The Muscular System 37muscle cannot lengthen on its own; rather a force
such as gravity or the contraction of another muscle
is required to create this elongation. This character-
istic of extensibility is key for allowing the dancer to
improve the range of motion permitted at a given
joint—that is, flexibility.
The study of this viscoelastic characteristic of
muscle has been instrumental in developing current
theories of muscle behavior and recommendations
for effective muscle conditioning programs such as
those for improving flexibility. When a muscle and
its related connective tissue are stretched, elonga-
tion of both the elastic and viscous elements occurs.
However, when the stretch is discontinued the elastic
elongation recovers, and only the plastic elongation
remains (Taylor et al., 1990). While the elastic ele-
ments are influenced only by the magnitude of the
applied force, the viscous elements are influenced
by temperature, as well as the rate and duration of
the applied forces. The behavior of this component
can be compared to that of Silly Putty or stiff taffy. A
force (i.e., pulling the taffy apart) that is applied slowly
and for a long duration, with the taffy warm, produces
greater elongation and less tendency for breaking.
Thus, to emphasize increases in flexibility that persist
over time, the goal is to maximize plastic elongation.
This can be achieved by the use of a slow, lower-force,
longer-duration stretch applied to warmed muscles.
In terms of duration, three repetitions of a 30-second
stretch appear to provide most of the potential length
changes associated with a given stretch (Garrett,
1991). Conversely, to emphasize greater force pro-
duction of a muscle, the goal is to maximize elastic
elongation. This can be achieved through applica-
tion of a rapid, higher-force stretch, immediately
preceding a shortening (concentric) contraction of
the same muscle (see Stretch-Shortening Cycle on
p. 54 for more information).
Microstructure of Skeletal Muscle and Muscle Contraction
The structural unit of a muscle is the muscle cell. It
has been estimated that the human body contains
approximately 270 million muscle cells (Wells and
Luttgens, 1976). Because these muscle cells are long
and very thin, they are often called muscle fibers. An
individual muscle cell generally has a diameter rang-
ing from approximately 0.0004 to 0.004 inches (0.01
to 0.1 millimeters). In contrast, many muscle cells
range between 1 and 3 inches (approximately 2.5-7.6
centimeters) in length, and select muscles may be
up to even 24 or 28 inches (60 or 70 centimeters)
in length (Hamilton and Luttgens, 2002; Rasch and
Burke, 1978; Smith, Weiss, and Lehmkuhl, 1996).
Skeletal muscle fibers grow in both length and diame-
ter from birth to adulthood, with a five times increase
in diameter possible during this period (Hall, 1999).
Strength training using heavy resistance and low
repetitions can also result in substantial increases in
muscle cell diameter, termed hypertrophy (G. hyper,
over + trophy, nourishment).
To understand how muscle cells are capable
of causing a contraction, it is necessary to look at
a single muscle cell on a microscopic level. Each
muscle fiber contains specialized protoplasm termed
the sarcoplasm, within which is embedded very thin
fibers called myofibrils (G. mys, muscle) that run
the length of the muscle cell but are only about
four-millionths of an inch (1-2 micrometers) wide
(Hamill and Knutzen, 1995). These myofibrils are
arranged in a parallel formation within the muscle
cell and consist of still finer threads called myofila-
ments (G. mys, muscle + L. filamentum, thread) that
can be either thick (primarily containing the protein
myosin) or thin (primarily containing the protein
actin). These myosin and actin filaments exhibit dif-
ferent light properties under the view of a polarizing
microscope and interdigitate in a manner that gives
rise to alternating dark and light bands, imparting
to skeletal muscle fibers their characteristic striated
appearance. As can be seen in figure 2.3, the lighter
I band contains only thin filaments (actin), while the
darker A band contains thick filaments throughout,
with thin filaments extending as far as the H zone.
The H zone of the A band contains only thick fila-
ments (myosin) and is lighter than the other portion
of the A band. Each I band is bisected transversely by
a Z line, and one end of each actin filament within
this I band is attached to this Z line. These actin
and myosin filaments are organized in repeating
segments longitudinally that are termed sarcomeres.
The sarcomere (G. sarco, muscular substance + meros,
part) is a compartment between consecutive Z lines
and is the functional unit of muscle contraction.The Sliding Filament Theory
The most widely held theory of muscle contraction is
called the sliding filament theory (Huxley, 1969). As
its name implies, this theory holds that the filaments
just discussed are the mechanism by which muscles
contract. Each myosin filament is surrounded by
six actin filaments. Myosin filaments contain cross-
bridges, and actin filaments contain active sites as
shown in figure 2.3. When the muscle is not activated,