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Muscles are capable of almost complete relaxation and of great contraction. When they contract they shorten. All muscles cause some movement when they contract. The majority of muscles start on one side of a joint and end via a tendon or ligament on another side of the joint. The purpose of most muscles is to make a joint move. A few muscles may just make the skin move as in the muscles of facial expression.
One might think that when a muscle is relaxed that it has no contraction at all. This is not the case. There are very few times when a muscle has no contraction at all in it. It happens after a severe injury to the spinal cord when the paralyzed muscles become completely flaccid. It can happen after a stroke and shortly after death. Even when a muscle is at total rest it has some contraction in it at all times. This amount of contraction is called the "resting muscle tone." In everyday life we refer quite accurately to the phenomenon of muscle tone. When someone is out of shape and flabby we can see and say that they have poor tone. The individual muscles are not visible and are mushy to the touch. When a person is buff and fit, we see the outlines of the muscles and the slight bulge of the belly of the muscle and we say they look great and "toned." The concept is the same in medicine.
There is a reason that all muscles always have some tone, some level of contraction, even when they are not being used. It takes a lot of energy to go from completely relaxed and long to contract enough to initiate movement. If we had to do this with every muscle every time we wanted to move, just the slightest movements would be exhausting. In children with abnormalities of low muscle tone we see this. "Floppy" babies move very little and become exhausted with the simple movements of controlling their head or with nursing. To prevent disabling fatigue, all of our muscles stay slightly contracted to be immediately ready for the command from the brain to move. Constant tone or "contraction" in the muscles is required for us to maintain ourselves upright or to remain standing. The resting tone varies with our posture, are internal environment and our activity. If we are lying back in a recliner our overall tone will be lower than if we are gearing up for a sprint or a fight. The muscles around an injury will often contract fiercely, called spasming, in the bodies attempt to immobilize the injured part.
Though muscle tone can be measured in the movement laboratory or by EMG, in day to day practice tone is measured subjectively by the doctor or the physical therapist by passively moving our limbs back and forth. It takes a long time and a lot of patients to become comfortable with what the range of normal tone" is. When you are sitting with your legs dangling off the examining table, and the examiner grabs your ankle and pumps your lower leg out and back, they expect to feel very little - but some - resistance. It should be pretty much the same resistance whether they pump it fast or slow. And the amount of resistance should be the same on both sides. This resistance is the current amount of tone in those muscles.
Now, at different times the resting muscle tone will be higher or lower, depending on the circumstances. If you sense you are in danger or are anxious, your muscle tone will rise readying you for immediate action. Chronic anxiety can result in muscles with abnormally high tone and tension. Pain will raise the resting muscle tone, especially close to the site of injury. If you have had an incredible massage and then a soak in a hot bath (pretend you don't have MS) your tone may dip very low. If someone were to shake your arm it at this time the muscles might flop around like it didn't belong to a live person. We have all experienced that. Also, if you decide at that moment (for some bizarre reason) to become very active, you would find that it takes a huge amount of effort to get going, and you would become quickly exhausted.
In certain neurological conditions the muscle tone in certain muscle groups is altered - when it is chronically increased you have a condition known as "spasticity." Spasticity causes an imbalance with the other muscle groups and interferes with smooth and easy movement. One of the concepts you need to understand in understanding spasticity is the existence of opposing muscle groups. For almost every action that muscles do, there is another set of muscles to do the opposite action.
The Example of Bending the Elbow
If you need to use your biceps to flex (bend) your elbow, you contract the biceps, and the brain sends a signal to the triceps to relax and allow the movement. But, to keep the action smooth and controlled, all muscles have a nerve endings called "stretch receptors." In our example of bending the elbow, the triceps relaxes, lengthens and its stretch receptors are activated causing it to pull back on the elbow slightly. This gives you the capability of making very fine movements and of quickly adjusting your motions. If you then want to straighten the arm, you contract your triceps to extend the elbow and make it straight again. The brain commands the triceps to contract and the biceps to relax to allow the arm to straighten. It is always a give and take as you move. All of this happens automatically and generally below your conscious level.
In spasticity the whole thing goes awry. I am going to use my own experience as an example. My first presenting symptom of MS was spasticity in my right leg all the way from my right lower back to my foot. The groups of muscles called the "extensors" had an increased tone in them at all times. Their stretch receptors are always activated causing the extensors to actively contract. The extensors arch the lower back, swing the leg backwards at the hip, straighten the knee, and pull the toes of the foot downward. I found that it took a lot of effort to lift my right foot off the floor when I was standing. Stairs were suddenly difficult. My brain would command the hip "flexors" to bring the knee up, but the signal to relax the hip and leg extensors never happened. They didn't relax, but instead continued to "pull" against my efforts. As I walked, every stride with the right leg felt as though I had to forcibly drag the leg forward. The simple act of walking became tiring. If I laid on my back to sleep or nap, the increased tone in the right side paraspinal muscles (back extensors) would cause those muscles to keep contracting until they were spasmed into a knot. I learned I had to sleep with my hips bent and began sleeping in my recliner.
I no longer could bend my knee to bring my foot back up toward my buttocks like you do to look at the sole of the foot. The non-stop tone, or spasticity, of the knee extensors (the quads, which work to keep the knee straight) kept me from performing that action.
I also tripped all the time catching my right toe on the floor. I would trip over a simple grout line! It didn't make sense, because I could bend my toes all the way up toward my knee. I should have been able to bring the toes up to clear the floor, but it was hard. When I walked, though, the extensor tone (in my calf) was stronger than my ability to bring up my toes. Because of this my toes pulled down as I walked, constantly tripping me. I had a "functional" foot drop. That is why I was fitted for a brace.
Spasticity is Velocity-Dependent
Spasticity is highly influenced by the speed with which one tries to move. This means that the faster the message is to contract against the tight muscles, the stronger the signal is to keep them rigid. Use the example of the person sitting on an exam table dangling the legs. If the examiner pumps the leg out and back slowly they may feel a little increased resistance over the normal level. But, the faster they pump the leg, the higher the resistance to their attempts. Many of you have seen this in action. You have a leg that often doesn't want to move. It feels like you have a 20# weight on your foot keeping your pace slow. But, you can move. However, if you need to step with that foot fast (say to catch yourself off balance or to dodge a thrown object) the foot is stuck, like it is literally nailed down. That signal to move fast, instantaneously increases the tone in the spastic muscles.
My spasticity was mild. But, as spasticity worsens, movement becomes harder and harder. The muscles become rigid, and the person can no longer overcome the higher tone. As the signal grows to keep the tone up the muscles can became extraordinarily painful and knotted. Eventually the muscle can shorten and "freeze." Finally, the joints can actually become rigidly stiff. They may be straight or twist permanently into unnatural shapes - a condition called "contracture." To prevent permanent shortening and contracture, spastic muscles must be continually stretched multiple times a day. Trained professionals, such as physiatrists and physical therapists are needed to guide this treatment.
Sometimes, spasticity works somewhat to a person's advantage. A common problem that accompanies spasticity is muscle weakness. The muscle doesn't get a clear signal to contract, or some of the fibers get no signal at all, and the muscle shows weakness. If a person has extensor spasticity, this tends to help them maintain an upright posture. If they are unfortunate to have increased tone in the flexors, it can, for example bend them over, bend the knees, and pull the elbows into tight flexion. This is incapacitating and very disabling.
Spasms are the nonstop contraction of a muscle or muscle group. They are caused by the non-stop signal from a damaged nervous system to "Keep on Contracting." It can happen without spasticity, but more often the underlying cause is spasticity with unrelenting, increased tone in that group of muscles. Muscle groups may spasm then release over and over or be a sustained contraction.
Related to spasms are muscle cramps. In a muscle cramp the muscle fibers contract into a knot. But, the cause is different. Cramps are the result of a disturbance of the muscle itself. It may be severely fatigued, the blood electrolytes like potassium and magnesium may be low, or the blood calcium may be low. Or perhaps the oxygen supply to the muscle is decreased for some reason. A cold muscle is more likely to cramp up. Correction of the abnormal condition will relieve the cramps.
In both spasms and cramps the muscle holds a painful contraction. In spasms the muscle is "driven" to contract by a faulty motor nerve signal. In cramps, the contraction is caused by abnormal internal conditions within the muscle itself.