Balance and Postural Stability

Balance and Postural Stability

Introduction

• Balance and postural stability involve a complex interaction of various body systems:
  • Cardiovascular system
  • Neuromuscular system
  • Musculoskeletal system
  • Environment
  • Cognitive factors (fear, anxiety, attention, executive functioning)

Key Concepts

• Balance and postural stability: Maintaining the center of mass over the base of support.
  • Center of mass: The point of perfect weight equilibrium; the center of the body's mass.
  • Base of support: The contact area between the body and the supporting surface.
  • Center of gravity: The vertical projection of the center of mass to the ground.
• Larger base of support increases stability because the center of mass can move a greater distance within that base.
• Limit of stability: The point at which a person must move their base of support to avoid losing balance.

Neuromuscular and Musculoskeletal Contributors

• Key systems for sensing movement and position in space:
  • Visual system: Indicates where the head is in space; compensates for inadequate vestibular or proprioceptive input.
  • Vestibular system: Provides information about the position and movement of the head in relation to gravity.
  • Somatosensory (proprioceptive) system: Provides information about joint position sense (flexion, extension, movement).
    • Muscle spindles, not joint receptors, are primarily responsible for joint position sense.

Balance Strategies for Postural Sway

• Body employs different strategies to manage postural sway:
  • Ankle strategy: Small body motions at the ankle to keep the center of mass over the base of support; used for small anterior or posterior perturbations.
    • Anterior perturbation: Posterior muscles contract in a distal-to-proximal sequence (gastrocnemius, hamstrings, paraspinal muscles).
    • Posterior perturbation: Anterior muscles contract in a distal-to-proximal sequence.
  • Hip strategy: Used for large or rapid perturbations that push the body to its limits of stability.
    • Anterior force: Hips rapidly flex.
    • Muscle activation is proximal-to-distal.
  • Stepping strategy: Used when a large force displaces the center of mass beyond the limits of stability, requiring a step to regain balance.
  • Suspension strategy: Quick lowering of the center of mass by flexing the knees, causing hip flexion and ankle dorsiflexion (e.g., surfing).
  • Weight shift strategy: Used when a perturbation comes from the side; the body steps to the opposite side to maintain the center of mass over the base of support.

Perturbation

• Perturbation: A deviation from a regular or normal position caused by an outside influence.
  • Examples: Being pushed, a bus moving suddenly, rapid muscle contraction.
• AP: Shorthand for anterior-posterior, referring to forward-backward or sagittal plane motion.

Practical Applications: Balance During Lifting

• When lifting an object in front of the body:
  • The body leans forward, displacing the center of mass forward.
  • Grasping the load further disrupts the center of mass.
• The central nervous system estimates the momentum needed to lift the load based on previous experience with similar objects (size, shape, weight).
  • Generates a backwards horizontal linear momentum to keep the center of mass over the base of support.
• Loss of balance occurs when the central nervous system's estimate is incorrect (load is lighter or heavier than expected).

Balance Interventions

• Key principles for initiating balance interventions:
  1. Identify the activity or movement where the balance deficit is present.
  2. Identify the structure or system contributing to the deficit.
  3. Start at a level that is challenging but achievable for the patient.
• Example: For a stroke patient with sitting balance issues, start with basic sitting interventions (varying base of support, sensory inputs) instead of advanced exercises.

Key Takeaways

• Identify the systems contributing to balance and postural stability (visual, vestibular, proprioceptive, etc.).
• Define key terms: center of mass, center of gravity, base of support, limits of stability.
• Describe balance strategies used when upright.
• Understand balance maintenance during lifting.
• List key principles for choosing balance interventions.