Complex Sustained Contraction

The muscular "twitch" just described plays an insignificant part in the voluntary action of muscles. Its physiology has largely been determined by the electrical stimulation method on the excised^[1] muscle. If, instead of a single stimulus, a series of stimuli in rapid successions be used, so that the second stimulus arrives before the effect of the first stimulus has worn off, we get, instead of a series of separate contractions, a sustained contraction known as physiological tetanus. The muscle remains contracted and shows a condition which we should expect if the stimulus were continuous. Moreover, in tetanus, the muscle shows a greater degree of contraction, a more powerful contraction, than in simple stimulation. This is but another example of the principle of summation of stimuli. All voluntary movement is tetanic in character.
The very shortest voluntary contraction is not induced by a single neural impulse, but a rapid series of such impulses. This is a fundamental characteristic of the neural current. The resulting action of the muscle, therefore, though in appearance a single unit of movement, results from the application of a series of stimuli. The shortest staccatissimo is not an exception and the reason for this speed of neural stimulation is found in the rate of the motor nerve, which has its own speed of neural discharge. Since this rate is physiologically predetermined and beyond our control, the fact that every voluntary or habitual movement is tetanic has no further direct bearing upon our problem.

Question: If a series of stimuli are applied to a muscular contraction in rapid succession, so that the second stimulus arrives before the effect of the first stimulus has worn off, do we get a series of separate contractions?
When a series of stimuli are applied to a muscle in rapid succession, such that the second stimulus arrives before the effect of the first stimulus has worn off, the resulting phenomenon is known as temporal summation, or wave summation. Instead of producing a series of separate contractions, the muscle responds with a sustained, forceful contraction that is stronger than the individual contractions would be if they occurred independently.
Temporal summation occurs because the second stimulus arrives before the muscle has had a chance to fully relax after the initial contraction. As a result, the muscle fibers are still partially contracted when the next stimulus arrives, leading to a progressive buildup of tension within the muscle. This continuous increase in tension can ultimately lead to a state of maximum, sustained contraction called tetanus.
There are two primary forms of temporal summation:

1. Unfused (incomplete) tetanus: In this type of summation, the stimuli are delivered at a frequency that allows for partial muscle relaxation between contractions. This results in a series of progressively stronger contractions that are not fully separate, but also not fully fused together.
2. Fused (complete) tetanus: In this form of summation, the stimuli are delivered at a high enough frequency that there is no relaxation between contractions. This leads to a smooth, continuous, and maximally forceful contraction of the muscle.

In summary, when stimuli are applied to a muscle in rapid succession, so that the second stimulus arrives before the effect of the first stimulus has worn off, the muscle does not produce a series of separate contractions. Instead, it exhibits temporal summation, resulting in progressively stronger contractions that can ultimately lead to a state of sustained, forceful contraction called tetanus.

Temporal summation is a phenomenon in muscular contraction where the force of a muscle contraction is increased by increasing the frequency of stimulation. This occurs because the muscle fibers do not have enough time to relax completely before the next stimulus arrives, resulting in the muscle fibers contracting more strongly.
Temporal summation is a type of muscle twitch, which is a single contraction of a muscle fiber in response to a single stimulus. A muscle twitch has three phases:

1. Latent period: The time between the stimulus and the onset of contraction.
2. Contraction period: The time during which the muscle fiber is contracting.
3. Relaxation period: The time during which the muscle fiber is relaxing.

If a second stimulus is applied to the muscle fiber before the relaxation period of the first stimulus is complete, the second contraction will begin before the first contraction has ended. This will result in a stronger overall contraction, as the two contractions summate.
The force of a muscle contraction can be increased by increasing the frequency of stimulation up to a certain point, called the tetanus frequency. At the tetanus frequency, the muscle fibers are contracting so quickly that they are unable to relax completely between stimuli, resulting in a smooth, sustained contraction.
Temporal summation is an important mechanism for controlling muscle force. For example, when you lift a heavy object, your muscles must contract more forcefully than when you lift a light object. This is achieved by increasing the frequency of stimulation to the muscle fibers.
Temporal summation is also important for other activities, such as running and jumping. When you run, your muscles must contract and relax very quickly in order to produce a smooth, rhythmic movement. This is achieved by increasing the frequency of stimulation to the muscle fibers.
Here is an example of temporal summation in action: Imagine you are trying to lift a heavy box. Your brain will send a signal to your muscles to contract, which will result in a single muscle twitch. If the box is too heavy for you to lift with a single muscle twitch, your brain will send more signals to your muscles to contract at a higher frequency. This will result in temporal summation, and your muscles will be able to produce a stronger contraction.
Temporal summation is an important physiological mechanism that allows us to control muscle force and perform a variety of activities.

[1]excise: [Latin excisus, pp. of excidere: see EXCIDE] to remove, as a tumor, by cutting out or away. - excision n.