54 Dance Anatomy and Kinesiology
of the stretch-shortening cycle, the different roles
muscles can play when they are acting simultane-
ously, how muscles can work as force couples, and
the unique challenges that arise with muscles that
cross multiple joints.Stretch-Shortening Cycle
In some movements, a muscle is used eccentrically
immediately preceding use of the same muscle
concentrically. This is termed the stretch-shorten-
ing cycle (SSC), or prestretch. When an active
muscle is stretched, mechanical energy is stored
in the elastic component of the muscle, which is
then released during the immediately following
shortening contraction, resulting in greater force
production (Asmussen and Bonde-Petersen, 1974;
Bosco and Komi, 1979; Komi, 1979). Recall that
the elastic components of muscle can be modeled
as a spring. So, you can envision this phenomenon
by imagining stretching a spring. When you let go,
the spring will recoil and pull back together. Addi-
tional factors, including neural considerations and
chemical energy from preloading the muscle, also
probably contribute to enhanced force output; and
the relative potential contribution of these and other
factors is still under investigation (Cronin, McNair,
and Marshall, 2000; Enoka, 2002; Smith, Weiss, and
Lehmkuhl, 1996). Whatever the mechanism, this
enhanced force can be marked; and in very rapid,
small jumps from both feet, it was calculated that
only 40% of the force was due to the concentric
contraction of the muscle while approximately 60%
of the force was due to these elastic and other factors
(Thys, Cavagna, and Margaria, 1975). In addition,
release velocity and jump height have been shown
to improve 12% to 18%, and mean power output in
a strength training exercise improved 8% to 16%
with the use of SSC (Cronin, McNair, and Marshall,
2000). Furthermore, use of SSC has been shown to
allow for lower energy requirements (e.g., greater
efficiency) in a given movement.
An example of use of the SSC in dance is the use
of a quick demi-plié prior to a jump. The gluteus
maximus, hamstrings, quadriceps femoris, and calf
muscles would work eccentrically on the down-phase
of the plié and then concentrically on the up-phase.
This principle is used frequently in dance, allowing
for greater movement efficiency and potentially
contributing to the “effortless” aesthetic desired
in some dance forms, as well as greater force pro-
duction for explosive movements. To optimize
use of this property, the prestretch should be of
a relatively small magnitude (e.g., lowering thebody 8-12 inches [20-30 centimeters] in the plié),
rapidly applied, with minimal delay (less than 0.4-
1.0 second), and without a pause or relaxation of
the muscle at the end of the stretch (e.g., at the
bottom of the plié) between the eccentric and con-
centric contraction. While some dancers appear to
naturally utilize a timing that facilitates enhanced
force from the SSC, other dancers could benefit
by small changes in their preparatory movements.
Common errors include hesitating rather than uti-
lizing a quick reversal of directions at the bottom of
the plié and suboptimal depth of the preparatory
plié. Training regimes that focus on this response
of muscle (e.g., plyometrics) have been shown to
enhance performance and may reduce injury inci-
dence in muscles such as the hamstrings that are
required to perform rapid stretch-shortening phases
in movements like jumping and sprinting (Smith,
Weiss, and Lehmkuhl, 1996).Muscle Roles
When muscles work together at the same time, there
are four primary potential roles they can play—mover
(agonist), antagonist, synergist, or stabilizer. These
roles are specific to a given movement and not a
given muscle. Hence, the same muscle can serve in
a different role with different movements.Mover (Agonist)A mover, or agonist (G. agon, contest), is a muscle
or muscles whose contraction actually produces the
desired joint movement. There may be many muscles
that are capable of producing this desired movement.
Traditionally the term prime mover(s) or primary
muscle(s) is reserved for those muscles that are
most important or effective in producing the move-
ment, and the terms assistant mover(s) or secondary
muscle(s) are used for those muscles that are less effec-
tive or that are called into play in specialized circum-
stances such as when more force is needed. However,
these distinctions often are controversial and complex,
and so which agonists are considered primary or
secondary may differ between different sources. In
raising the leg to the back (e.g., parallel attitude), the
agonists are the hip extensors including the hamstrings
as seen in figure 2.14.AntagonistAn antagonist (G. anti, against + agon, contest) is a
muscle or muscles with an action opposite to the
action of the prime mover. Antagonists are often
positioned on the side of the joint opposite to the