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Mechanical
properties. The physical aspects
Vertebrates
are able to move about and to exert and bear forces because
of the contraction of the striated muscles. These activities
usually involve several structures operating in different
ways. The skeleton to which the muscles are attached operates
as a lever system. When a muscle shortens, it moves the
joints that it spans. In addition, in coordinated movement
usually several muscles contract in different ways. As some
muscles shorten, others develop a force while at a fixed
length, and still others may be lengthened by an external
force even as they contract.
The
force that a muscle develops is a “pulling” force, never
a “pushing” force. If the load is small enough, the muscle
can shorten and produce a pulling motion (an isotonic condition).
If the load is just equal to the maximum force the muscle
can develop, the length of the muscle will remain the same
(an isometric condition). An even larger load will stretch
the muscle.
The
size and the rate of the mechanical responses to stimulation,
whether by a nerve in the body or by direct electrical shocks
of an isolated muscle, depend on the muscle and the temperature.
In a frog sartorius muscle (of the leg) at 0° C (32° F),
the action potential reaches its peak of depolarization
about 1.5 milliseconds after the stimulus.
The
very early tension changes require much more rapid and sensitive
measuring and recordinginstruments than are necessary for
studying other aspects of the contraction process. The latent
period, the first seven milliseconds, is the amount of time
needed for the electrical signal,which appears as an action
potential at the surface membrane, to be translated and
travel to the contractile apparatus within the muscle fibres.
The explanation for latency relaxation (a four-millisecond
period during which the tension drops slightly), however,
is not so clear. It may be related to a change in shape
of the sarcoplasmic reticulum, which releases a large amount
of calcium ions at about the time latency relaxation occurs.
The tension begins to rise after 15 milliseconds.
