Lever Systems. Classified systems of torque Relative positions of force, resistance, and axis of rotation vary in the different types or classes of levers As with any torque calculations, operations on levers determine the tendency for some force to produce rotation around a fixed point.
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Classified systems of torque
Relative positions of force, resistance, and axis of rotation vary in the different types or classes of levers
As with any torque calculations, operations on levers determine the tendency for some force to produce rotation around a fixed point.
This is the effectiveness of a lever at moving a resistance. It is a calculated value:
Because of their different configurations, the mechanical advantage of a first class lever can favor the force or resistance depending on the placement of the fulcrum. A second class lever always favors the force arm. A third class lever always favors the resistance arm.
The fulcrum in a first class lever system can often vary in position to favor the force arm or the resistance arm.
In a second class lever system, the mechanical advantage favors the force arm. (The force arm will always be longer.)
The mechanical advantage of a third class lever system favors the resistance arm. (The resistance arm is always longer.)
ALevers In The Musculo-Skeletal System
The musculo-skeletal lever systems generally favor speed over strength. Although the mechanical advantage favors the resistance arm, in the time that the muscle insertion moves a given distance (red arrow), the resistance moves a much greater distance (blue arrow).
If DFA < DRA the mechanical advantage is < 1
A muscle inserts 3 cm from a joint axis (DFA = 3 cm)
The distance to the weight that the muscle is resisting is 30 cm (DRA = 30 cm)
mechanical advantage = 3 cm / 30 cm = .1
This means that, when FM and R are both perpendicular to the limb, FM must be 10 times greater than R to move the resistance.
vR = (w rad/sec)(DRA)
vM = (w rad/sec)(DFA)
= (w rad/sec)(DRA)/ (w rad/sec)(DFA) = DRA/DFA
The relative speed of the resistance to the muscle insertion = DRA/DFA = (30 cm)/(3 cm) = 10
This means that the resistance is moving at 10 times the velocity of the muscle insertion.