40 Dance Anatomy and Kinesiology
such muscles has led to the use of the terms tonic or
postural to describe muscles that have a greater pres-
ence of slow-twitch fibers and phasic or nonpostural
to describe muscles that have a greater presence of
fast-twitch fibers.
In addition to varying between muscles, percent-
ages of slow-twitch fibers and fast-twitch fibers differ
between individuals. Most sedentary individuals have
a similar proportion of fast-twitch and slow-twitch
fibers in many muscles. However, since fast-twitch
fibers are important for generating fast, powerful
muscle contractions, athletes like sprinters gen-
erally have high proportions of fast-twitch fibers
(55-75%). In contrast, since slow-twitch fibers are
important for producing repetitive contractions
without fatigue, endurance athletes such as distance
runners have high proportions (60-90%) of slow-
twitch fibers (Powers and Howley, 1990; Takashi,
Kumagai, and Brechue, 2000). For example, the
gastrocnemius muscle in some elite sprint runners
has been shown to be composed of 73% fast-twitch
fibers while the gastrocnemius in elite female dis-
tance runners contained 69% slow-twitch fibers
(Wilmore and Costill, 2004). Ethnicity may also be a
factor. For example, individuals of African American
descent have been shown to have a higher percent-
age of fast-twitch fibers than individuals of Caucasian
descent (ACSM, 2001).
How much of this composition of fibers is geneti-
cally determined and how much of it can be changed
by training is still an area of controversy. At this point
it appears that genetics is the most fundamental
determinant of quantity and distribution of fibers,
but heavy training may alter some of the proper-
ties of given fibers to allow them to better meet the
demands produced by the specific training regime
(Gordon and Pattullo, 1993; Nieman, 1999; Wilmore
and Costill, 2004). So, for example, dancers who
genetically have a higher percentage of fast-twitch
fibers may be able to naturally generate more force
and potentially jump higher, while dancers with
higher percentages of slow-twitch fibers may have
advantages in adagio or repetitive movements such
as relevés. However, with appropriate training, all
dancers can improve their ability to some degree to
meet specific dance demands.Muscle Architecture
In addition to fiber type, the architecture of a given
muscle is important for meeting specific demands.
Two architectural characteristics that are particularly
important are muscle cross-sectional area and fiber
arrangement.Muscle Cross-Sectional Area
Basically, a muscle with more muscle fibers will be
capable of producing more force than one with fewer
fibers. Muscle fibers usually lie parallel to each other,
and so the cross-sectional area reflects to some degree
the number of fibers and relates to force production.
Although this idea of a direct relationship between
cross-sectional area and the number of muscle fibers
is complicated by different fiber arrangements and the
fact that different fibers have different diameters due
to fiber types and hypertrophy, the concept still holds
that a larger muscle with a greater cross-sectional
area can produce more force than a smaller muscle
with a smaller cross-sectional area. So, for example,
the gluteus maximus can produce more force than
one of the hamstring muscles, in part due to a greater
cross-sectional area. Furthermore, strength training
will generally cause an increase in the cross-sectional
area within the same muscle (hypertrophy) and will
allow for greater force production.Fiber Arrangement
Muscle fibers in a whole muscle occur in two primary
arrangements—fusiform and penniform—with many
variations within each of these types. With fusiform
(L. fusus, spindle + forma, form), also termed longitu-
dinal arrangements, muscle fibers run close to paral-
lel with the muscle’s long axis as seen in figure 2.5A.
This structure allows for relatively few fibers per unit
area, and so offers a disadvantage in terms of force
production capacity. However, in this arrangement
muscle fibers are generally longer, and so there is an
advantage in terms of how much shortening of the
muscle can occur. When sarcomeres are fully con-
tracted, they can reduce the length of a muscle fiber
by 30% to 70% of its original resting length, with the
average muscle fiber shortening about 50% (Hamill
and Knutzen, 1995; Levangie and Norkin, 2001; Pitt-
Brooke, 1998). So, for example, a 50% shortening
of a muscle fiber of the sartorius, which is about
17.6 inches (448 millimeters) long (Enoka, 2002),
would represent about 8.8 inches (224 millimeters)
of shortening; 50% shortening of the vasti muscle
fibers (penniform muscles), which are about 2.8
inches (72 millimeters) long, would only represent
a decrease in length of about 1.4 inches (36 millime-
ters). Furthermore, due to the lengthwise arrange-
ment of fibers, shortening of fibers translates into
almost equivalent shortening of fusiform muscles as
a whole. Hence, muscles with such an arrangement
favor moving the limbs through space with greater
range or speed. Examples of fusiform muscles are