Stance and Stability

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COM, COG, and COP
Stability and Equilibrium
Stability at Quiet Stance
Stability at Externally-Perturbed Stance
Stability at Self-Perturbed Stance

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  1. Hamilton, N., & Luttgens, K. 2002. Kinesiology, Scientific Basis of Human Motion, 10thed. Boston: McGraw-Hill. Chapter 14, pp. 371-394 and Chapter 15, pp. 399-411
  2. Chaffin & Andersson, 1999¡G Chapter 17
  3. Hall, 2003¡GChapter 13

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Objectives: After studying this topic, the students will be able to

  • identify the center of mass, center of gravity, and center of pressure of human body and distinguish their differences
  • describe the methods to measure limit of stability and the factors that affect stability and equilibrium
  • explain the changes in center of mass and center of pressure at quiet stance and during different perturbed tasks

    • COM, COG, and COP

    Posture and Balance

  • posture¡G a term to describe the orientation of any body segment relative to the gravitational vector
  • balance¡G a term to describe the dynamics of body posture to prevent falling

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    Definition of Center of Mass (COM)

  • the point where all the mass of a body is concentrated
  • the point about which a body would balance without a tendency to rotate
  • All the linear forces acting on the body is balanced, i.e. SF = 0
  • All the rotary forces acting on the body is balanced, i.e. SM = 0

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    Location of Center of Mass

  • its precise location depending on
  • individual's anatomical structure
  • habitual standing posture
  • current position
  • external support
  • NOTE¡G Location of COM remains fixed as long as the body does NOT change the shape
  • location in human body
  • generally accepted that it is located at
  • ~57% of standing height in males
  • ~ 55% of standing height in females
  • varies with body build, posture, age, and gender
  • infant > child > adult (in % of body height from the floor)
  • methods to estimate the COM of an object
  • suspension method¡G to suspend an irregular-shaped object by a string and let it hang until it ceases to move



  • segment modeling method¡G weighed average of every segment of the entire body
  • kinetic method¡G double integration of shear forces from the force platform
  • clinical method¡G measurement of the PSIS (posterior superior iliac spine) level in the sagittal plane
  • methods to locate the COM of one segment


  • COM parameters
  • absolute position of the COM in the AP and ML positions
  • excursion of the COM
  • linear acceleration of the COM equals to the difference between the COP and COM
    COP - dCOM = ka
    where k = constant
               a = linear acceleration of the COM
    since   (GRF) (COP) - (BW) (dCOM) = Ia    and , , so

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    Center of Pressure (COP)

  • the point where the resultant of all ground reaction forces act
  • COP parameters
  • absolute position of the COP in the AP and ML directions
  • excursion of the COP
  • safety margin
  • measurement of the position of the COP
  • single-force-platform method
           
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  • two-force-platform method¡G measurement the COP with one foot standing on one force plate and the other foot on the second force plate

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    Definition of Centroid and COG

  • centroid
  • the point that defines the geometric center of a body
  • If the material composing a body is homogeneous, the weight can be neglected, i.e. centroid = COM
  • Note: human body is not homogeneous
  • center of gravity (COG)
  • the vertical projection of the center of mass to the ground

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    Stability and Equilibrium

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    Classification of Equilibrium

  • stable equilibrium
  • occurs when an object is placed in such a position that any disturbance effort would raise its COM
  • tend to fall back its original position
  • e.g. ­BOS or ¡ÂCOM
  • unstable equilibrium
  • occurs when an object is placed in such a position that any disturbance effort would lower its COM
  • tend to fall into a more stable position
  • neutral equilibrium
  • occurs when an object is placed in such a position that any disturbance effort would not change the level of its COM
  • tend to fall into a more stable position

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    Factors Affecting Stability

  • size and shape of base of support (BOS)
  • wide-base stance
  • tandem stance: standing with one foot ahead the other
  • stance with crutches


  • Pai et al., 1997¡G effects of velocity and position of COM on bas of support
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  • height of COM
  • relationship of COG to BOS
  • mass of body
  • friction
  • segmental alignment
  • sensory input
  • visual
  • vestibular system
  • proprioception
  • other somatosensory system
  • psychological or mental status
  • muscle activities
  • postural muscle¡G the muscle that acts to prevent collapse of the skeleton
  • slow twitch
  • fatigue resistant
  • phasic muscle¡G fast muscle
  • physiological and pathological factors

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    Tasks Used to Study the Stability of Erect Posture

  • quiet stance¡G to maintain static stability
  • externally-perturbed stance¡G to regain dynamic stability
  • self-perturbed stance¡G to maintain dynamic stability

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    Stability at Quiet Stance

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    Postural Sway

  • the body sways back and forth like an inverted pendulum, pivoting about the ankle, at quiet stance
  • AP sway (anteroposterior sway)
  • sway in the sagittal plane
  • ~ 5-7 mm at quiet stance in young adults
  • ML sway (mediolateral sway)
  • sway in the frontal plane
  • ~ 3-4 mm during quiet stance in young adults
  • inverted pendulum model
  • the trunk sways around the ankle joint like an inverted pendulum
  • (GRF) (dCOP) = (BW) (dCOG) + Ia
  • assumptions
    1. BW = GRF
    2. body sway around ankle only
    3. ankle acts as a hinge joint
  • relationship of COG and COP during quiet stance
  • In the case if the COP ahead the COG (see the sketch below), a counter-clockwise moment (Ia) is present at the ankle joint, resulting in backward rotation of the trunk and the balance is regained.
  • In the case if the COP behind the COG, a clockwise moment is present at the ankle joint, resulting in forward rotation of the trunk and the balance may be lost and possibly fall forward.

  • postureal sway strategy
  • the timing and amplitude of the coordinated motor patterns at many joints in order to adjust (reactive or proactive) posture and balance
  • ankle strategy vs. hip strategy
  • factors affecting postureal sway strategy
  • age¡G highly correlated to falls in the elderly
  • fatigue
  • injury
  • bracing
  • obesity
  • stability of the external environment

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    Stability at Externally-Perturbed Stance

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  • dynamic balance¡G the ability that the body regains balance at the moment of giving any externally-perturbed situation
  • methods of external perturbation
  • changes in direction of perturbation by standing on a moving platform
  • horizontal translation
  • sagittal plane translation
  • changes in surrounding environment
  • horizontal translation on a moving platform
  • Nashner (1977)¡G first researcher to study the effect of a moving platform
  • COM sways backwards when the platform moves backwards
  • NOTE¡G Actually, what he did is to measure the COP rather than the COM.
  • bottom-up sequence of activities of the participating muscles

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    Stability at self-Perturbed Stance

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  • dynamic balance¡G the ability that the body maintains balance during a functional task
  • methods of self perturbation
  • stance with external support, e.g. using crutches or using canes
  • change in base of support, e.g. wide-base stance, tandem stance, or one-leg stance
  • moving one of body parts, e.g. fast arm raise, reach, or leaning
  • closing eyes
  • relationship of COG and COP during forward reach movement
  • CNS regulates COG by controlling the net ankle moment that is expressed by COP (Fung and Winter, 1996)

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    Established on 03/19/2002 and Last Updated 05/20/2003 by ®ã´f±Ó
    © 2003 Huei-Ming Chai, PT, PHD at School of Physical Therapy, National Taiwan University, Taipei
    All Right Reserved