1

Stability and Balance

Objectives:

• Define the concepts of stability and balance

• Define center of mass, center of pressure, base of support

• Understand requirements for static & dynamic balance

• Understand the 5 factors that influence stability

Stability and BalanceEquilibrium• an object is at rest and will remain at rest

Stability• ability to resist a disruption of equilibrium

• ability of a body to return to its original state if disturbed

• more stable = able to resist a larger force

Balance • the ability to control equilibrium

Center of Mass• Geometric point about which every particle of a

body's mass is equally distributed

• The “average” location of the mass of a body

xcom =Σ xi mi

Σ mi

mbody

xcom

marm

mhead

mforearm

mthigh

mleg

mfoot

mtorso

xhead

xarmxtorso

xforearm

xthigh

x leg

xfoot

• Segment mass and center of mass locations available in anthropometrictables

Center of Pressure• Reaction forces between the body and support

surface are distributed over the entire contact area

• The force can be summed into a single net force acting at a single point: the center of pressure

• Center of pressure = point about which the ground reaction force is balanced

Fground

Fground

Center of Pressure

2

Computing Center of Pressure• Center of pressure is the point about which the

net torque created by the ground reaction forces is zero

• Example:

F1 = 100 N F2 = 50 N

15 cm

5 cm 10 cm

ΣT = (0.1 m)(50 N) – (0.05 m)(100 N) = 0

Center of Pressure

Base of Support• Base of support: area enclosed by all the points

at which the body contacts a supporting surface

• A person can move the center of pressure to any point within the base of support, but cannot move it outside the base of support

during left foot stance

toe

heel

during bipedal stance feet and hand on

support surface(s)

up on toes of one foot

Static Balance• To remain in equilibrium, the center of pressure

must be directly below the body center of mass

• Thus, to remain in equilibrium, the body center of mass must be within the boundaries of the base of support

Base of support Fgrf

W

Line of Gravity

Backward Balance

Loss Region

Forward Balance

Loss Region

Dynamic Balance• When center of mass is moving

forward, must apply posterior-directed force and torque to arrest anterior motion and rotation before a loss of balance

• If moving forward faster →→ greater decel. impulse needed→→ greater displacement during decel.→→ COM must start off more posterior

• Greater horizontal distance from a boundary of the base of support →→ less likely to lose balance in that direction

Fgrf

W

vω

3

Factors Increasing Stability1. Greater mass and moment of inertia

Requires a larger force or torque to accelerate

2. Increased surface frictionFriction forces used to prevent or stop horizontal motionIncreased friction →→ Greater ability to resist / stop

horizontal motion

Ffriction ≤ µ N

N = W

W

m Fapplied

x

ax =Fapplied - Ffriction

m

Factors Increasing Stability3. Increased base of support

Greater base of support in a given direction

Center of mass can travel farther before crossing the boundary

→→

4. Increased horizontal distance between center of mass and base of support boundaryCenter of mass must travel farther before a balance loss; provides more time for deceleration

dist. for balance loss forward dist. for balance loss backward

dist. for balance loss right

dist. for balance loss left

Factors Increasing Stability

5. Lower center of mass height

Less destabilizing torque produced by a given angular displacement

W

W T = d⊥ W

d⊥

less opposing torque needed to restore balance

→→

Dynamic Stability• Many dynamic movements, including walking and

running, are unstable

• Walking and running essentially consist of a series of interrupted falls

• Difficult to quantify stability during such tasks

Path of Center of Mass

Path of Center of Pressure