Covering, Support, and Movement of the Body
Increasing the stimulus intensity beyond the maximal stimu-
lus does not produce a stronger contraction. In the body, the
same phenomenon is caused by neural activation of an increas-
ingly large number of motor units serving the muscle.
Te recruitment process is not random. Instead it is dictated
. In any muscle:
Te motor units with the smallest muscle ﬁbers are activated
ﬁrst because they are controlled by the smallest, most highly
excitable motor neurons.
As motor units with larger and larger muscle ﬁbers begin to
be excited, contractile strength increases.
Te largest motor units, containing large, coarse muscle ﬁ-
bers, have as much as 50 times the contractile force of the
smallest ones. Tey are controlled by the largest, least ex-
citable (highest-threshold) neurons and are activated only
when the most powerful contraction is necessary.
Why is the size principle important? It allows the increases
in force during weak contractions (for example, those that
maintain posture or slow movements) to occur in small steps,
whereas gradations in muscle force are progressively greater
when large amounts of force are needed for vigorous activities
such as jumping or running. Te size principle explains how the
same hand that lightly pats your cheek can deliver a stinging
slap at the volleyball during a match.
the motor units of a muscle may be recruited
simultaneously to produce an exceptionally strong contraction,
motor units are more commonly activated asynchronously. At
a given instant, some are in tetanus (usually unfused tetanus)
while others are resting and recovering. Tis technique helps
prolong a strong contraction by preventing or delaying fatigue.
It also explains how weak contractions promoted by infrequent
stimuli can remain smooth.
Muscle Response to Changes in Stimulus Strength
summation contributes to contractile force, but its primary
function is to produce smooth, continuous muscle contractions
by rapidly stimulating a speciﬁc number of muscle cells.
, also called
multiple motor unit summation
trols the force of contraction more precisely. In the laboratory,
recruitment is achieved by delivering shocks of increasing volt-
age to the muscle, calling more and more muscle ﬁbers into play.
Stimuli that produce no observable contractions are
Te stimulus at which the ﬁrst observable contraction occurs
is called the
. Beyond this
point, the muscle contracts more and more vigorously as the
stimulus strength increases.
is the strongest stimulus that in-
creases contractile force. It represents the point at which all
the muscle’s motor units are recruited.
Proportion of motor units excited
Strength of muscle contraction
Stimuli to nerve
Relationship between stimulus intensity (graph
at top) and muscle tension (tracing below).
voltage, the tracing shows no muscle response (stimuli 1 and 2).
Once threshold (3) is reached, increases in voltage excite (recruit)
more and more motor units until the maximal stimulus is reached
(7). Further increases in stimulus voltage produce no further increase
in contractile strength.
The size principle of recruitment.
of motor neurons controlling skeletal muscle ﬁbers is orderly and
follows the size principle.