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Muscles
that work skeletons
Figure
4 shows a simple system, in which a skeleton is worked by
muscles. The two rigid parts of the clam shell (Figure 4A)
are hinged together. They canbe closed to protect the animal
within or allowed to open. A block of rubbery protein, the
inner hinge ligament, lies just inside the hinge. When the
adductor muscle contracts it closes the shell, but in so
doing it compresses the inner hinge ligament. When it relaxes,
the ligament recoils elastically, reopening the shell. This
is an unusual system, in that it is worked by just one muscle.
Most other skeletal systems need muscles in antagonistic
pairs, in which each muscle is paired with a muscle of the
opposite effect.
This antagonism is illustrated by the human ankle (Figure
4B). The tibialis anterior muscle flexes the ankle (raising
the toes) and the soleus muscle extends the ankle. These
muscles make up an antagonistic pair. In this particular
case there is another muscle, the gastrocnemius, which cooperates
with the soleus, helping it to extend the ankle. (The gastrocnemius,
however, crosses the knee as well as the ankle and affects
both joints.)
The ankle is not a simple hinge joint. As well as flexion
and extension, it can exhibit inversion (the sole of the
foot faces the other leg) or eversion (the opposite movement).
These movements are controlled by the tibialis posterior,
which inverts the ankle, and the peronaeus muscles, which
are antagonistic and evert it (Figure 4B).
A
hinge such as the clam joint or the human knee performs
just one kind of movement, flexion/extension, expressed
in technical terms as allowing one degree of freedom of
movement. The human ankle performs two kinds of movement,
flexion/extension and inversion/eversion, allowing two degrees
of freedom. Ball-and-socket joints, such as the human hip,
allow three degrees of freedom. Most animal joints have
at least two muscles (an antagonistic pair) for each degree
of freedom.
Seldom are muscle fibres as long as a muscle, but many muscles,
such as the biceps in the human arm, are composed of relatively
long fibres lying nearly parallel to each other. These parallel
muscles are attached to tendons or apodemes only at their
extreme ends. Since muscle fibres can contract about one-third
of their resting length, this arrangement is suitable to
an extensive and quick movement. The deltoid muscle in the
human shoulder is said to be pennate: relatively short fibres
attach diagonally onto a tendon that penetrates far into
the muscle. The ankle muscles shown in Figure 4 are pennate
muscles, but most of the hamstring muscles (at the back
of the thigh) are parallel. The adductor muscles of the
shells of clams are parallel, but most of the leg muscles
of arthropods are pennate. A pennate muscle may contain
many more and shorter fibres than a parallel muscle of equal
mass. Therefore, the pennate muscle can exert a greater
force but cannot shorten a great deal; the parallel-fibred
muscle can exert only a relatively small force but can shorten
significantly. The presence of pennate muscle in a given
structure may have the same effect as a longer lever arm.
In the slender legs of arthropods, with insufficient space
for bulky muscles or long lever arms, many of the muscles
are pennate.
Tendons
and apodemes (in arthropods, chitinous rods that serve as
sites for muscle attachment) have elastic properties. Tendons
in the legs of mammals serve as springs, reducing the energy
cost of running: energy that is lost as the foot hits the
ground and decelerates the body is stored as elastic strain
energy in tendons and is subsequently returned in an elastic
recoil. An apodeme in the hind legs of locusts is one of
the important elastic elements in the catapult mechanism
that powers jumping.
