Week 2 Flashcards
First Class Lever
Internal forces and External forces are on the opposite side of the lever/axis
Internal force can be equal to, greater or less than external force
Second Class Lever
Internal forces and External forces are on the same side of the axis
IMA >EMA
Internal force is always less than external force
Third Class Lever
Internal forces and Externalforces are on the same side of the lever
IMA <EMA
Internal force is always greater than external force
Inertia
resistance to acceleration (change in velocity)
resistance to movement
MASS – quantifies the amount of matter composing an object units = kg)
objects with greater mass resist movement more
Stability
the ability to resist acceleration (movement)
Three factors influence stability:
1) magnitude of mass
2) distribution of mass
3) location of COM within BOS
Magnitude of mass
Greater mass = greater stability
Center of Mass (COM)
the point about which all mass of an object is distributed evenly
the “balance point”
Generally, the COM is located close to the largest concentration of mass
Sometimes called the center of gravity (COG) – but this refers only to the position of the center of mass in the
vertical axis
Location of the COM in humans
In anatomical position the COM is located approximately at
the level of the 2nd sacral segment in the center of the pelvis
Distribution of Mass
Specifically, height of center of mass
Lower center of mass = Greater stability
e.g. athletic stance, bracing to push/pull a heavy object
How could we improve stability of
this position?
Move COM
towards center of
BOS
Increase size of
BOS
Location of the Center of Mass relative to the Base of Support
(BOS)
Larger BOS = Greater stability
Body is stable as long as COM remains within the BOS
More stability closer to the center of the BOS
Base of Support
The maximum contact area of the body with the ground surface
An area bounded posteriorly by the tips of the heels and
anteriorly by the tips of the toes
Assistive devices increase stability through enlarged BOS
Assistive devices increase stability through enlarged BOS
Stability-mobility trade-offs
Gait deviations tend to increase energy expenditure
Using standard walkers requires significantly more energy
expenditure than using rolling walkers or canes
BOS
For static stability the COG of a body must project within the BOS
Balance on a smaller BOS increases the demand on the
neuromuscular system
Wider BOS increases stability
Lower COG increases stability
Where is the LOG within the BOS? (at the center – increased stability)
Stability-Mobility
Stability precedes mobility
Can the patient hold a position in a desired fashion?
Does the patient have adequate ROM/strength to attain the
position?
Progression from stable to mobile is typical in human motor
development and in rehabilitation
1. Maintain a position
2. Attain position
