1. Stability and Balance
Equilibrium
Stability and Balance
• an object is at rest and will remain at rest
Stability
Objectives: • ability to resist a disruption of equilibrium
• ability of a body to return to its original state if
• Define the concepts of stability and balance
disturbed
• Define center of mass, center of pressure, base
• more stable = able to resist a larger force
of support
• Understand requirements for static & dynamic Balance
balance • the ability to control equilibrium
• Understand the 5 factors that influence stability
Center of Mass Center of Pressure
• Geometric point about which every particle of a • Reaction forces between the body and support
body's mass is equally distributed surface are distributed over the entire contact area
• The “average” location of the mass of a body • The force can be summed into a single net force
acting at a single point: the center of pressure
Σ x i mi x head
mhead • Center of pressure = point about which the ground
x com =
Σ mi reaction force is balanced
x arm marm
x torso Center of
• Segment mass x forearm mtorso mforearm
xcom Pressure
and center of x thigh mthigh mbody
mass locations
available in x leg
mleg
anthropometric Fground
x foot mfoot
tables Fground
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2. Computing Center of Pressure Base of Support
• Center of pressure is the point about which the • Base of support: area enclosed by all the points
net torque created by the ground reaction forces at which the body contacts a supporting surface
is zero • A person can move the center of pressure to any
• Example: point within the base of support, but cannot move
Center of Pressure it outside the base of support
toe
F1 = 100 N F2 = 50 N
5 cm 10 cm
15 cm
heel
ΣT = (0.1 m)(50 N) – (0.05 m)(100 N) = 0 during left during bipedal up on toes
feet and hand on
foot stance stance of one foot
support surface(s)
Static Balance Dynamic Balance
• To remain in equilibrium, the center of pressure • When center of mass is moving
must be directly below the body center of mass forward, must apply posterior-
• Thus, to remain in equilibrium, the body center of directed force and torque to arrest
mass must be within the boundaries of the base ω anterior motion and rotation before a
of support v loss of balance
• If moving forward faster
Backward Forward → greater decel. impulse needed
Balance Balance W → greater displacement during decel.
Loss Loss
Region W Region → COM must start off more posterior
• Greater horizontal distance from a
Fgrf boundary of the base of support
Line of Gravity
→ less likely to lose balance in that
Base of direction
support Fgrf
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3. Factors Increasing Stability Factors Increasing Stability
3. Increased base of support
1. Greater mass and moment of inertia
Requires a larger force or torque to accelerate Greater base of → Center of mass can travel
support in a farther before crossing the
2. Increased surface friction given direction boundary
Friction forces used to prevent or stop horizontal
4. Increased horizontal distance between center
motion
of mass and base of support boundary
Increased friction → Greater ability to resist / stop
horizontal motion Center of mass must travel farther before a balance
loss; provides more time for deceleration
m Fapplied Fapplied - Ffriction dist. for balance loss left
ax =
m
Ffriction ≤ µ N W dist. for balance loss backward dist. for balance loss forward
x
dist. for balance loss right
N=W
Factors Increasing Stability Dynamic Stability
5. Lower center of mass height • Many dynamic movements, including walking and
running, are unstable
Less destabilizing torque produced by a given • Walking and running essentially consist of a
angular displacement
series of interrupted falls
→ less opposing
torque needed to • Difficult to quantify stability during such tasks
restore balance W
Path of Center
of Pressure
T = d⊥ W
W
d⊥ Path of Center
of Mass
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