The Muscular System 51Dynamic Muscle Contraction
Dynamic muscle contraction or tension occurs when
there is a change in length of the involved muscle and
accompanying observable joint movement. There are
two types of dynamic (historically termed isotonic)
contractions—concentric and eccentric.Concentric Contraction
A concentric muscle contraction or tension involves
a shortening of the muscle and resultant visible joint
movement (e.g., flexion, abduction) in the direction
of the action of the primary muscle. Concentric (G.
con, with + kentron, center) loosely means “toward
the center”; and with this type of contraction, both
attachments of the muscle will tend to be pulled
toward each other as the muscle shortens. On the
sarcomere level, actin is being pulled toward the
center of the sarcomere with each successive cross-
bridge formation. This is the classic way we think of
muscle contraction, and the concentric contraction
is the basis for charts that list the action or actions
of a given muscle. With concentric contractions the
torque from the muscle or muscle group is greater
than that of the resistance, and joint movement is in
the same direction as the torque generated by the
muscle or muscle group. For example, a concentric
contraction occurs in a biceps curl exercise on the
up-phase of the movement (table 2.3A) because the
torque of the elbow flexors (including the biceps bra-
chii) is greater than the resistance torque produced
by the weight of the dumbbell and forearm.
Concentric contractions are commonly used on
the up-phase of movements in dance such as rising
from a plié, the takeoff phase of a jump, or raising
the arms overhead.Eccentric Contraction
An eccentric contraction or tension involves a
“lengthening” of the muscle (i.e., the distance
between origin and insertion gets greater) as vis-
ible joint motion is occurring. Eccentric (G. ek, out
- kentron, center) loosely means “away from the
center,” and with eccentric contractions the resis-
tance is “lengthening” the muscle while the muscle
is contracting to control the effect of the resistance.
On the sarcomere level, the actin go away from
the center of the sarcomere, and cross-bridges are
broken and then reformed as the muscle lengthens
(Levangie and Norkin, 2001). In most cases, the
muscle is not actually “lengthening” beyond resting
length, but rather gradually decreasing the degree of
contraction from its shortened (contracted position)
FIGURE 2.13 Influence of angle of muscle attachment toward its resting length. With eccentric contractions,
on rotary and parallel components of muscle force.