Anatomy and Physiology - Introduction to Biomedical Engineering - page 123

CLASS 2

CLASS 1

CLASS 3

FIGURE 3.36 Depending on the muscle in use, the location of the load, and the location of the fulcrum, the
humerus can act as a class 1 lever, a class 2 lever, or a class 3 lever.

a ball in front of the body, the applied force is generated by the contraction of the biceps
brachii muscle. The weight to be moved includes the ball and the weight of the forearm
and hand, and the elbow acts as the fulcrum.
The three types of muscle tissue—cardiac, skeletal, and smooth—share four important
characteristics: contractility, the ability to shorten; excitability, the capacity to receive and
respond to a stimulus; extensibility, the ability to be stretched; and elasticity, the ability to
return to the original shape after being stretched or contracted. Cardiac muscle tissue is
found only in the heart, whereas smooth muscle tissue is found within almost every other
organ, where it forms sheets, bundles, or sheaths around other tissues. Skeletal muscles are
composed of skeletal muscle tissue, connective tissue, blood vessels, and nervous tissue.
Each skeletal muscle is surrounded by a layer of connective tissue (collagen fibers) that
separates the muscle from surrounding tissues and organs. These fibers come together at
the end of the muscle to form tendons, which connect the skeletal muscle to bone, to skin
(face), or to the tendons of other muscles (hand). Other connective tissue fibers divide the
skeletal muscles into compartments called fascicles that contain bundles of muscle fibers.
Within each fascicle, additional connective tissue surrounds each skeletal muscle fiber and
ties adjacent ones together. Each skeletal muscle fiber has hundreds of nuclei just beneath
the cell membrane. Multiple nuclei provide multiple copies of the genes that direct the pro-
duction of enzymes and structural proteins needed for normal contraction so contraction
can occur faster.