The Muscular System 55FIGURE 2.14 Muscle roles in whole body movement
(lateral view).
agonists. In raising the leg to the back, the antagonists
are the hip flexors including the rectus femoris as
seen in figure 2.14. Generally, antagonists relax while
the prime movers contract. However, antagonists
sometimes work together with agonists—termed co-
contraction—when a part must be held rigid, when
very precise movement is required, and during decel-
eration of body parts. An example of the latter occurs
during running, where the antagonist (hip extensors)
would initially relax to allow the prime movers (hip
flexors) to bring the thigh forward in the swing phase
but then would contract eccentrically to decelerate the
thigh before it reaches its full height to the front.
Synergist (Neutralizer)
A synergist (G. syn, together + ergon, work) is a muscle
that works together with the agonist(s) to help
achieve the movement goal. The role of the synergist
is described differently in different texts. This text
will confine the meaning of synergist to a muscle
whose action serves to neutralize an undesired sec-
ondary action of the prime mover(s). When raising
the leg to the back in a parallel attitude, the medial
hamstrings (semimembranosus and semitendinosus)
have actions of hip internal rotation as well as hip
extension. To achieve the desired parallel position of
the leg, the lateral hamstring (biceps femoris) can act
as a synergist, with its secondary action of hip external
rotation neutralizing the undesired internal rotation
of the other hamstrings as shown in figure 2.14.
A further useful distinction is between a true and a
helping synergist. If the synergist only neutralizes the
undesired action and does not help with the desired
action, it is called a true synergist. However, if it helps
with the desired action and neutralizes the undesired
action, it is called a helping synergist. The back atti-
tude synergy just described is an example involving
a helping synergist, since the lateral hamstring aids
with the desired action of hip extension while it neu-
tralizes the undesired hip internal rotation.Stabilizer (Fixator)
A stabilizer, or fixator, is a muscle that contracts iso-
metrically to support or steady a body part against
forces related to muscle contraction, gravity, soft
tissue constraints, momentum, or recoil from the
movement. The first of these potential functions,
stabilization against the forces related to muscle con-
traction, occurs with most movements. Remember
that when a muscle contracts, it tends to pull both of
its ends toward its center. However, often the desire
is to have movement occur only at one end of the
muscle, and stabilizers can work to anchor the neces-
sary bone or body part to allow that to happen. For
example, in a kick to the front (grand battement),
stabilizers (abdominal muscles) work to prevent the
proximal attachment of the iliopsoas muscle (a hip
flexor that attaches to the lower spine proximally)
from arching the low back, as the iliopsoas’ distal
attachment on the femur effects the desired action
of lifting the thigh (hip flexion).
An example in which gravity is the primary factor
occurs with push-ups. Abdominal muscles must be
contracted to stabilize the pelvis and spine and pre-
vent the tendency for the low back to arch and the
body to sag due to the effect of gravity. An example
in which soft tissue constraints are instrumental occurs
during raising of the leg to the back in a parallel atti-
tude. As the leg is raised backward, the hip flexors
offer passive resistance as they are being stretched and
tend to pull the pelvis into an anterior tilt and pro-
duce an arching of the low back unless the abdomi-
nal muscles are used to stabilize the pelvis as seen
in figure 2.14. Appropriate timing and magnitude
of stabilization are an important element of skilled
dance performance, and very specific positioning
and movement of many joints are often desired.