Balance control types:
  • Steady state: Stability in predictable conditions (sitting, standing, slow walking)
  • Reactive: Response to sudden changes (tripping, getting bumped)
  • Proactive: Anticipatory adjustments (stepping onto a step, lifting a heavy object)
• Tasks vary in orientation and stability demands
  • Goalie: Oriented towards the ball, potentially losing stability
  • Tightrope walker: Center of mass always within base of support
  • Reading on a bench: Different posture and orientation from the previous examples
• Postural control is complex, involving multiple systems:
  • Musculoskeletal: Joints, spine, muscles, range of motion, flexibility, muscle properties
  • Neuromuscular: Motor processes, muscle synergies
  • Sensory: Vision, vestibular system, proprioception
  • Cognitive: Anticipatory mechanisms, adaptive mechanisms, body awareness (proprioception)
• Feedback: Body reacts to disturbances
• Feedforward: Body anticipates actions
• Center of mass (CoM): Weighted average of body segment masses
• Center of gravity (CoG): Point for perfect balance, changes with movement and carrying objects
• Base of support: Body area in contact with the supporting surface
• Postural stability: CoM control relative to base of support
• Stability limits: Points where the base of support needs to be adjusted for stability
• Center of pressure (CoP): Distribution of force on the supporting surface (often at the feet in standing)
• CoP moves around CoM, keeping CoM within base of support

Types of Postural Control

• Steady state balance: Maintaining CoM within base of support in static conditions
  • Ideal alignment minimizes energy expenditure
  • Small postural sway occurs naturally within the base of support
  • Muscle tone and postural tone contribute to steady-state balance
  • Muscle tone: Inherent stiffness of muscles, influenced by neural contributions
  • Postural tone: Muscle activation against gravity in quiet stance
    • Requires active sensory processing
  • Muscles active in quiet stance:
    • Soleus and gastrocnemius
    • Tibialis anterior
    • Gluteus medius and tensor fasciae latae
    • Iliopsoas
    • Thoracic extensors
    • Intermittent abdominals
• Ankle strategy: Leg and trunk segments move in phase, sway focused around ankle
• Hip strategy: Leg and trunk segments move out of phase, sway at hip, higher sway frequencies
• Sensory systems provide information about body position and motion:
  • Vision: Provides position and motion of head in the environment, reference for uprightness and self-motion
  • Somatosensory: Provides position and motion relative to supporting surfaces
    • Less helpful on unstable surfaces (boats, ramps)
  • Vestibular: Provides position and motion of the head relative to gravity
    • Cannot always distinguish head movement relative to trunk from forward bend
• Sensory reweighting: Central nervous system modifies importance of senses based on environment and accuracy
  • When one sense is inaccurate, the CNS relies more on other senses to maintain balance
  • Sensory reweighting occurs with learning new motor skills
    • Visual input is important initially, decreasing as the skill becomes automatic

Reactive Balance Control

• Response to unexpected perturbations
• Muscle synergies: Muscle groups working together to simplify CNS demands
• Fixed support strategies: Ankle and hip strategies, used for smaller perturbations
• Change in support strategies: Taking a step or reaching to regain balance
• Ankle strategy:
  • Used for anterior/posterior perturbations
  • Requires good ankle range of motion and strength
  • Muscles involved:
    • Tibialis anterior
    • Gastrocnemius
    • Quadriceps
    • Hamstrings
    • Abdominal muscles
    • Paraspinal muscles
• Hip strategy:
  • Used for larger perturbations
  • Higher sway frequencies
  • Muscles involved:
    • More proximal muscles (hip flexors and extensors)
    • Trunk muscles

Hip Strategy

• Hip strategy controls center of mass with quick movements at hip joints, with ankle rotations happening in opposite directions.
• Used during unstable support surfaces, like standing on a beam.
• With forward sway, the quadriceps and abdominal muscles activate more.
• With backward sway, the hamstrings and paraspinal muscles activate more.
• Change in support balance reactions include stepping and reaching, both occurring in half the time of a similar voluntary movement.
• Voluntary limits can be pre-planned, while change in support strategies must be programmed due to unpredictable body movements.
• Step strategy involves stepping and realigning the base of support under the center of mass.
• Reach and grasp extends the base of support by using the arms.

Lateral Pelvic Movement

• Medial, lateral or side to side stability issues result in lateral movements of the pelvis.
• One leg abducts, the other abducts.
• Hip joint is used for recovery, as the ankle and knee have limited medial to lateral movement.

Multi-Directional Instability

• Rectus Femoris and Tensor Fascia Lata are most active during lateral perturbations.
• Other leg and trunk muscles are most active during diagonal perturbations.

Seated Balance

• Balance recovery in a seated position is similar to standing.
• Muscle activation changes depending on the direction of imbalance.
• Muscle responses are quick, with coordinated muscle synergies and minimal antagonistic muscle activity.
• With feet dangling in seated position and anterior-posterior perturbations, the trunk muscles are more active.
• With feet planted in seated position and anterior-posterior perturbations, the leg muscles are more active.
• Multi-directional perturbations see muscle activity influenced by the direction of the instability.

Postural Muscle Synergies and Adaptability

• Postural muscle synergies are responsive and change in fine tune to task and environmental demands.
• Studies show individuals without nerve pathology can adapt their postural movements.
• As platform movement velocity increases, ankle strategy is replaced with hip strategy.
• Further velocity increases transition to change and support strategies.
• Even if task demands remain constant, individuals adapt responses to repeated perturbations by reducing muscle activity amplitude.
• Anticipation of the size or amplitude of imbalance influences postural adjustments.

Clinical Application

• Practice should include predictable and unpredictable perturbations, varying in size and direction.
• This helps avoid habituating postural responses and promotes broader reactivity.

Sensory Information and Reactive Balance

• Visual information helps assess step space and reaching objects.
• Somatosensory information helps control body sway and provides information regarding support surface changes.
• Vestibular system plays a minor role in recovery on unstable surfaces because it doesn't offer reliable information.

Proactive or Anticipatory Balance Control

• Proactive or anticipatory balance control adapts to different tasks and environments.
• Central nervous system uses anticipatory processes to control movement.
• Central nervous system forms representations and pre-tunes systems based on past experiences, anticipating tasks.
• Mistakes demonstrate that the central nervous system utilizes anticipatory processes in movement control.

Preparatory Phase

• Postural muscles pre-activate before prime mover muscles to compensate before a planned movement.
• Experiment showed that left muscles activated prior to right leg lifting, anticipating stability maintenance while the right leg was lifting.

Compensatory Phase

• Postural muscles activate after prime movers in a feedback manner to stabilize the body.

Factors Influencing Anticipatory Muscle Activity

• Sitting decreases anticipatory postural activity due to increased support.
• Increased task load increases anticipatory muscle activity.

Cognitive Systems and Balance Control

• Most daily activities involve multitasking, like walking and talking.
• Postural control typically occurs automatically, leading to the assumption of minimal cognitive or attentional resource need.

Attentional Resources

• Attentional resources are information processing resources required for task completion.
• Dual-task interference occurs when competition for attentional resources reduces performance on one or more tasks.

Dual-Task Research

• Dual-task research shows significant attentional demands for postural control.
• Attentional needs vary based on the task, individual age, and balance abilities.
• Memory tasks while balancing were found to be very attentionally demanding for young adults.

Influence of Sensory Context

• Decreasing sensory inputs for postural control, like standing with eyes closed, dim environments, or uneven surfaces, increases attentional demands.

Performance of Secondary Tasks

• Secondary task performance is not always detrimental to balance.
• Fixating on a visual target and performing a secondary task like counting letters resulted in less sway compared to inspecting a blank target.
• Focusing on a near target reduces sway compared to focusing on a distant target.
• Postural control can be enhanced by secondary tasks, implying an integrated perception and action system.

Postural Prioritization

• Under dual-task conditions, balance maintenance prioritization depends on the stability threat.
• With significant stability threats, young adults prioritize postural control over other tasks, known as postural prioritization or "posture first" strategy.

Clinical Implications of Dual-Task Performance

• Assess balance under both single and dual-task conditions.
• Treatment should focus on improving postural stability under both single and dual-task conditions.

Aging and System Changes

• Aging process is heterogeneous, meaning individuals of the same chronological age can have vastly different physical functions.
• Factors contribute to health and longevity.

Continuum of Function among Older Adults

• Physically elite older adults participate in competitive sports and experience optimal aging.
• Physically fit older adults engage in sports, games, and hobbies, capable of moderate physical work.
• Physically independent older adults are active but engage in less demanding activities.
• They're independent in basic activities of daily living (BADLs) and instrumental activities of daily living (IADLs).
• Physically frail older adults are independent in BADLs, but may require assistance with IADLs.
• Physically dependent and disabled older adults may require assistance with BADLs and are usually dependent for all IADLs.

Age-Related Neuromuscular Changes

• Muscle strength declines with age, reducing lower extremity strength by up to 40% between ages 30 and 80.
• Strength gains have been observed in some individuals, highlighting aging heterogeneity.
• Decline in muscle strength is more severe in nursing home residents with a history of falls.
• Muscle endurance also decreases with age, but is more preserved than strength.

Muscle Changes

• Muscular aging involves decreased muscle mass, more pronounced in lower extremities.
• Muscle cell death is replaced with connective tissue and fat tissue.
• There's age-related loss of both type one and type two muscle fibers, although previously thought to be primarily type two fiber loss.
• Many muscle fibers instead transform into blended types with characteristics of both.

Motor Unit and Neuromuscular Junction Changes

• Number of motor units decreases with age, affecting both large and small myelinated fibers.
• Age-related changes occur in the neuromuscular junction.

Functional Consequences of Muscle Changes

• Maximum isometric force decreases.
• Muscle fatigue occurs more quickly.
• Rate of tension development slows down.
• Concentric contractions are more affected by aging than eccentric contractions.
• Rapid contractions are impacted more than slow contractions.

Other Age-Related Changes

• Decreases in range of motion and spinal flexibility can lead to flexed or stooped postures, shifting the center of mass.
• Somatosensation or tactile sensitivity decreases.

Age-related changes in walking mechanics

• Somatosensation: Older adults with peripheral neuropathy have decreased sensation.
• Vision: Age-related decline in vision affects gait performance, particularly obstacle negotiation.
• Vestibular Function: Decreased vestibular response impacts walking speed, especially on narrow walkways.
• Cognitive Control: Normal aging requires increased cognitive control for sensory and motor functions.
• Fear of Falling: Repeated falls can lead to a fear of falling, which causes slower walking speed and higher anxiety levels.

The ICF model for movement analysis

• Body Functions and Structure: Addresses physiological functions like sensation, balance, cognition, and anatomical parts like limbs, eyes, and nerves.
• Activity: Focuses on tasks individuals perform, like walking or reaching, and limitations in executing those tasks.
• Participation: Concerns involvement in life situations, like employment or social activities. It also considers limitations due to disability, environmental factors, or personal perceptions.
• Environmental Factors: Physical, social, and attitudinal environment, including base of support, surface, obstacles, and ramps/stairs.
• Personal Factors: Internal aspects unique to each person, including injury history, pain, mobility, and fatigue.
• Mobility and Gait: The ability to change and maintain body position, handle objects, walk, and move within the environment.
• Upper Extremity Function: Encompasses reaching, grasping, and manipulation, crucial for self-care and domestic activities.
• Contextual Factors: Impact mobility by influencing gait patterns and upper extremity use, including environmental factors such as terrain and personal factors such as age and sex.

Motion capture technology and its applications

• Motion Capture: Used to record movement, often in gait analysis, sports therapy, neuroscience, and research.
• Reflective Marker Systems: Place markers on specific body parts, tracked by cameras, to analyze movement patterns.
• Force Plates: Measure ground reaction forces during movement, providing information on stability limitations and center of mass.
• Surface EMG: Measures muscle activity through electrodes placed on the skin, providing insight into muscle activity timing, intensity, and coordination.
• Force Plates: Measure the forces exerted on the ground, used for analyzing gait, running, jumping, and other activities.
• Pressure-Sensitive Walkways: Provide temporal and spatial parameters of walking, detecting gait abnormalities.
• Accelerometers: Measure acceleration, quantifying gait characteristics.
• IMU (Inertial Measurement Unit): Measures specific gravity and angular rates through gyroscopes and accelerometers.
• Smartwatches: Collect data on walking speed, step length, asymmetry, and stair speed, offering insights into real-world mobility.

Test your knowledge on the mechanisms of postural control and balance. This quiz covers topics like muscle tone, the role of the central nervous system, and various strategies used to maintain balance. It is essential for understanding the principles of human movement and stability.