Human Physiology, 14th edition (2016)

374 Chapter 12

in response to the activation of motor units. By this means,

the contractions of skeletal muscles can be graded, or varied—

a requirement for the proper control of skeletal movements.

Because stimulation of muscle fibers usually results in their

full contractions, stronger muscle contractions are produced

mostly by recruitment of more muscle fibers into the con-

traction. Through variations in the numbers of muscle fibers

12.3 Contractions of Skeletal Muscles

Contraction of muscles generates tension, which allows

muscles to shorten and thereby perform work. The con-

traction strength of skeletal muscles must be sufficiently

great to overcome the load on a muscle in order for that

muscle to shorten.

LEARNING OUTCOMES

After studying this section, you should be able to:

6. Distinguish between the different types of muscle
   contractions.


7. Identify the series elastic component, and explain
   the length-tension relationship in striated muscles.

The contractions of skeletal muscles generally produce move-
ments of bones at joints, which act as levers to move the loads
against which the muscle’s force is exerted. The contractile
behavior of the muscle, however, is more easily studied in vitro
(outside the body) than in vivo (within the body). When a mus-
cle—for example, the gastrocnemius (calf muscle) of a frog—
is studied in vitro, it is usually mounted so that one end is fixed
and the other is movable. The mechanical force of the muscle
contraction is transduced (changed) into an electric current,
which can be amplified and displayed on a recording device
( fig. 12.18 ). In this way the contractile behavior of the whole
muscle in response to electric shocks can be studied.

Twitch, Summation, and Tetanus

Contractions of isolated muscles in response to electrical
shocks mimic the behavior of muscles when they contract
within the body. When the muscle is stimulated with a single
electric shock of sufficient voltage, it quickly contracts and
relaxes. This response is called a twitch ( fig. 12.18 a ). The
shock produces an action potential that is conducted along the
sarcolemma and stimulates the release of Ca^2 1 from the sarco-
plasmic reticulum. These events occur during the latent period
between the stimulus and the contraction; the Ca^2 1 then binds
to troponin and stimulates the muscle twitch. If a second elec-
tric shock is delivered before the muscle has had a chance to
fully relax from the first twitch, the second twitch will “ride
piggyback” on the first. This response is called summation
( fig. 12.18 b ).
Increasing the stimulus voltage increases the frequency of
action potentials and the amount of Ca^2 1 in the sarcoplasm,
increasing the strength of each fiber’s contraction. Increasing
the stimulus voltage also activates more muscle fibers, recruit-
ing them into the contraction. Analogous events occur in vivo

Figure 12.18 Muscle twitch and summation.

( a ) A single electrical shock to a muscle stimulates a muscle

twitch. There is a latent period of a couple of milliseconds

between the shock and the twitch, which can last from several to

a hundred milliseconds, depending on the muscle. ( b ) A second

shock delivered to the muscle before it has had a chance to relax

fully from its first twitch results in a second twitch that summates

with the first to produce a stronger contraction.

Muscle twitch

Stimulus

Summation

Twitches

Stimuli

(a)

(b)