a motionless state where fores and moments cancel each other.
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
Center of Mass (COM)
the point around which a body's mass is equally balances in ALL directions
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)
combination of the nervous and muscular system
muscle shortens during contractions
muscle lengthens during contractions
muscle stays the same length while contracting
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
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?
which fiber arrangement has a greater range of motion?
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
inability to develop tension
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?
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
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,
2.in 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?
What happens to acceleration when mass increases?
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?
when a person is standing quietly, what is the vertical force?
friction acts in what direction?
opposite to motion
what are the two types of friction?
static and kinetic
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