38 Dance Anatomy and Kinesiology
the active sites on the actin are blocked. The sliding
filament theory holds that activation of a muscle
causes a release of calcium from within the muscle
fiber. This calcium release changes the configuration
of protein molecules (troponin and tropomyosin) so
that the active sites on actin are exposed and become
available for the myosin cross-bridges to attach to.
This attachment, termed coupling, triggers a split-
ting of the energy molecule adenosine triphosphate
(ATP), producing rapid “flexion” of the myosin
cross-bridge, which pulls the actin filaments a short
distance toward the center of the sarcomere. The
cross-bridge then uncouples, and retracts, and the
myosin is recharged with another molecule of ATP.
It is now ready to react with another active actin site.
In this process of cross-bridge coupling, flexion, and
uncoupling, the Z lines are drawn in toward the A
bands, and the H zone narrows or even disappears
as seen in figure 2.4.
Although the amount of shortening of each sar-
comere unit is small, the cumulative effect of short-
ening of the many sarcomere units in series can be
marked. For example, a muscle fiber similar in length
to that of the biceps brachii has been estimated to have
about 40,000 sarcomere units in series; and the sum
effect of their shortening would be approximately
1.6 inches (4 centimeters), equivalent to about
40% of the length of the muscle from its position at
rest (Smith, Weiss, and Lehmkuhl, 1996). These cou-
pling, flexion, uncoupling, retraction, and recharg-
ing processes, known as cross-bridge cycling, areA bandZ line H zone Z lineSarcomereI band I bandThin (actin) filamentThick (myosin) filamentThick (myosin) filamentThin (actin) filamentCross-bridgeFIGURE 2.3 Microstructure of a skeletal muscle fiber.
Reprinted, by permission, from R.S. Behnke, 2006, Kinetic anatomy, 2nd ed. (Champaign, IL: Human Kinetics), p. 14.FIGURE 2.4 Schema of muscle contraction. (A) Resting
state, (B) slight contraction, (C) greater contraction.