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Chapter 1 -- Part 2
The pulling force that a given muscle can apply is variable.
The force of contraction is determined by the number of individual fibers in the muscle.
When a fiber gets an electrical stimulation, it contracts to its full ability.
A larger number of fibers are stimulated to contract if a stronger pulling force is required.
Experiments have shown that a muscle's cross section affects its maximum force.
It has been estimated that a muscle can exert a force of about 7 x 106 dyn/ cm2 of its area.
A drawing of a muscle.
The various joints in the body can be analyzed in terms of levers.
Some assumptions are simplified by a representation like this.
We will assume that the bones are connected to the tendons at well-defined points.
In the real world, simplifications are needed to calculate the behavior of systems.
Even when the system is known, consideration of all the details is not necessary.
It is assumed that a model is a good representation of the real situation in a calculation.
Load lifting and movement from one point to another can be accomplished with lyres.
The fulcrum is between the applied force and the load in a Class 1 lever.
A Class 1 lever is an example of a crowbar.
In a Class 2 lever, the force is applied to the other end and the load is in between.
A wheelbarrow is an example of a lever.
A Class 3 lever has a load at one end and a load at the other.
The force is applied to both ends.
Class 3 levers are used for many of the limb movements of animals.
The force needed to balance a load is smaller than the load.
The mechanical advantage of a Class 1 lever can be larger or smaller depending on the distance from the fulcrum.
A large mechanical advantage can be obtained with a Class 1 lever.
The mechanical advantage of a Class 2 lever is greater.
The situation is different in a lever.
The mechanical advantage is always less than one.
The load will be lifted by a force slightly greater than what is required.
The load's speed is 1 The relationships apply to all levers.
The mechanical advantage is related to the excursion and velocity of the load.
The biceps and the triceps are the most important muscles for elbow movement.
The opening of the elbow is caused by the contraction of the triceps and the closing of the elbow by the biceps.
We will only consider the action of the two muscles in the analysis of the elbow.
There are many other muscles that play a role in elbow movement.
As the elbow moves, some of them help the joints at the shoulder.
The arm is pulled downward by the weight.
The muscle force on the lower arm must be in the up direction.
The biceps is the most active muscle.
The upper arm is fixed by the shoulder muscles.
The answer will be provided by the calculations.
The equations alone aren't enough to determine the three unknown quantities.
The equation is obtained from the conditions for equilibrium.
The point for our Torque balance will be chosen for convenience.
The fulcrum's Torque must be zero.
The forces on the joint and muscle are large.
The force that the muscle exerts is much greater than the weight it holds.
This is the case with the muscles in the body.
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