Skeletal Muscle Tone, Tonus, Tonicity, and Motor Control

By Greg Mack

Muscle Tone (n.) – the normal level of firmness or slight contraction in a resting muscle. (Physiology) – the normal level of activity in a nerve fiber. (Oxford American Dictionary)

The state of activity or tension of a muscle beyond that related to its physical properties, that is, its active resistance to stretch. In skeletal muscle, tonus is dependent upon efferent innervation. (Stedman, 25th ed.)

Tone – 1. That state of a body or any of its organs or parts in which the functions are healthy and normal. In a more restricted sense, the resistance of muscles to passive elongation or stretch. 2. Normal tension or responsiveness to stimuli, as of arteries or muscles, seen particularly in involuntary muscle (such as the sphincter of the urinary bladder. (Taber’s Cyclopedic Medical Dictionary 1989)

Tonus (n.) – That partial steady contraction of muscle which determines tonicity or firmness. Opposite of clonus. SYN: tone, tonicity. (Taber’s Cyclopedic Medical Dictionary 1989)

Clonus (n.) muscular spasm involving repeated, often rhythmic, contractions.

“Muscle tone, the force with which a muscle resists being lengthened, …”

Kandel, Schwartz, Jessel – “Principles of Neural Science”, 4th Edition, Pg. 730

“The thixotropic property of muscle and its associated connective tissues endows it with a passive stiffness that resists changes in length. Clinicians refer to this resistance to stretch by a relaxed muscle as muscle tone.”

Enoka, R. M., Neuromechanics of Human Movement, 4th Ed., 2008, Pg. 309

“Muscle displays excitability – the ability to respond to stimuli and, by means of a stimulus, to be able to actively contract, extend (lengthen) or to elastically recoil from a distended position, as well as to be able to passively relax when stimulus ceases. Lederman (1998) suggests that muscle tone in a resting muscle relates to biochemical elements – a mix of fascial and connective tissue tension together with intramuscular fluid pressure, with no neurological input (therefore, not measurable by EMG). If a muscle has altered morphology, due to chronic shortening, for example, or compartment syndrome, then muscle tone even at rest will be palpable. He differentiates this from Motor Tone, which is measurable by means of EMG and which is present in a resting muscle only under abnormal circumstances”

Chaitow, L., DeLaney, J.W., “Clinical Application of Neuromuscular Techniques – Volume 1 – The Upper Body”, 2002, Pg. 20.

By the various descriptions and definitions referenced above it can be concluded that Muscle Tone reflects two properties of living muscle tissue; 1) its firmness under haptic compression and, 2) its resistance to change in length. Both of these properties are qualitative in their descriptions. A muscle’s “firmness” may be qualitatively assessed via a practitioner’s haptic compression performed at an angle perpendicular to its pennation and length. This haptic judgment of firmness can be applied and interpreted along a continuum of whole muscle tension – from relaxed to maximally contracted at some specified length. Apparently, the amount of ‘tone” in a specific muscle under conditions is represented in the reaction force of the tissue response felt by the practitioner’s tactile interpretation, hence, the terms “judgment” and “qualitative” in determining the tone of a muscle.

Judging muscle tone as a function of its resistance to change in length from some specified position of relative short is reflected in the concept of muscle “stiffness”.

* Of note is the reference contrasting the term “Muscle Tone’ with the term “Motor Tone” in the Chaitow quote inferring the difference is based on neurological input.

“Because muscle has elastic elements in series and parallel that resist lengthening, it behaves like a spring. In addition to this intrinsic stiffness, however, there is a neural contribution to muscle tone; the stretch reflex feedback loop also acts to resist lengthening of the muscle. The neural circuits responsible for stretch reflexes provide the higher centers of the nervous system with a mechanism for adjusting muscle tone under different circumstances.”

Kandel, Schwartz, Jessel – “Principles of Neural Science”, 4th Ed., Pg. 730

This latter quote from Principles of Neural Science infers that whole muscle tone is a dynamic neurological attribute observed under a variety of conditions whereas the former quote from Neuromechanics of Human Movement limits the description to the static condition of “relaxed muscle”.

“Normal individuals have very low, barely perceptible muscle tone when fully relaxed.”

Fahn, S., “Principles and Practice of Movement Disorders”, 2nd Ed., Pg. 45

“It is the muscle stretch receptors that are responsible for the impression of “tone” that can be elicited by passive manipulation of a limb; in this case, extension of a joint will activate Ia afferent nerve endings in the flexor muscle and will evoke a reflex contraction of the muscle. If the nerve endings are compromised by disease or injury, so as to be relatively in-excitable, then the reflexes will be diminished, and the tone perceived by an examiner will be decreased. Conversely, an increase in spindle activity, as in patients with certain types of brain or spinal cord lesion, will give an impression of greater resistance when the limb is moved (spasticity)”

Macintosh, Gardiner, McComas,“Skeletal Muscle: Form and Function”, 2006, Pg. 47

“The hind-brain provides two major functions for general motor control of the body: (1) maintenance of axial tone of the body for the purpose of standing and (2) continuous modification of the different degrees of tone in the different muscles in response to continuous information from the vestibular apparatus for the purpose of maintaining equilibrium.”

Guyton, A. C. “Textbook of Medical Physiology”, 9th Edition, 1996, Pg. 730

The two previous citations apparently provide different reasons for the presence or absence of “Muscle Tone”. One is based on the passive structural properties of the peripheral afferent sensory receptors and their influence on muscle state when manipulated and the other under efferent neurological influence from the cerebellum. Two citations provided below challenge the validity of the use for the term in assessing and describing this particular attribute of living muscle.

“Clinicians have defined spasticity as a disorder of spinal proprioceptive reflexes manifested as profound changes in reflexes to muscle stretch with a strong velocity-dependent component, emergence of pathological reflexes and uncontrolled spasms, an increase in muscle tone, and impairment of voluntary motor function. This is a descriptive definition in terms of signs and symptoms rather than underlying mechanisms. Furthermore, it contains a couple of vague terms, such as “strong velocity-dependent component” (what is strong?), and one of the worst misnomers, “muscle tone.” Clinicians define “increased muscle tone” as a “feeling of increased resistance when you try to move a joint.”

Latash, Mark L., “Neurophysiological Basis of Movement”, 1998, Pg. 214

“At one extreme, the muscle may be fully relaxed, and under such conditions much electromyographic evidence has accumulated to show that there is no motor-unit activity and the muscle is electrically silent – these views contrast with many earlier accounts of electrophysiologists who held that the nearest approach to full relaxation was still accompanied by as low ‘background’ rotational activity of a few motor units. On such accounts many misconceptions concerning the use of the term ‘muscle tonus’ were based. However, it must be realized that even a fully relaxed muscle is a highly complex three dimensional structure endowed with passive viscous, elastic, and other mechanical properties which provide some degree of resistance to deformation (e.g. by stretching). During activity also, such properties of the inactive motor units and their connective tissues provide an initial internal source of resistance, to be taken up and overcome by the active units, before external forces can be opposed and work performed.”

Gray’s Anatomy, 35th British Edition, 1973, Pg. 493

In spite of Gray’s apparent dismissal of the term “Muscle Tone” in the latter quote the textbook does refer to it (Muscle Tone) later in the text …

“Without reference to particular cerebellar regions the more obvious effects of cerebellar dysfunction may include; (a) disturbances in equilibrium of the whole body; (b) disturbances in muscle ‘tone’ or their resistance to stretch, tendon reflexes and ability to stabilize joint positions; (c) in coordination of movements (ataxia) due to irregularities in timing of onset, rate and force of contraction of synergistic muscle groups.”

Gray’s Anatomy, 35th British Edition, 1973, Pg. 877

Conclusion

Even a cursory exploration of established reference literature on the subject of myology – oft referenced as authoritative work in human physiology – presents an intellectual challenge for workers in the exercise sciences as they seek to deepen their comprehension and come to understand the implications for the practical application to human muscle. Clearly there exist conflicting definitions for the terms “muscle tone, tonus, and tonicity”. These references tend to describe the terms under two different qualitative domains. One describing muscle tone as a result of nervous system influence and the other as an intrinsic feature attributed to its non-contractile material components. As an extra factor, the following references to nervous system influences over muscle tone contribute to the conflict as practitioners explain its presence or absence by neuromuscular disease conditions…

 “Spasticity” is defined as hypertonia in which 1 or both of the following signs are present: 1) resistance to externally imposed movement increases with increasing speed of stretch and varies with the direction of joint movement, and/or 2) resistance to externally imposed movement rises rapidly above a threshold speed or joint angle. “Dystonia” is defined as a movement disorder in which involuntary sustained or intermittent muscle contractions cause twisting and repetitive movements, abnormal postures, or both. “Rigidity” is defined as hypertonia in which all of the following are true: 1) the resistance to externally imposed joint movement is present at very low speeds of movement, does not depend on imposed speed, and does not exhibit a speed or angle threshold; 2) simultaneous co-contraction of agonists and antagonists may occur, and this is reflected in an immediate resistance to a reversal of the direction of movement about a joint; 3) the limb does not tend to return toward a particular fixed posture or extreme joint angle; and 4) voluntary activity in distant muscle groups does not lead to involuntary movements about the rigid joints, although rigidity may worsen.”

PEDIATRICS Vol. 111 No. 1 January 1, 2003, pp. e89 e97,(doi:10.1542/peds.111.1.e89)

“It is the muscle stretch receptors that are responsible for the impression of “tone” that can be elicited by passive manipulation of a limb; in this case, extension of a joint will activate Ia afferent nerve endings in the flexor muscle and will evoke a reflex contraction of the muscle. If the nerve endings are compromised by disease or injury, so as to be relatively in-excitable, then the reflexes will be diminished, and the tone perceived by an examiner will be decreased. Conversely, an increase in spindle activity, as in patients with certain types of brain or spinal cord lesion, will give an impression of greater resistance when the limb is moved (spasticity)”

Macintosh, Gardiner, McComas,  “Skeletal Muscle: Form and Function”, 2006, Pg. 47

… and electro-physiologists refer to tone as twitch (non-tetanized motor unit activity);

“Muscle Tonus. 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, or tonus. All the muscles in a living organism possess such tone. Even in a relaxed limb, the motor nerves are activated at low frequency. The resulting tone is detectable as a resistance to passive bending of the limb.

The tone of muscles is chiefly involved in their postural function. Even when we sit relaxed, our limbs are not fully passive; instead they adopt a certain posture. This posture is determined by the relative degree of tone in the various groups of muscles.”

Schmidt, Robert F., Fundamentals of Neurophysiology, 2nd Ed, 1978, Pg. 154.

The prefixes “hyper” and “hypo” attached to the base words “tone” or “tonicity” infer that the base word possesses a clear definition from which a qualitative change is “over” or “under” some normatively understood or accepted value of “tone”. As previously shown in a brief review of the literature containing the subject there is a lack of definitive consensus for the definition except that which is found in the quote from the Pediatrics Journal Article.

Voluntary human movement under all conditions is fundamentally dependent on the state of, and the abilities of, alpha motor unit pools, and the material properties of muscle in order to generate tension that ultimately generates the torques necessary to change and/or maintain the positions of the human lever system. As important as the quantified abilities of muscle are, the qualitative features of the sensory-motor apparatus should not be ignored despite the inaccurate definitions for the material property of “Muscle Tone”. Therefore, it is reasonable to acknowledge that Muscle Tone, under its various definitions, may influence the ability of the motor control system to meet intentional needs under diverse conditions. The caveat being that it is a challenge to make any specific claims as to how those influences contribute to the total observed output of motor activity. Therefore, it is recognized that the term “Muscle Tone” as a qualitative description of skeletal muscle is a useful term applied to the material and/or neurological condition of muscle reflecting its current state and the resulting capabilities of function at a specified point in time under a specified set of circumstances. Using the term ought to be made with qualification and it is recommended that one avoid any strict cause and effect conclusions without corroborating data.

“Changes in muscle tone affect a person’s ability to control movement, resulting in poor selective control of muscles, poor regulation of activity and muscle groups, decreased ability to learn unique movements, inappropriate sequence of movements, and delayed postural response. Most often, the timing and sequence of muscle activity are also affected.”

Goodman, C.C., Fuller, K.S., “Pathology: Implications for the Physical Therapist”, 3rd Edition, Pg. 1522.