The space between overlapping thick and thin ﬁ la-
ments is bridged by projections known as
These are portions of myosin molecules that extend from
the surface of the thick ﬁ laments toward the thin ﬁ laments
(see Figure 9–2). During muscle contraction, the cross-
bridges make contact with the thin ﬁ laments and exert force
of Skeletal Muscle Contraction
as used in muscle physiology, does not
necessarily mean “shortening.” It simply refers to activation of the
force-generating sites within muscle ﬁ bers—the cross-bridges.
For example, holding a dumbbell at a constant position requires
muscle contraction, but not muscle shortening. Following con-
traction, the mechanisms that generate force are turned off, and
tension declines, allowing
of the muscle ﬁ ber.
When force generation produces shortening of a skeletal mus-
cle ﬁ ber, the overlapping thick and thin ﬁ laments in each sar-
comere move past each other, propelled by movements of the
cross-bridges. During this shortening of the sarcomeres, there
is no change in the lengths of either the thick or thin ﬁ
). This is known as the
of muscle contraction.
During shortening, each myosin cross-bridge attached
to a thin ﬁ
lament actin molecule moves in an arc much like
an oar on a boat. This swiveling motion of many cross-bridges
forces the thin ﬁ laments attached to successive Z lines to
move toward the center of the sarcomere, thereby shorten-
ing the sarcomere (
). One stroke of a cross-bridge
produces only a very small movement of a thin ﬁ lament rela-
tive to a thick ﬁ lament. As long as a muscle ﬁ ber remains acti-
vated, however, each cross-bridge repeats its swiveling motion
many times, resulting in large displacements of the ﬁ laments.
It is worth noting that a common pattern of muscle shorten-
ing involves one end of the muscle remaining at a ﬁ xed posi-
tion while the other end shortens toward it. In this case, as
ﬁ laments slide and each sarcomere shortens internally, the
center of each sarcomere also slides toward the ﬁ
xed end of
(a) Electron micrograph of a cross section through three
myoﬁ brils in a single skeletal muscle ﬁ ber. (b) Hexagonal
arrangements of the thick and thin ﬁ laments in the overlap
region in a single myoﬁ bril. Six thin ﬁ laments surround each
thick ﬁ lament, and three thick ﬁ laments surround each thin
ﬁ lament. Titin ﬁ laments and cross-bridges are not shown.
From H. E. Huxley,
J. Mol. Biol.,
Draw a cross-section diagram like the one in part b for a
slice taken: (1) in the H zone, (2) in the I band, (3) at the
M line, and (4) at the Z line (ignore titin).
Answer can be found at end of chapter.
The sliding of thick ﬁ laments past overlapping thin ﬁ laments shortens the
sarcomere with no change in thick or thin ﬁ lament length. The I band
and H zone are reduced.