Skeletal muscles interact to create movements by method of anatomical positioning and the combination summation of innervation signals.

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Learning Objectives

Explain the summation interactions of skeletal muscles and how they impact movement

Key Points

Muscle contractions have the right to be twitch, summation or tetanus. A twitch convulsion is the duration of contraction and relaxation that a muscle after ~ a single stimulation. Summation is the occurrence of additional twitch contractions prior to the vault twitch has totally relaxed. Summation deserve to be achieved by raising the frequency that stimulation, or through recruiting additional muscle fibers within a muscle. Tetanus occurs when the frequency the muscle convulsion is such the the maximal pressure is tension is generated without any kind of relaxation that the muscle.

Key Terms

tetanus: once the frequency the muscle convulsion is such the the maximal pressure is stress is generated without any relaxation that the muscle. summation: The incident of extr twitch contractions prior to the ahead twitch has fully relaxed. twitch: The period of contraction and also relaxation that a muscle ~ a solitary stimulation.

Skeletal muscle contractions can be grouped based upon the length and frequency that contraction.


When engendered by a single action potential a muscle contracts and then relaxes. The time in between the stimulus and also the initiation of convulsion is the latent period, i beg your pardon is followed by the convulsion period. At top contraction the muscle relaxes and returns to its resting position. Taken all together these three periods are a twitch.


Muscle Twitch Contraction: The time between stimulation and also contraction is the latent period. After convulsion the muscle relaxes back to a relaxing level the tension. With each other these 3 periods kind a single muscle twitch,


If secondary action potential were to stimulate a muscle contraction prior to a vault muscle twitch had totally relaxed climate it would amount onto this previous twitch enhancing the total amount the tension created in the muscle. This addition is summation. In ~ a muscle summation have the right to occur throughout motor units to recruit an ext muscle fibers, and additionally within motor devices by enhancing the frequency of contraction.

Multiple fiber summation

When a weak signal is sent by the main nervous device to contract a muscle, the smaller motor units, being more excitable than the bigger ones, are stimulated first. As the toughness of the signal increases, more (and larger) engine units room excited. The largest motor units have as lot as 50 times the contractile strength as the smaller ones; thus, as much more and bigger motor units room activated, the force of muscle contraction becomes progressively stronger. A ide known as the dimension principle permits for a gradation the muscle force throughout weak contraction to occur in little steps, which end up being progressively bigger as greater quantities of force are required.

Frequency summation

For skeletal muscles, the force exerted by the muscle can be regulated by varying the frequency at which action potentials are sent out to muscle fibers. Action potentials perform not come at muscle synchronously, and, during a contraction, just a certain percentage the the fibers in the muscle will certainly be contracting at any kind of given time. In a common circumstance, once a human is exerting as much muscular pressure as they are consciously able, about one-third that the yarn in the muscle will be contracting at once. This relatively low level of convulsion is a protective device to prevent damage to the muscle tissue and also attaching tendons and also structures.


If the frequency of activity potentials produced increases to together a point that muscle tension has reached the peak and also plateaued and also no be safe is observed then the muscle contraction is defined as a tetanus.

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Summation and Tetanus Contractions: repetitive twitch contractions, wherein the ahead twitch has actually not relaxed fully are dubbed a summation. If the frequency of these contractions rises to the allude where maximum stress and anxiety is generated and no be safe is observed climate the convulsion is a tetanus.

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