The Muscular System 65sion. In contrast, the down-phase would be produced
by gravity, and the knee extensors would be working
eccentrically to control knee flexion and prevent
the dancer from collapsing to the floor. So, even
though opposite motion is occurring at the joint
(knee extension on the up-phase and knee flexion
on the down-phase), the same muscle group is
being used for both phases (concentrically on the
up-phase to produce the movement and eccentri-
cally on the down-phase to control and resist the
flexion tendency produced by gravity). So, one
approach to movement analysis is to determine
what muscle group is working on the concentric
phase of the movement; this will tell you the muscle
group responsible for the movement both on the
up-phase (concentrically) and on the down-phase
(eccentrically). A basic schema for this approach is
provided in table 2.5 with the plié performed in a
parallel position and analysis limited to the hip and
knee joints for purposes of simplicity. Another way
of thinking of this is to note that when eccentric con-
tractions are involved, muscles are working that have
the opposite action to the direction of movement
that is actually occurring. Also remember that when
gravity would tend to produce a given movement
and no movement is occurring, isometric muscle
contractions are generally in play.
In slow movements of body segments that are
perpendicular to gravity (horizontal or parallel to
the floor), gravity does not have the same effect, and
muscles are often used concentrically to produce
movements and to maintain the limb in the horizon-
tal plane in both directions of the movement (e.g.,
horizontal abduction and adduction). In fast move-
ments, the interplay of muscles and gravity becomes
more complex; and concentric use of muscles to
accelerate segments, co-contraction of antagonists
to control movement, and eccentric contractions to
decelerate body segments combined with gravita-
tional forces often come into play. Understanding
the importance of gravity on muscle function is
essential for accurate movement analysis and will be
further discussed in chapter 8.- Application to exercise design. Another way to
reinforce the understanding of a muscle’s action(s)
and location is to design exercises for strengthening
a given muscle, stretching a given muscle, or prevent-
ing injury to that muscle or a related structure. Chap-
ters 3 to 7 contain samples of such exercises. When
one is designing a strength exercise, at least one of
the primary actions of the muscle must be opposed
by the resistance. In order for the exercise to be effec-
tive, this muscle must be challenged (overloaded)
sufficiently such that muscle failure is approached
within relatively few repetitions but muscle injury is
avoided. The American College of Sports Medicine
recommendations (1998) are to perform 8 to 12
repetitions of a variety of exercises. Some of the more
difficult examples of strength exercises provided in
the following chapters may initially require that fewer
repetitions be performed (four to six repetitions) so
that excessive muscle stress is avoided.
The resistance utilized in strength exercises can
take many forms, such as body weight, ankle weights,
FIGURE 2.21 Types of muscle contraction in which gravity plays a primary role.