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# Stability and Balance

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

Equilibrium • 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 x com =

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
Center of Pressure mbody

Σ x i mi Σ mi

• Segment mass and center of mass locations available in anthropometric tables

x arm x torso x forearm x thigh x leg x foot

marm mtorso mthigh mleg mfoot mforearm

xcom

Fground Fground

1

Computing Center of Pressure
• Center of pressure is the point about which the net torque created by the ground reaction forces is zero • Example:
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
toe

F1 = 100 N

5 cm

10 cm 15 cm

F2 = 50 N
heel

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

during left foot stance

during bipedal stance

up on toes of one foot

feet and hand on support surface(s)

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
Backward Balance Loss Region Forward Balance Loss Region
Line of Gravity Base of support

Dynamic Balance
• When center of mass is moving forward, must apply posteriordirected 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

ω v

W

W

Fgrf

Fgrf

2

Factors Increasing Stability
1. Greater mass and moment of inertia Requires a larger force or torque to accelerate 2. Increased surface friction Friction forces used to prevent or stop horizontal motion Increased friction → Greater ability to resist / stop horizontal motion m Ffriction ≤ µ N W x N=W Fapplied ax = Fapplied - Ffriction m

Factors Increasing Stability
3. Increased base of support Greater base of → Center of mass can travel support in a farther before crossing the given direction boundary 4. Increased horizontal distance between center of mass and base of support boundary Center of mass must travel farther before a balance loss; provides more time for deceleration
dist. for balance loss left dist. for balance loss backward dist. for balance loss right dist. for balance loss forward

Factors Increasing Stability
5. Lower center of mass height Less destabilizing torque produced by a given angular displacement → less opposing torque needed to W 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 Pressure

W

T = d⊥ W d⊥
Path of Center of Mass

3
The text passage is an educational excerpt that defines and explains concepts related to stability and balance, including center of mass, center of pressure, base of support, and factors that influence stability. It does not contain any explicit mathematical questions or answers.
no_question
Given a text passage, determine:
    1. If it contains a mathematical question (including word problems or calculations)
    2. If an answer to that mathematical question is provided in the text

First, describe the content structure you found in 1-2 sentences.

Then provide your analysis in the following format:
TYPE: <one of the following values>
- NO_QUESTION
- SINGLE_QUESTION
- SINGLE_QUESTION_WITH_ANSWER
- MULTIPLE_QUESTIONS
- MULTIPLE_QUESTIONS_WITH_ANSWERS
system
You are on page 1of 3

# Stability and Balance

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

Equilibrium • 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 x com =

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
Center of Pressure mbody

Σ x i mi Σ mi

• Segment mass and center of mass locations available in anthropometric tables

x arm x torso x forearm x thigh x leg x foot

marm mtorso mthigh mleg mfoot mforearm

xcom

Fground Fground

1

Computing Center of Pressure
• Center of pressure is the point about which the net torque created by the ground reaction forces is zero • Example:
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
toe

F1 = 100 N

5 cm

10 cm 15 cm

F2 = 50 N
heel

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

during left foot stance

during bipedal stance

up on toes of one foot

feet and hand on support surface(s)

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
Backward Balance Loss Region Forward Balance Loss Region
Line of Gravity Base of support

Dynamic Balance
• When center of mass is moving forward, must apply posteriordirected 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

ω v

W

W

Fgrf

Fgrf

2

Factors Increasing Stability
1. Greater mass and moment of inertia Requires a larger force or torque to accelerate 2. Increased surface friction Friction forces used to prevent or stop horizontal motion Increased friction → Greater ability to resist / stop horizontal motion m Ffriction ≤ µ N W x N=W Fapplied ax = Fapplied - Ffriction m

Factors Increasing Stability
3. Increased base of support Greater base of → Center of mass can travel support in a farther before crossing the given direction boundary 4. Increased horizontal distance between center of mass and base of support boundary Center of mass must travel farther before a balance loss; provides more time for deceleration
dist. for balance loss left dist. for balance loss backward dist. for balance loss right dist. for balance loss forward

Factors Increasing Stability
5. Lower center of mass height Less destabilizing torque produced by a given angular displacement → less opposing torque needed to W 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 Pressure

W

T = d⊥ W d⊥
Path of Center of Mass

3
user
The text passage is an educational excerpt that defines and explains concepts related to stability and balance, including center of mass, center of pressure, base of support, and factors that influence stability. It does not contain any explicit mathematical questions or answers.

TYPE: NO_QUESTION