Effects of Gravity on Human Movement

Overview

• Gravity helps keep humans grounded, but also poses challenges through the gravitational pull.
• Infants learn movement by overcoming gravity, starting from lifting their heads to standing up and walking.

Key Concepts to Understand Gravity's Influence

Center of Gravity (CG)

• Definition: Balance point where the three movement planes intersect.
• Location: Generally around the navel (belly button) when standing upright.
• Line of Gravity: A line passing through the CG, indicating the gravitational pull's direction and strength.
• Body Mass Influence: CG is affected by body mass distribution.
  • Example: A person with strong lower extremities will have a lower CG, whereas someone with a well-built upper body will have a higher CG.

Center of Gravity Variations

• Changes with body position and movement.
  • Raising arms: CG moves higher.
  • Bending over: CG moves forward and downward, possibly outside the body.
• Reaching for an object: CG shifts from symmetric to the weight-bearing foot.
• Wheelchair users: CG moves due to leg position, requiring wheelchair design considerations for stability.
• Infants: High CG due to heavier head, leading to instability.

Importance of Understanding CG

• Stability: Knowledge of CG aids in understanding a person's stability.
  • Example: Difficulties faced when maneuvering a wheelchair over curves.
• Base of Support: The area of contact with the ground, critical for stability.
  • Standing normally: Base of feet defines the support area.
  • Changing posture: Modifies base of support and affects stability.

Combining CG and Base of Support

• A wider base of support increases stability.
• A higher CG reduces stability.
• Example: A book is stable when CG is within its base of support; adding weight or reducing the base can cause it to fall.
• Human posture and movement similarly affect stability; e.g., standing on one foot can destabilize CG.

Factors Influencing Balance and Stability

• Central Nervous System (CNS): Processes information and triggers movement.
• Musculoskeletal System: Requires proper motion range, muscle strength, tone, and endurance.
• Sensation: Key for accurate judgment and maintaining balance.

Special Considerations

• Stroke Patients: Increased fall risk due to weakness and impaired sensation on one side.
• Amputees: Higher CG leading to increased instability.

Practical Application

• These concepts of CG and base of support are crucial for training patients on safety and stability.