HUMAN BIOLOGY

the MUSCULar SYSteM 113

tension.” When a muscle contracts isotonically, it shortens
as it moves a load (Figure 6.15A). Isometric means “same
length.” When a muscle contracts isometrically it con-
tracts and develops tension but doesn’t shorten. This hap-
pens when you attempt to lift an object that is too heavy
(Figure 6.15B).

a tired muscle can’t generate much force

When steady, strong stimulation keeps a muscle in a state
of tetanus, the muscle eventually becomes fatigued. Then
the muscle’s ability to generate force—that is, to develop
tension—plummets. After a few minutes of rest, however, a
fatigued muscle will be able to contract again.
How long does this recovery take? That depends in part
on how long and how often the muscle was stimulated

Figure 6.14 each motor unit contraction is a muscle

twitch. This figure shows recordings of twitches in

muscles artificially stimulated in different ways. A A single

twitch. b About twenty twitches per second cause tetanic

contraction. c Monitoring activity in a patient’s muscle.

repeated stimulation

Time

Force

Force

Astimulus twitch

B twitch

tetanic contraction

c

before. Muscles trained by

a pattern of brief, intense

exercise fatigue recover rap-

idly. Regular weight lifting

provides this sort of muscle

training. Muscles used in

prolonged, moderate exercise

fatigue slowly but take lon-

ger to recover, often up to a

day. In addition to oxygen

debt, other factors may con-

tribute to muscle fatigue. For

instance, a muscle’s supply of

glycogen, the form in which muscles hold glucose in reserve

for energy, may run out. In addition, research suggests that in

tiring muscles, calcium starts to leak out of the SR (sarcoplas-

mic reticulum, Section 6.4). As a result there is less calcium

available to spark muscle contractions. It’s possible that such

mechanisms are “safety valves” that help prevent serious

muscle damage from overexercising.

What are the properties of Whole muscles?

• In a whole muscle, the fibers are organized into motor units. This
  arrangement permits variations in how whole muscles contract.


• A motor unit consists of a motor neuron and the muscle cells
  it serves. In a muscle twitch, the cells contract simultaneously.


• The number of motor units in a muscle correlates with how
  precisely the nervous system must control a muscle’s activity.


• Skeletal muscles normally contract in a sustained manner called
  tetanus. Healthy muscles maintain good muscle tone even when
  they are relaxed.

takE-homE mEssagE

Figure 6.13 Muscle cells are organized into motor units.

Spinal cord (section)

Axons of

two neurons

Neuromuscular

junctions

Muscle

fibers

One motor unit

Another motor unit

Muscle

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© Cengage Learning

Figure 6.15 Muscle contractions may be isotonic or isometric.
A In an isotonic contraction, the load is less than a muscle’s peak
capacity to contract, so the muscle can contract, shorten, and
lift the load. b In an isometric contraction, the load exceeds the
muscle’s peak capacity. The muscle contracts but can’t shorten.

b Isometric contraction.

Muscle tension is less

than the opposing force

and the muscle remains

at the same length,

rather than shortening.

A Isotonic contraction.
Muscle tension is greater
than the opposing force
and the muscle shortens,
as when you lift a light
weight.

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