Biomedical Engineering Reference
In-Depth Information
Fig. 1.10 Two bodies of
different masses on the
surface of the earth,
therefore with different
weights and respective
reaction forces
body of mass m 1
W 1
R 1
R 2
W 2
Earth of mass M
Figure 1.10 shows the weights of two bodies with different mass and respective
reaction forces exerted by bodies on the earth.
1.7.2 Muscle Forces
Animal posture and motion are controlled by forces produced by muscles. There are
approximately 600 muscles in the human body that are responsible for all the
motions of the body from very subtle movements in the facial expression to moving
the tongue in speech, to circulating the blood in the vessels of the body, and to the
beating of the heart, whose main function is muscle contraction.
A muscle consists of a large number of fibers whose cells are able to contract
when stimulated by nerve impulses coming from the brain. A muscle is usually
attached to two different bones by tendons.
The maximum force that a muscle can exert depends on its cross-sectional area
(perpendicular cut) and is inherent to the structure of muscle filaments. This
maximum force per unit area varies from 30 to 40 N/cm 2 . It does not depend on
the size of the animal and, therefore, has the same value for a muscle of a rat or of an
elephant. Under the microscope, the muscle of an elephant is very similar to that of
a rat, except in the quantity of mitochondria, which is larger in smaller animals.
Example 1.2 Representation of motionless (at rest) biceps muscle, bones of arm,
and forearm with an object in the hand can be seen in the figure of Example 1.2. The
forces that act on both forearm and hand are drawn in the figure of Example 1.2.
Find the magnitude of force exerted by the biceps muscle, by adding all the forces,
observing that once the system is in equilibrium, the resultant must be equal to zero.
 
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