How Muscular Relaxation Works

March 12, 2016    BY David Coulter

Relaxation is one of the most important skills in hatha yoga. It has many dimensions—mental, emotional, autonomic, and muscular. We’ll pay most attention to relaxation of skeletal muscles, because their connections with the nervous system are relatively simple, and because they can generally be brought under conscious control. We’ll defer comment for the moment on mental, emotional, and autonomic complications, and assume that you are calm, healthy, and free of pain.

You can start to work with muscular relaxation in savasana, the corpse posture. Lie on a padded but relatively firm surface, one that allows your spine to flatten slightly against the floor, but still provides some support for the lumbar region. A down pillow is helpful. It supports your neck while allowing the back of your head to be at about the same level as the rest of your body. Hands are away from the body, palms up. Feet are apart. If the room isn’t warm, cover up. Thrust your hips, heels, hands, and head away from your torso, flattening your spine against the floor. Then adjust your body a little this way and that to get settled. Get perfectly situated, and don’t move again until you are ready to break the relaxation. If you feel a need to move in a minute or so, you’ll know you didn’t get positioned ideally in the first place.

Breathe naturally. In the supine corpse posture, this means your abdomen will rise with each inhalation and fall with each exhalation. Your chest will not be moving. There are other possibilities for breathing, but for a beginning exercise, the motion should be only in the abdomen.

To relax, there is one thing to do and two things not to do. Keep your mental attention actively focused: do a mental exercise—any exercise—that holds your attention in the body and not on outside concerns such as lunch, football, job, or relationships. Second, be still. Moving fires up the nervous system and sabotages relaxation. Third, don’t sleep. Your goal is active relaxation, not a nap.

To relax, there is one thing to do and two things not to do.

Any one of dozens of exercises can hold your attention. You can concentrate sequentially on different parts of the body for a “systematic relaxation.” You can do various exercises concentrating on selected points on the surface of the body. You can sweep your attention down and up the spine with each exhalation and inhalation. You can concentrate on the rise and fall of your abdominal wall or on your heart center. The main idea is to keep your mind occupied and your attention within the body.

Two Questions

The essential questions are: how do muscles relax, and what determines their relaxation? You may be thinking that it must involve some complex planned activity. That is not the case. It is elegant and simple. If you hold your attention appropriately, and neither move nor sleep, most of your skeletal muscles will relax themselves. How that happens is the subject of this column.

Three Elements

If you wanted to analyze a machine, let’s say an electric typewriter, you would take note of three things: its output, its structure, and its input. Its output is the typewritten page, its structure includes keys, electronic circuitry, and type bars, and its input is the person typing. Here we’ll be looking at three analogous elements, all within the body, to analyze relaxation. I’m going to define the output as the muscle fibers that are active or relaxed, the structural component as the class of cells in the nervous system called motor neurons, and the input as all of the factors that influence the motor neurons.

Muscle Tissue

The skeletal muscles account for almost half of our body weight. They execute all of our actions, gestures, speech, respiration, and every willful or habitual movement of the body. They bring the output of the machine we call “body” to fruition.

The Motor Neuron 

Motor neurons innervate skeletal muscles, meaning they supply them with nerve fibers. Motor neurons are absolute dictators; they control skeletal muscles as actors hidden overhead control puppets. You see only the activity of the muscles, just as you see only the movements of puppets. When motor neurons transmit nerve impulses, the muscle fibers contract, and when they don’t, the muscle fibers relax. The motor neuron is like the typewriter in the prior analogy, responding to input and delivering output.

Marching Orders 

Although the motor neurons exert absolute control over the muscles, they are only agents of the body and mind as a whole, just as the typewriter is the agent of the person doing the typing. The sources of the orders to the motor neurons are many: habits, addictions, and willful decisions of the mind; hearing, sight, taste, smell, and touch; and internal discomforts from stretched muscles, sore ankles, or an overloaded stomach.

The Motor Unit

The functional entity in which we are interested is the motor unit—a single motor neuron plus all the individual muscle fibers that it innervates. Motor units in large postural muscles can contain 500-1,000 muscle fibers. Every time a nerve impulse travels down the neuron, all the muscle fibers in the motor unit contract, and when many motor units act repetitively and in concert, the entire muscle contracts. In the kind of biofeedback training that utilizes electromyography, the subject gains control of the gross muscles by gaining control of the individual motor units.

Neuronal Parts

To understand muscular relaxation, we need to look at the input to the motor neuron, its component parts, and its output. We’ll start with the parts. It has three: dendrites, cell body, and axon. Dendrites and cell bodies are located in the spinal cord and brain, and axons travel in nerves to the skeletal muscle fibers of the motor units.

To understand muscular relaxation, we need to look at the input to the motor neuron, its component parts, and its output.

The axon and its branches are the parts of the neuron that carry out the action. They are like type bars in a typewriter. Axons carry nerve impulses to muscle cells, exciting them to activity. A very active neuron might carry 40-50 nerve impulses per second to its muscle fibers, and a moderately active neuron might carry 10-20 nerve impulses per second. A completely relaxed muscle indicates that no nerve impulses are being carried down the axon.

(A misconception still common among both clinicians and biologists is that muscles always continue to receive input from their motor neurons, even at rest. Four decades of study with electromyography, using needle and thin-wire electrodes, has proven this is not the case. Most motor units can become totally silent with biofeedback training.)

Dendrites and cell bodies are like the keyboard. They receive information. The typewriter keyboard receives information from the person typing, and dendrites and cell bodies receive information from thousands of other neurons.

Facilitation and Inhibition

Dendrites and cell bodies are specialized to receive information from axon terminals of other neurons from all over the nervous system. The plot now thickens. A motor neuron is a lot smarter than a typewriter. Some axon terminals signal the motor neuron to increase its number of nerve impulses per second, and other axon terminals signal it to decrease them. Input calling for increased activity of the motor neuron is called facilitory, and input calling for decreased activity is called inhibitory. The motor neuron integrates the input from other neurons to reflect a summary of their “wishes” in firing off its nerve impulses.

During relaxation, you will decrease the number of nerve impulses per second to the muscle fibers in three ways: by decreasing the rate of firing of the facilitory neurons’ axon terminals to the dendrites and cell bodies of the motor neurons, by increasing the rate of firing of the inhibitory neurons’ axon terminals, or by both factors in combination. All of the marching orders to the motor neurons—habits, addictions, will, and sensory input from all sources—are translated into simple facilitory and inhibitory inputs to their dendrites and cell bodies, and their axons respond in lockstep to that summated and cumulative input.

And in Practice

The most important factor in muscular relaxation is remaining still. If you are able to keep your mind awake while holding your attention within the body, and if you don’t move for some minutes, your motor neurons become completely inactive. They not only become inactive as reflected by the number of nerve impulses dropping to zero; they become so inhibited (remember the restricted physiological use of this term) that a large facilitory input is required to fire them into activity.

This extreme inhibition can have unexpected results. Until you become experienced in quickly gearing up the motor neurons after a long relaxation, you may find that you can’t move immediately on command. For example, a doorbell may ring after you are well into a relaxation. You think, well, I’ll just jump right up and answer the bell. Surprise. You can’t do it. You feel paralyzed. It may take you 4-5 seconds before you are able to move. It can be quite startling.

This temporary paralysis confers a warning. Don’t overdo relaxation. Be moderate. It is tempting, especially among meditators, to do excessive relaxation. Pretty soon you are relaxing before and after meditation, before and after meals, and even before and after a night’s sleep. Relaxation is best balanced with hatha. Get a good night’s sleep, go to the bathroom, take a shower, and practice asana enthusiastically for an hour. Then relax fully in the corpse posture. Or, if you are interested in aerobic exercise, you can do a short relaxation, stretching, vigorous exercise, more stretching, and another relaxation afterwards.

Don’t overdo relaxation.

Why these precautions? Because doing long relaxations several times a day without other practice seems to diminish one’s control over the motor neurons. The motor portion of the nervous system gets lazy. Most of us will do better to get more vigorous activity, and to practice relaxation only once or twice a day.

So, coming back to our two initial questions, how do muscles relax, and what determines their relaxation? First, they relax when, and only when, their motor neurons become silent. Second, motor neurons are silenced when inhibitory input to those neurons exceeds the facilitory input. Third, those influences are summed from all our habits, will, addictions, and sensory input.

Complications

Other dimensions of relaxation can sometimes reveal themselves when you relax your skeletal muscles. If you lie down to relax, and find yourself experiencing increased anxiety rather than mental calm and clarity, you may be experiencing “relaxation-induced anxiety.” It is not uncommon. If the symptoms do not dissipate with practice, psychological issues may have to be addressed. Vigorous exercise and hatha yoga may also be helpful; such activities are generally conducive to pleasant mental states.

The autonomic nervous system helps regulate functions of internal organs. A few of these are important to relaxation. If you lie down to relax your skeletal muscles and notice that your hands and feet are cold and sweaty, or if you get dizzy or headachy, or if your heart is beating too fast, those findings are evidence that certain autonomic functions are overly active. In such a case, biofeedback techniques (other than electromyography) can be helpful in teaching you how to diminish the symptoms. Such training highlights the multidimensional nature of relaxation and can help individuals learn to relax more globally.

Pain stops relaxation in a hurry. It magnifies undesirable mental states, disrupts autonomic functions, and prevents muscular relaxation. If you have low back pain, shoulder pain, neck pain, gas pain, arthritis, or any other physical discomfort, your mind will be diverted to your pain as soon as you lie down to relax. If you are in more than the mildest discomfort in the supine corpse posture or the prone crocodile posture, the discomfort will prevent relaxation. So first address the pain; then work with relaxation.

What to Do

Practice relaxation regularly, and practice in moderation. If you have problems, do more therapeutic exercise and hatha yoga. These not only alleviate unwanted mental and emotional states; they can also benefit the autonomic nervous system and miscellaneous aches and pains. The time, patience, and intensity of effort needed are roughly proportional to your age. You might as well start. If you don’t make an effort, you’ll definitely get worse, and more likely sooner than later.

This article originally appeared in the January/February 1993 issue of Yoga International.

David Coulter
David Coulter, Ph.D., studied and taught in the field of anatomy and physiology of hatha yoga.

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