Did you know that posture results from rapid stimulation of skeletal muscles? This lesson describes the nature of a muscle twitch and explains how individual contractions are added together resulting in tetanus and good posture.
Whole Muscle Contraction
We all know that muscular contraction is needed to stand up and have good posture. We take it for granted that we can stand or sit with little thought or even effort. Have you ever wondered how our postural muscles contract for long periods of time so we can stand or sit? In this lesson, we’re going to describe how our nervous and muscular systems work together for good posture.
Before we examine how postural muscles maintain a contraction, we need to first look at a single muscle twitch, which is a single contraction in response to a brief threshold stimulation. Let me quickly note that a twitch in this context is different from involuntary twitching of muscle that we experience on occasion. A threshold stimulation is the smallest amount of stimulation that will actually result in a contraction.
Think of a threshold as being like a minimum amount of pressure needed to activate a key on your computer. Just resting your fingers on the keyboard has no effect. You need to apply a slight pressure in order to get a response. Likewise, a minimum amount of stimulation is needed to induce a muscular contraction.
We can observe a muscle twitch by removing the gastrocnemius from a frog and administering a small stimulus to a muscle. Let’s do an experiment using the gastrocnemius of a frog. First, remove the muscle from the frog. Hook the muscle up to a force transducer, which will then record force or tension development when the muscle contracts.
We can stimulate the muscle with an external power source. By gradually increasing the amount of stimulus strength, we can find the threshold level of stimulation. Remember, the tension developed from the threshold stimulus is a single muscle twitch, and that’s what we’re looking for. Ah, there it is – a muscle twitch.
The image you see on the screen now illustrates the different parts of a single muscle twitch, with time on the x-axis and tension or force development on the y-axis. The red arrow you see at the bottom marks the time at which the muscle is stimulated. Experimentally, muscle can be stimulated with a small electrical shock.
As you look closely, you will see a short period of time between stimulation and tension development, and that’s referred to as the latent period. This latent period is due to slack in the muscle that must be removed before the tension can develop and therefore be realized. Once the slack is removed, the muscle contraction develops tension, and we call this the contraction period. The relaxation period is the time during which tension is removed from the muscle. Note that the relaxation period is longer than the contraction period and the entire twitch is over in a matter of milliseconds.
So, what does a muscle twitch have to do with whole muscle contraction and even posture? In short, muscles maintain prolonged contraction by adding numerous individual contractions back-to-back, resulting in temporal or wave summation. Let’s do another experiment with our frog’s gastrocnemius to see how this works.
If we administer a single threshold stimulus, we get a single muscle twitch in response. What happens if we administer a second threshold stimulus after the muscle relaxes? As you can see, the muscle forms a second twitch, which is just like the first. That’s not very exciting. What happens if we stimulate the muscle before it has a chance to completely relax? Let’s stimulate the muscle during the relaxation period and see what happens.
Notice the second contraction is added, or summed, to the first contraction, yielding a prolonged contraction with more tension, or twitch summation. This staircase-like contraction is also referred to as treppe.
What happens if we apply repeating stimuli before the muscle has a chance to relax? Let’s stimulate the muscle repeatedly during the contraction period to find out.
In short, the muscle maintains a smooth contraction as long as it is stimulated. Tetanus or a tetanic contraction is a prolonged contraction with no relaxation. The tetanic contraction will continue until either the stimulus stops or the muscle fatigues. This is exactly how our postural muscles maintain a prolonged contraction. Our nerves stimulate the muscles in rapid fire fashion, resulting in the tetanic contraction so that we can stand or sit without jerking around or even falling down.
In summary, a muscle twitch is a single contraction resulting from a threshold stimulus, where a threshold stimulus is the smallest stimulus strength that actually results in a contraction. The latent period is the delay between the stimulation and the physical contraction. The contraction period is the time during which tension develops in the muscle, and the relaxation period is the time during which tension is removed from the muscle.
Twitch summation or treppe is the addition of a second twitch, resulting in greater tension, and it results from stimulating the muscle before it has a chance to relax completely. Tetanus is prolonged contraction without relaxation and results from repeating stimulation before the muscle has a chance to relax at all. Our postural muscles produce tetanic contractions, thus maintaining posture for long periods of time.
Once you have viewed this video, you should be able to describe what a muscle twitch is and what it can do, what the different stimulation periods are, and how a twitch summation works.