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Static equilibrium

a motionless state where fores and moments cancel each other.

Dynamic equilibrium

a state of motion with a balance between applied forces and internal forces.


equations of equilibrium


equations of motion

Is there such thing as a balance score?

No, too complex to have a single score. Multifunctional skill.


resistance to disruption of equilibrium, resistance to linear and angular accelerations


ability to control equilibrium

5 factors to increase stability

increase mass, increase friction, larger BoS, lower COM

Is standing still mechanically stable or unstable?


Center of Pressure (COP)

location of ground reaction forces

Units of COP


Center of Mass (COM)

the point around which a body's mass is equally balances in ALL directions

Units of COM


If the COP is ahead of the COM

longer moment arm, largest moment, pushes forward, person falls back

If the COM is ahead of the COP

person moves forward, COP moves forward to keep balance

In a passive system, if the COM is pushed outside of the base of support, what happens?

it falls over

When a person initiates gait, the COM moves outside the BOS, why don't they fall?

the step to move the COM forward

Three mechanical reasons humans are unstable

COM is high, BoS is small, leg joints can lead to vertical collapse

Controller (sheep dog)


neuromuscular system

combination of the nervous and muscular system

concentric contraction

muscle shortens during contractions

eccentric contraction

muscle lengthens during contractions

isometric contraction

muscle stays the same length while contracting

musculotendinous unit

muscle + tendon

Four behavioral properties of a musculotendinous unit

extensibility, elasticity, irritability, ability to develop tension/force


ability to increase in length


ability to return to normal length


ability to respond to a stimulus

when a person initiates a forceful stretch of a muscle group immediately before contracting that muscle group

strech-shortening cycle

motor unit

a single motor neuron and all the fibers it innervates

fast twitch fiber

reaches max tension more quickly than slow twitch

slow twitch fiber

important for endurance

parallel fiber arrangement

parallel to the long axis of the muscle

pennate fiber arrangement

fibers act at an angle to the long axis of the muscle

which fiber arrangement can generate more force?

pennate fibers

which fiber arrangement has a greater range of motion?

parallel fibers

what are the four roles of assumed by muscles during movement?

agonist, antagonist, stabilizer, neutralizer


causing the movement


opposing the movement


counteracts external force


a muscle causes an unwanted movement

active insufficiency

inability to develop tension

passive insufficiency

cannot stretch full range of motion due to tightness of stretched multi-joint muscle on opposite side

what factors affect muscular force generation

fiber type, muscle size, # of fibers, and pennation angle

what muscle length do we get optimal force?

150% resting length

what type of contraction generates optimal force?


what kinetic quantity do we describe muscular strength

moment of force, moment, torque

do we tend to describe the strength of individual muscles or muscles groups?

muscle groups

when body temp. increases, what 5 changes occur?

increase speed of nerve conduction, muscle function, oxygen supply, and waste removal. decrease in the number of motor units needed to sustain Cx

what contraction is most effective at developing muscle size and strength?


what contraction produces the most force?


what contraction produces the least force?


what contraction is most likely to result in muscle strain?


Can loss of strength be slowed and reversed?


1st degree muscle strain


2nd degree muscle strain


3rd degree muscle strain

100% tear

Law of Inertia

A body will maintain a state of rest or constant velocity unless acted on by an external force that changes the state

Law of Acceleration

An applied force causes an acceleration of the body:
1.with a magnitude proportional to the force, the direction of the force, and
3.inversely proportional to the mass.

Law of Reaction

For every action, there is an equal and opposite reaction

What happens to acceleration when force increases?

it increases

What happens to acceleration when mass increases?

it decreases

what is the direction of acceleration?

the direction of the force

two major factors of force?

mass and acceleration

GRF's represent the summation of the acceleration of what?

all of the body segments, COM

What can a person do to reduce GFR on a force plate?

bend down quickly

What can a person do to increase GFR?

push upwards

when a person is standing quietly, what is the vertical force?

body weight

friction acts in what direction?

opposite to motion

what are the two types of friction?

static and kinetic

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