Stance and Stability

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Posture and Balance
Quiet Stance
Externally-Perturbed Stance
Self-Perturbed Stance

1. Hamilton, N., & Luttgens, K. 2002. Kinesiology, Scientific Basis of Human Motion, 10^thed. Boston： McGraw-Hill. Chapter 14, pp. 371-394 and Chapter 15, pp. 399-411
2. Chaffin, D.B., Andersson, G. B., Chaffin, D. (1999).  Occupational Biomechanics, 3^rd ed. New York, USA: Wiley, John & Sons.  Chapter 17
3. Hall S.J. (2003). Basic Biomechanics, 4 ^th ed. Boston, USA: McGraw-Hill. Chapter 13
4. Smith L.K., Weiss E.L., Don Lehmkuhl L. (1996). Brunnstrom's Clinical Kinesiology, 5^th ed. Philadephia, USA: FA Davis. Chapter 12, pp.401-410.

Posture and Balance

Related Terminology

posture： a term to describe the orientation of any body segment relative to the gravitational vector balance： a term to describe the dynamics of body posture to prevent falling center of mass (COM)： the point where the entire mass of the body is concentrated center of gravity (COG)： the vertical projection of the center of mass to the ground center of pressure (COP)： the point where the resultant of all ground reaction forces act centroid： the point that defines the geometric center of a body base of support (BOS)： the area underneath and between both feet

Location of center of pressure (COP)

COP parameters absolute position of the COP in the AP and ML directions excursion of the COP (COPE) safety margin = BOS - COPE - COPinitial measurement of the position of the COP single-force-platform method： measurement the COP with both feet standing on the same force plate
two-force-platform method： measurement the COP with one foot standing on one force plate and the other foot on the second force plate

Location of center of mass at erect posture

methods to estimate the COM at quiet stance

segment zone approach： weighed average of every segment of the entire body

kinetic method： double integration of shear forces from the force platform

clinical method： measurement of the PSIS (posterior superior iliac spine) level in the sagittal plane

kinematic method

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 - d_COM = ka
where k = constant
           a = linear acceleration of the COM

since		and	,
get	,
so

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 ‾COM 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

Major sensory systems involved in posture and balance

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

height of COM

relationship of COG to BOS

Pai et al., 1997： effects of velocity and position of COM on base of support

mass of body

friction

segmental alignment

psychological or mental status

muscle activities

postural muscle： the muscle that acts to prevent collapse of the skeleton

slow twitch

fatigue resistant

phasic muscle： fast muscle

physiological and pathological factors

Tasks used to study the stability of erect posture

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Quiet Stance

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 at quiet stance in young adults inverted pendulum model the trunk sways around the ankle joint like an inverted pendulum (GRF) (dCOP) = (BW) (dCOG) + Ia where GRF = ground reaction force (N)             dCOP = moment arm of the COP to the ankle             BW = body weight (N)             dCOG = moment arm of the COG to the ankle             I = moment of inertia of the total body around the ankle (kg × m2)             a = angular acceleration of the pendulum (rad/s2) assumptions BW = GRF body sway around ankle only ankle acts like a hinged joint

Postural sway strategy

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Externally-Perturbed Stance

Definition

Methods of external perturbation

Horizontal translation on a moving platform Nashner (1977)： 1st researcher to study the effect of a moving platform COM sways backwards when the platform moves forwards COM sways forwards when the platform moves backwards muscle activity pattern： bottom-up sequence of activities of the participating muscles NOTE： Actually, what he did is to measure the COP rather than the COM.

Platform tilting up and down Nashner (1982) tilting-upward both gastrocnemius and hamstring muscles are stretched COM sways backwards and then forwards to regain balance titling downward stretched muscles? COM sways forwards and then backwards to regain balance

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Self-Perturbed Stance

Definition

Methods of self perturbation

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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