a. Trunk Musculature. The trunk musculature is arranged in two
ways--longitudinal muscles and oblique muscles. Together, they:
(1) Maintain trunk posture.
(2) Move the parts of the trunk.
(3) Adjust the internal pressures of the trunk to perform certain functions
such as breathing.
b. Limb Musculature. The limb musculature is arranged around the joints to
produce the appropriate motions of the limbs. Elementary mechanics are described in
the next section to help you to understand typical arrangements of limb musculature.
Section II. SOME ELEMENTARY SKELETO-MUSCULAR MECHANICS
Muscles and bones together work like machines within the laws of physics and
chemistry. Lever and pulley systems are examples of simple machines found
commonly in the human body.
5-6. LEVER SYSTEMS
See figure 5-3 for an illustration of the three classes of levers.
a. First Class. In a first class lever, the weight to be moved is at one end of the
lever, the applied force is at the other end, and the fulcrum (the pivot or turning point) is
between the two.
b. Second Class. In a second class lever, the weight to be moved is between
the applied force and the fulcrum. This type of lever enables a weight to be moved with
less force than would be required without a lever. (Many feel that there are no second
class levers in the human body.)
c. Third Class. In a third class lever, the weight to be moved is at one end of
the lever, the fulcrum is at the other end, and the applied force is between the weight
and the fulcrum. This type of lever provides speed, but a greater amount of force is
required for a given weight. This is the most common type of lever in the human body.