Lecture 9 Notes: Skeletal Muscle Continued
How do Isometric Contractions Occur?
When a muscle contracts isometrically, tension increases in the muscle but the overall length of
the muscle does not change. This occurs because there are two “components” to the muscle; a
contractile component consisting of the thick and thin filaments (myosin and actin, respectively)
and a series elastic component consisting of tendons and other connective tissues. During an
isometric contraction, the contractile component shortens while the series elastic component
lengthens such that there is no overall change in the length of the muscle.
During an isotonic contraction, tension initially increases (isometrically) until it is sufficient to
lift the load. At that point the muscle contracts isotonically (it shortens at a constant tension).
During this period, both the series elastic and contractile components shorten meaning that the
entire muscle shortens.
Effects of High Frequency Stimulation
A single stimulus to a muscle will lead to a muscle twitch. As the stimulus frequency increases
you begin to see various types of contraction such as treppe, summation, incomplete tetanus and
Treppe occurs when the stimulus frequency increases but the frequency is not so great as to
prevent muscle tension from returning to the baseline level following every contraction. However,
there does reach a point where a peak tension is reached. This is likely caused by increased
cytosolic levels of calcium due to incomplete removal of Ca++ back into the sarcoplasmic
Summation and tetanus occur when the stimulus frequency is high enough that individual
twitches begin to fuse together. This can occur because a muscle action potential is very short
(approximately 5 ms) compared to the duration of a muscle twitch (which can last over 100 ms).
Therefore, many muscle action potentials can arrive at the muscle during the time it takes for one
Summation occurs when the stimulus frequency is great enough that a second twitch begins
before the first twitch is complete and so on. This is due to the muscle APs arriving before the
twitch is over. In this case the twitches (increases in tension) superimpose on one another leading
to a larger amount of force production than with a single twitch.
As the stimulus frequency increases even further, tetanus develops. At first, incomplete (unfused)
tetanus occurs in which individual twitches are still discernable and the tension oscillates around
a constant average. With even greater levels of stimulus you get fused (complete) tetanus in
which the twitches are no longer discernable as individual events and tension is constant.