Muscle tone is a tricky subject. Gray’s Anatomy empirically denies that it exists, but perhaps, because it is a physiological topic rather than an anatomical one, his should not be the last word. He says it doesn’t show up on an electromyograph, so it can’t exist, but an electromyograph can’t pick up low level, steady-state action unless the motor units immediately adjacent to the contact are firing. It just reads as background noise.
Other authors postulate that there is an inherent elasticity in the muscles, and still others talk about muscle tone but don’t have a clear picture of what it is. They get it confused with muscle contraction to resist the constant pull of gravity, which is the static stretch reflex. Now even though this isometric contraction against gravity (internal against external forces), is a much greater contraction than muscle tone, it doesn’t show up very well on an electromyograph either, because it is also static and balanced; but no one denies its existence.
Because muscle fibers twitch, they have to do so in synergetic, co-active, symbiotic motor units. In other words there are overlapping motor units so that when one relaxes, another one comes on. So the contractions of muscle tone are turning on and off, but overlapping so smoothly and steadily, with no spiking contraction, that the electromyograph doesn’t read it. In regular skeletal muscle contraction of course, the electromyograph shows muscle contraction very clearly as spikes, and can measure how many pulses per second as well as the strength of each pulse.
True tonus is static and balanced, internal force against internal force, operated internally through the cerebellum and spindle. Muscle tone is a response to an inside pre-program in the cerebellum. The cerebellum is self-contained; it doesn’t want to know anything about outside sources, already has all the information it wants, and is just waiting to distribute it.
Muscle tone is usually defined as the amount of contraction in a resting muscle; a more detailed definition would be a static, balanced, isometric contraction between agonist and antagonist (both internal forces) in every muscle in the body, for the purpose of maintaining joint integrity and posture. Muscle tone is static because the job it has to do doesn’t change; it is a basal metabolic function. It never lets go completely, even in sleep, when all dynamic muscle activity is gone. The static activity remains, just sitting there. It operates when one does nothing, and doesn’t change very much from that point. Muscle-building exercise elevates tone, can even slam it up considerably, but at whatever level the involvement of contraction, tone still remains static and constant at any given time.
And as we know, muscle tone can get way out of whack and create unhealthy muscles to the extent of making the whole body ill. But it’s still static. So how is muscle tone increased? It’s done in two ways, which we discuss in classes focusing on the individual spindle fibers and nerves.
For the purpose of muscle tone maintenance, the spindle only measures the amount of stress, or force endured by the extrafusal muscle fibers. It does not measure the length of the fibers, it doesn’t care how long they are. The spindle wants to keep the skeletal muscles at a certain level of contraction regardless of their length. Nor does it measure the rate of change of length of the fibers, because muscle tone is not about the stretch reflex, it’s about a constant contraction. And because the function of muscle tone is to maintain minimal tone just enough to keep joint integrity - it measures extrafusal fiber stress only in a resting muscle.
When a healthy muscle is in motion, the alpha motor drive to the muscles is activated, and it produces more than enough skeletal muscle contraction to maintain joint integrity, so the low-level contraction of muscle tone fades into the background in a healthy muscle. However, once muscle tone becomes excessive, we have a sick muscle. For example a body builder is a mass of hypertonic spasm, and his muscle tone is more powerful than his stretch reflex.
All muscle contraction begins in the muscle core. As more contraction is needed, fibers further out towards the surface contract. Even in flat muscles like latissimus dorsi, contraction starts in the central plane, and then works its way out. In very thin muscles such as the eyelid, even low level contractions will involve the outer layers. But a quadriceps muscle standing against gravity, using only lower levels of contraction, will show a lot of activity in the deep layers of the muscle but none in the superficial layers. The core of the quadriceps has so much power that for this job it doesn’t need the superficial fibers.
Hypertonic contraction also begins at the core of the muscle; and as the muscle gets sicker, the permanent contraction extends further out toward the surface of the muscle. Minimal, healthy muscle tone contraction is 5 pulses per second. Summation of the twitches of many fibers excited asynchronously at low frequencies up to 5 per second generate a total force that does not fluctuate. This is proper muscle tone.
Here are some cites from the varying texts regarding muscle tone so you will be prepared for the confusion you are bound to meet:
Gray’s Anatomy: “At one extreme a muscle may be fully relaxed when much electromyograph evidence shows no activity, the muscle is electrically silent. This contrasts with earlier accounts by electrophysiologists who held that full relaxation was still accompanied by background activity rotating among a few units. Misconceptions concerning such muscle tonus were based on these views.
Anatomy of the Human Body by Lockhart, Hamilton and Fyfe: “Some writers discard the term ‘muscle tone’ or define it as the response of skeletal muscle to stretch.” (We know who describes it thus, we just read it; they are referring to Gray’s Anatomy, who discount the term ‘muscle tone’.) “Electromyograph registers no activity in a resting intact muscle, but such a muscle is certainly different from one with its nerve supply cut. It may be that muscle tissue itself has an intrinsic elasticity.” (But if that were true, we’d have to be fighting against that elasticity every time we used a skeletal muscle.)
Guyton’s Physiology (does at least accept that muscle tone is the residual contraction in the relaxed muscle.) “Even when muscles are at rest, a certain amount of tautness usually remains. This is called muscle tone. Since skeletal muscle fibers do not contract without an actual action potential to stimulate the fibers except in certain pathological conditions, skeletal muscle tone results entirely from nerve impulses coming from the spinal cord. These in turn are controlled partly by impulses transmitted from the brain to the appropriate anterior motoneurons and partly by impulses that originate in muscle spindles located in the muscle itself.”
Structure & Function of the Human Body, Memmler & Wood: “Muscle tone refers to a partially contracted state of the muscles which is normal even though the muscles may not be in use at the time. The maintenance of this tone or tonus is due to the action of the nervous system, and its effect is to keep the muscles in a constant state of readiness for action.”
And finally, Fundamentals of Neurophysiology by Schmidt: “Summation of the twitches of many fibers, excited asynchronously at low frequencies up to 5/s, generates a total force that does not fluctuate very much, with an amplitude that must be approximately proportional to the average frequency of excitation.The ‘background’ tension produced in this way by summation of the twitches of many fibers is called tone. All the muscles in a living organism possess such tone. Even in a relaxed limb, the motor nerves are activated at low frequency.” So it gets better, but the full picture is very hard to find.
As therapists, we need to be aware of a couple of things. The term ‘post isometric relaxation’ indicates that the muscles can be relaxed by first contracting them isometrically; but something else is really happening: skeletal muscles have both slow twitch fibers, which don’t fatigue easily, and fast twitch fibers, which fatigue rapidly. When one holds an isometric contraction, the fast twitch fibers soon fatigue to zero. Muscle contraction begins as a combination of both fast and slow twitch fibers, but when only that contraction from the slow twitch fibers can be felt, the muscle feels softer than it was before. This difference in muscle tension is not relaxation, however, it is fatigue. Once the initiating force is removed, circulation resumes in the tissue. Circulation is the mechanism that allows the fast twitch fibers to reset, or rejuvenate, and in about 15 minutes the tension, or contraction, returns.
Furthermore, we must not fail to notice the background tension of muscle tone, such as resistance to passive bending of a limb. Nor can we fail to notice resistance in the superficial muscle if we’ve turned on the stretch reflex. Deep tissue therapist think they are reaching the deep muscle when they feel that resistance, so they go in deeper, push and hold deep into the muscle, until they gradually feel the muscle start to soften and melt. And what is that softening again? It’s the fast twitch fibers fatiguing. And when it comes back, it’s back with vengeance. Why? Because they’ve activated the stretch reflex.
Chiropractic adjustment not only fatigues the fast twitch fibers, it also inhibits the superficial muscles by increasing the activity of the deep muscles, and in jerking the deep layers and activating the stretch reflex, it activates the Renshaw cells (interneurons in the spinal cord that inhibit motor neurons). The Renshaw cells are automatically coupled in so that the more activity there is in the core of the muscle, the greater the inhibition to the superficial muscle. Therefore the feeling of softness is even more exaggerated post adjustment than post deep tissue massage.
So proper muscle tone is how many contractions per second? Five. And how many muscles have five pulses per second in the relaxed state? Not very many. Why? Because most of most of us are hypertonic and sick. This excludes cardiac muscle, and also flat muscles, which do stay at about 5/s because they can’t poison themselves. Flat muscles resist becoming hypertonic because they can’t squeeze off their veins so they can’t encapsulate lactic acid. Cardiac muscle is non-spastic because it can burn lactic acid, and the number of pulses in cardiac muscle all depends on what it’s doing. It gets excited to react to the conditions of the body. The heart is never in a relaxed state, except between beats. But neither are skeletal muscles; tone works 24 hours a day. Those who stand in awe of the heart muscle could give a few kudos to skeletal muscle as well.