Chapter 5 The Muscular System 165
Maximum Tension and Return to
Relaxation
When receiving an action potential, a given
motor unit always develops maximum tension, a
physiological principle known as the all-or-none
law. But because each whole muscle includes
multiple motor units, simultaneous activation
of many motor units is required for the muscle
to develop maximum tension. The diagram in
Figure 5.8 displays the relationship between
number and frequency of action potentials and
the development of tension in the muscle. With
high-frequency stimulation, the muscle develops
a sustained, maximal level of tension called
tetanus.
Almost all skeletal motor units develop
tension in a twitch-like fashion, generating
maximum tension very briefl y and then
immediately relaxing. After the action potential
has traveled the length of the muscle fi ber,
chemical processes return the fi ber to its resting
state. Sodium ions diffuse back out of the cell
into the interstitial fl uid, and calcium ions
return to storage sites within the cell. The actin
fi laments slide back to their original positions as
the cross bridges release them, and the muscle
fi ber returns to a state of relaxation.
Skeletal Fiber Types
Why are some athletes especially good at
events or tasks that require endurance, whereas
others excel at activities that require explosive
strength or speed? The answer may have
something to do with the ways in which these
individuals train, but that is only a small part of
the explanation. In fact, a big part of why certain
people are better at particular activities and
sports may relate to the characteristics of their
skeletal muscle fi bers.
Skeletal muscle fi bers may be divided into
two umbrella categories—slow-twitch (Type I)
and fast-twitch (Type II). As the names suggest,
1. What structures make up a motor unit?
2. Describe the neuromuscular junction.
3. What is an action potential?
4. Why do you want a small motor unit for
fi ne motor skills?
5. Explain the all-or-none law.
6. How do muscles relax?
Check Your Understanding
Figure 5.8 Tension developed in a muscle in response to a single stimulus (A), in response to repetitive
stimulation (B), and in response to high-frequency stimulation, or tetanus (C). Do you think this graph represents
the activation of one or many motor units? Explain.
0
S = Stimulus S S S S S S S S S S S S S S S S S S S S S
Tension
Time (msec)
A B C