simultaneously. To record how a whole muscle responds to
stimulation, a skeletal muscle can be removed from a frog or
other small animal and mounted on a special device. The mus-
cle is then electrically stimulated, and when it contracts, it pulls
on a lever. The lever’s movement is recorded as a myogram.
The myogram results from the combined twitches of muscle
F bers taking part in the contraction, so it looks essentially the
same as the twitch contraction depicted in F gure 9.15.
Sustained contractions of whole muscles enable us to
perform everyday activities, but the force generated by those
contractions must be controlled. ±or example, holding a styro-
foam cup of coffee F rmly enough that it does not slip through
our F ngers, but not so forcefully as to crush it, requires pre-
cise control of contractile force. In the whole muscle, the force
developed reﬂ ects (1) the frequency at which individual mus-
cle F bers are stimulated and (2) how many F bers take part in
the overall contraction of the muscle.
The force that a muscle F ber can generate is not limited to
the maximum force of a single twitch
(f g. 9.17
. A muscle
ber exposed to a series of stimuli of increasing frequency
reaches a point when it is unable to completely relax before
the next stimulus in the series arrives. When this happens,
the individual twitches begin to combine, and the contrac-
tion becomes sustained. In such a
force of individual twitches combines by the process of
). When the resulting forceful, sustained
contraction lacks even partial relaxation, it is called a
Recruitment of Motor Units
The number of muscle F bers in a motor unit varies consider-
ably. The fewer muscle F bers in the motor units, however,
the more precise the movements that can be produced in a
particular muscle. ±or example, the motor units of the mus-
cles that move the eyes may include fewer than ten muscle
contract at their optimal lengths. Some activities, such as
walking up stairs two at a time or lifting something from an
awkward position, put F
bers at a disadvantageous length
and compromise muscle performance.
A muscle f
ber brought to threshold under a given set oF conditions
contracts completely, and each twitch generates equal Force. This
response. However, “all-or-none” is misleading,
because in normal use oF muscles, the Force generated by muscle
bers and by whole muscles must vary.
Understanding the contraction of individual muscle
F bers is important for understanding how muscles work, but
such contractions by themselves are of little signiF cance in
day-to-day activities. Rather, the actions we need to perform
usually require the contribution of multiple muscle F
Muscle fiber length
Force of contraction
A myogram oF a single muscle twitch.
The Force a muscle f
can generate depends on the length to
which it is stretched when stimulated.