Muscles and Muscle Tissue
Finally, as the stimulation frequency continues to increase,
muscle tension increases until it reaches maximal tension. At
this point all evidence of muscle relaxation disappears and the
contractions fuse into a smooth, sustained contraction plateau
tense) (Figure 9.15c). (Note that this term is o±en confused
with the bacterial disease called tetanus that causes severe invol-
untary contractions.) In the real world, fused tetanus happens
infrequently, for example, when someone shows superhuman
strength by li±ing a fallen tree limb oﬀ a companion.
Vigorous muscle activity cannot continue indeﬁnitely. Pro-
longed tetanus inevitably leads to muscle fatigue, a situation in
which the muscle cannot contract and its tension drops to zero.
Extraocular muscle (lateral rectus)
(a) Myogram showing the three phases of an isometric twitch
(b) Comparison of the relative duration of twitch responses of
The muscle twitch.
(a) A single stimulus is delivered. The muscle contracts and
Maximal tension of a single twitch
(b) If another stimulus is applied before the muscle relaxes
completely, then more tension results. This is wave (or
temporal) summation and results in unfused (or incomplete)
(c) At higher stimulus frequencies, there is no relaxation at all
between stimuli. This is fused (complete) tetanus.
Low stimulation frequency
unfused (incomplete) tetanus
High stimulation frequency
fused (complete) tetanus
A muscle’s response to changes in stimulation
(Note that tension is measured in grams.)