Musculoskeletal System and Postural Control

Postural Control Mechanisms

• There are different postural control mechanisms for different parts of the spine:
  • Hip: ligaments vs extension gravitational moment
  • Lumbar: ligaments vs extension gravitational moment
  • Thoracic: ligaments + extensors vs flexion gravitational moment
  • Cervical: ligaments vs extension gravitational moment

Muscle Torque

• Muscle torque has continuous variations to control balance (sway)
• It increases to maintain postural control in case of external perturbances

Identifying Muscle Groups Activation

• To identify muscle groups activation, consider:
  • Base of Support
  • Line of gravity
  • Gravitational moment: opposing muscle activations
  • Imagine what movement would occur if all muscle groups around a specific joint suddenly relax

Postural Control Mechanisms

• Sensorimotor control under load perturbances involves:
  • Feedback control: stimulation of a corrective response after sensory detection
  • Feedforward control: CNS predicts the effect of a voluntary movement and plans a sequence of anticipatory muscle activity

Balance and Posture

• Center of Mass/Gravity (COG) location changes constantly and is anterior to the S2 vertebra in standing position
• Line of Gravity is important for balance
• Base of Support is important for balance
• Centre of Pressure is important for balance

Balance

• Balance is the ability to maintain the line of gravity within the base of support with minimal postural sway
• Equilibrium is when all acting forces are cancelled by each other, resulting in a stable balanced system
• Balance improves with:
  • Wider base of Support
  • Lower center of gravity
  • Longer distance between the line of gravity and the border of the base of Support

Posture

• Posture is the attitude assumed by the body either with support during the course of muscular activity or as a result of the coordinated action performed by a group of muscles working to maintain stability
• There are two types of posture:
  • Dynamic posture
  • Static posture

Standing Posture

• Standing posture involves a small base of Support and a high center of mass
• Ideal posture involves the best alignment possible, lowest muscle activity, minimal energy consumption, and high variability based on anatomy
• Poor posture leads to postural stress

External and Internal Forces

• External forces in standing posture include gravity and ground reaction forces
• Internal forces in standing posture include:
  • Muscle activity
  • Tensión of noncontractile structures (ligaments, capsule, bones)

Line of Gravity

• The line of gravity should be on or close to joints axis to distribute compressive forces and reduce passive tensión or muscle activation
• The plumb line is an external representation of the line of gravity and passes through:
  • Mastoid process
  • Cervical vertebrae
  • Shoulder
  • Lumbar vertebrae
  • Greater trochanter
  • Anterior to the knee
  • Anterior to the ankle

Postural Control

• Postural control is a complex sensorimotor skill involving:
  • Musculoskeletal system
  • Somatosensory system (vision, vestibular, proprioception, cutaneus or tactile)
  • Central nervous system integration and processing (afferent and efferent)

Postural Sway

• Postural sway is the unconscious, small movements that happen around the body's center of gravity in order to maintain balance
• Continuous muscle activation with a stabilizer role is seen in:
  • Ankle
  • Knee
  • Hip
  • Spine