University of Scranton - Department of Physical Therapy
Edmund M. Kosmahl, PT, EdD
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GRAVITY AND EQUILIBRIUM

Objectives:
  1. Define gravity.
  2. Explain the interaction of center of gravity, base of support, and gravity line.
  3. Explain the factors that promote stability or mobility.
  4. Explain how the principles of stability and mobility can be applied to lifting, carrying, moving, exercise, gait training.
  5. Explain the factors that lead to poor body mechanics.
  • GRAVITY
    1. FORCE of attraction one mass exerts on another
    2. Larger mass attracts smaller
    3. F = m x a, W = m x g
    4. Acceleration of earth's gravity = 32 ft/sec2
  • CENTER OF GRAVITY (COG)
    1. Point about which a body's mass is equally distributed
    2. "Action point" of a body's weight
    3. Anterior to S2 for standing human
    4. Changes when body segments move
    5. Described for each segment (e.g. HAT anterior to T11)
  • BASE OF SUPPORT (BOS)
    1. Those points of the body contacting the supporting surface and the area in between
  • GRAVITY LINE
    1. Vertical projection from the center of gravity
    see center of gravity and gravity line picture (96K)
     
  • STABILITY
    • see low center of gravity picture (94K)
     
  • MOBILITY (UNSTABLE)
    1. High center of gravity
    2. Gravity line away from center of base of support
    3. Narrow base of support
    4. Small mass (light weight)
    see wide base of support picture (96K)
     
  • LIFTING, CARRYING, MOVING
    1. Keep object COG close to yours
    see carrying picture (98K)
     
  • STANDING FROM CHAIR
    • see standing from chair picture ( 120 K)
     
  • EXERCISE
    1. Move limbs and weight away from COG to increase resistance
    2. Keep limbs and weight close to COG to decrease resistance
  • GAIT TRAINING
    1. Sensory deprived limb (hemiplegia) encourage even cadence to keep gravity line near center of BOS
    2. Assistive devices encourage correct placement, sequence, and force application to keep gravity line near center of BOS
  • POOR BODY MECHANICS
    1. Working with segments and weights far from COG
    2. Requires increased muscle tension forces
    3. Leads to fatigue, strain, sprain, articular damage, etc.
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