Muscle Tone

Muscle tone is defined as the continuous and passive-partial contraction of the muscle or the muscle's resistance to passive stretch during the resting state.

From: Cooper's Fundamentals of Hand Therapy , 2020

Muscle Tone

Mark L. Latash , Vladimir M. Zatsiorsky , in Biomechanics and Motor Control, 2016

Abstract

Muscle tone is arguably one of the most commonly used and least commonly defined notions in studies of movement, posture, and movement disorders. While most researchers imply under this expression something like "state of relaxed muscle under the spontaneous excitation by the central nervous system," methods of assessment of muscle tone commonly used in clinical practice are likely to reflect a host of factors including properties of peripheral tissues that do not receive neural excitation. The lack of a clear and unambiguous definition for muscle tone has resulted in much misunderstanding in the literature and the emergence of various devices that claim to measure muscle tone objectively. Most commonly, these devices measure resistance of tissues to deformation applied to the surface of a body part; so, they measure apparent stiffness (see Chapter 2) of all the tissues, which is defined by numerous factors related and unrelated to the neural control of muscle state.

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Muscle Tone

T.L. Katner , E.J. Kasarskis , in Encyclopedia of the Neurological Sciences (Second Edition), 2014

Neurophysiological Regulation of Muscle Tone

Muscle tone is regulated by the local spinal cord reflexes at the segmental level innervating that muscle and also by suprasegmental influences. The neural circuits subserving the spinal reflex are illustrated in Figure 1. The afferent limb of the circuit (toward the central nervous system) consists of three elements: (1) annulospiral mechanoreceptors (nuclear bag fibers) and their myelinated type Ia axons, (2) flower-spray mechanical stretch receptors (nuclear chain fibers) and their type II axons, and (3) Golgi tendon organ mechanoreceptors and their type Ib myelinated axons. The annulospiral and flower-spray receptors are interposed between flanking intrafusal muscle fibers that collectively form the muscle spindle. The muscle spindle is oriented in parallel with the extrafusal skeletal muscle fibers, and together they merge into a common tendonous attachment to bone. The parallel arrangement allows these receptors to encode the event of muscle stretch (flower-spray receptors and type II axons) as well as the rate of passive elongation (annulospiral receptors and type Ia axons) of the extrafusal muscle fibers. In contrast, Golgi tendon organs are aligned in series with the entire mass of the muscle by virtue of its localization within the tendonous attachment of muscle. In this way, the Golgi tendon organ encodes the stretch on the tendon generated by the total force of a given muscle during contraction.

Figure 1. Relationship between afferents from the muscle spindle and efferents from the α-motorneuron. This also illustrates the muscle spindle afferents and the λ-motor efferent innervation of intrafusal fibers. LMN, lower motor neuron.

 Reproduced from Felten DL and Shetty AN (2003) Netter's Atlas of Neurosciences, 2nd edn., p. 358. Philadelphia: Saunders.

α-Motor neurons have been long recognized as the final common efferent (outgoing) pathway for voluntary movement. These large neurons receive widespread synaptic input via 20   000–50   000 synaptic sites, including direct excitatory contacts from Ia and II afferents from the muscle spindle. Important inhibitory contacts arise from interneurons driven by type Ib afferents from the Golgi tendon organ, interneurons interposed between the corticospinal projections and the α-motor neuron, and from Renshaw cells. Thus, inhibitory interneurons play an important role in coordinated movements by inhibiting α-motor neurons innervating antagonist muscles (a phenomenon termed as reciprocal inhibition).

Descending fibers originating from the reticulo-, vestibulo-, rubro-, and corticospinal tracts converge on α-motor neurons and excite them. These supraspinal excitatory pathways also converge on γ-motor neurons that innervate the muscles in the intrafusal muscle spindle and cause their contraction and stretch of the annulospiral mechanoreceptors. Therefore, the net effect of γ-activation is stimulation of the Ia afferents and secondary reflex contraction of extrafusal muscle via the activation of α-motor neurons. This has been termed the γ-loop, and it results in shortening of the muscle due to the supraspinal influences as opposed to reflex contraction initiated by passive stretch of the muscle. As one can see, coordinated coactivation of the γ and α motor systems is essential in maintaing a sustained voluntary muscle contraction against a load.

Therefore, the overall impression of muscle tone is dependent on a multiplicity of voluntary and reflex, excitatory and inhibitory influences on the basic segmental arrangement of motor units.

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Lower Motor Neuron Lesions

A.A. Ramahi , ... M. Devereaux , in Encyclopedia of the Neurological Sciences (Second Edition), 2014

Tone

Muscle tone is assessed by examining its response to passive stretch. Lower motor neuron lesions reduce muscle tone, whereas upper motor neuron lesions increase muscle tone resulting in spasticity as seen in pyramidal lesions, or rigidity as observed in extrapyramidal lesions. Regulation of muscle tone is normally mediated by the reticulospinal fibers accompanying the pyramidal tract that exert inhibitory effects on the stretch reflex. Flaccidity, or hypotonia, is a typical feature of lower motor neuron damage, and in the extreme form of total flaccid paralysis, all resistance to passive muscle stretch is lost and the limbs become flail-like. Hypotonia can also be seen in cerebral or spinal shock resulting from acute extensive brain, spinal cord, or cerebellar lesions. It is also seen in peripheral nerve or root lesions affecting sensory afferent pathways. In clinical practice, observation of flaccidity is of limited diagnostic usefulness compared to more important physical signs such as wasting or tendon reflex loss.

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DISORDERS OF SKELETAL MUSCLE

Rodger Laurent , in The Musculoskeletal System (Second Edition), 2010

Control of muscle function

Each fibre has a motor endplate on which the nerve fibre terminates. The functional unit of activity is a motor unit, which combines numerous fibres that are supplied by a single anterior horn cell and its axon. The individual muscle fibres that make up the motor unit are scattered throughout the muscle but contract together under the influence of the anterior horn cell. All muscles fibres supplied by a single motor neuron are of the same histochemical type, either type 1 or type 2.

All of the muscle fibres controlled by a single motor neuron form a motor unit. Small motor units where a motor neuron may control two or three muscle fibres are found in muscles where fine control is required. The converse is found in muscles that do not need fine control, for example the gastrocnemius or gluteus maximus. The amount of tension produced in a contracting muscle depends on the frequency of stimulation and the number of muscle units involved.

The nervous system controls the force of the contracting muscle by varying the number of motor neurons activated at any one time. For each movement, there is a progressive increase in the number of motor units contracting to provide an even increase in tension. Maximum tension in a muscle occurs when all the motor units are contracting.

Muscle tone is the resting tension in a skeletal muscle. It occurs because there are always a few motor units contracting in a resting muscle. These contractions do not cause enough tension to produce movement. Muscle tone is maintained by a normal reflex arc, whereby a signal is sent from the muscle spindles to a lower motor neuron in the posterior root ganglion which then sends a signal to the appropriate muscles to adjust the extent of their contraction. Changes in tension in a muscle result in activation of the muscle spindles so that the contraction of other muscles is altered to correct the tension in that muscle. This reflex arc is also under the control of the central nervous system.

Resting muscle tone is important for maintaining normal posture, and provides support for the joints to stabilize their position and help prevent sudden changes in the position. Muscle tone is increased in upper motor neuron lesions, for example in cerebral cortical damage that occurs in cerebrovascular accident. This is thought to be due to loss of cortical control of motor neurons, which increase their activity. There is no muscle wasting. A reduction in muscle tone, hypotonia, occurs in lower motor neuron disorders. These occur in spinal and/or peripheral nerve damage. This results in muscle atrophy. Examination of muscle tone provides important clues to the cause of muscle weakness.

Interesting facts

Myotonia is delayed relaxation of the muscle after contraction. It is an important feature of dystrophia myotonica but can be due to other causes, where it is less severe, which include hypothyroidism, prolonged cold exposure, extreme physical exercise and medication, e.g. propranolol.

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Selected Applications

Sverre Grimnes , Ørjan G Martinsen , in Bioimpedance and Bioelectricity Basics (Third Edition), 2015

Electrotonus

Tonus is the natural and continuous slight contraction of a muscle. Electrotonus is the altered electrical state of nerve or muscle cells from the passage of a DC. Subthreshold DC currents through nerves and muscles may do the tissue more (excitatory effect) or less (inhibitory effect) excitable. Making the outer nerve cell membrane less positive lowers the threshold and has an excitatory effect (at the cathode, catelectrotonus), the anode will have a certain inhibitory effect (anelectrotonus). This is used in muscle therapy with diadynamic currents (see the following section).

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Neuroanatomy of the Spinal Cord

Susan A. Darby , Robert J. Frysztak , in Clinical Anatomy of the Spine, Spinal Cord, and Ans (Third Edition), 2014

Muscle Tone

Muscle tone is the resistance of a muscle to active or passive stretch, or the overall stiffness of the muscle. Skeletal muscle has an intrinsic resistance to stretch resulting from the elastic properties of the tendons, connective tissue, and the muscle tissue itself. Therefore muscle behaves much like a spring. Reflexes also function to counteract the active or passive stretch of the muscle tissue via the monosynaptic connections from the spindles to the alpha motor neurons, and work with the elastic components of muscle to resist stretch. Normal muscle tone serves three important functions. First, it assists in maintaining posture, or the resistance of the muscle to the forces of gravity. Muscle tone helps to ensure that the center of gravity is aligned over the base of support. Second, because of a muscle's inherent ability to act as a spring, it can store energy and release it at a later time. This is particularly important for movements such as walking. When a leg pushes off, some of the stored energy is released and helps propel the leg and body forward, thereby assisting the muscles that normally pull the leg forward. Lastly, because muscles act like springs, they help dampen jerky movements and allow for more "fluidlike" movements of most muscles ( Ghez, 1991).

Control of muscle tone is achieved largely through feedback mechanisms. Negative feedback helps to counteract deviations from the desired muscle position. Overall muscle length is chosen by the CNS and regulated by descending connections to the motor neuron pool in the spinal cord. Deviations in the intended position are detected by the muscle spindles and relayed back to the motor neuron pool. Increases in the length of the muscle result in an increased output from the muscle spindle and increased stimulation of the motor neuron pool, which result in an increase in the force of contraction of that muscle to counteract the increase in length. Decreases in muscle length have the opposite effect. Therefore the stretch reflex functions continuously to keep the muscle position as close as possible to the length chosen by the CNS. Two crucial elements of the feedback system are the gain of the system and the loop delay (Gordon & Ghez, 1991). Gain of the system is largely determined by the fusimotor set discussed earlier, and relates to the overall sensitivity of the muscle spindles. The higher the gain (greater sensitivity of spindles), the larger is the reflexive force of contraction by a muscle to counteract a given change in length. Gain can be adjusted by the overall level of fusimotor (intrafusal) activity, by presynaptic modulation of excitatory and inhibitory interneurons (activated by various forms of sensory input), and by direct connections to the motor neuron pool by descending input. The loop delay is the time between the detection of a disturbance or error and the actual compensatory response by the muscle to counteract that error. The loop delay is a sum of the conduction times from the sensory afferents, the motor neurons leading back to the muscle, and the mechanical response from the muscle. The loop delay usually is insignificant when movements are slow. During rapid or extremely precise movements, the loop delay can play a significant role in accurately regulating movement around a joint. Muscle tone also plays a substantial role in most postural control mechanisms.

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Nervous System

Hillary A. Reinhold , Michele P. West , in Acute Care Handbook for Physical Therapists (Fourth Edition), 2014

Muscle Tone

Muscle tone has been described in a multitude of ways; however, neither a precise definition nor a quantitative measure has been determined. 20 It is beyond the scope of this book to discuss the various definitions of tone, including variants such as clonus and tremor. For simplicity, muscle tone is discussed in terms of hypertonicity, hypotonicity, or dystonia. Hypertonicity, an increase in muscle contractility, includes spasticity (velocity-dependent increase in resistance to passive stretch) and rigidity (increased uniform resistance that is present throughout the whole range of motion and is independent of velocity) secondary to a neurologic lesion of the CNS or upper motor neuron system. 7 Hypotonicity, a decrease in muscle contractility, includes flaccidity (diminished resistance to passive stretching and tendon reflexes) 21 from a neurologic lesion of the lower motor neuron system (or as in the early stage of spinal cord injury [SCI] known as spinal shock). Dystonia, a hyperkinetic movement disorder, is characterized by disordered tone and involuntary movements involving large portions of the body resulting from a lesion in the basal ganglia (as in Parkinson's disease with excessive L-dopa therapy). 7 Regardless of the specific definition of muscle tone, clinicians agree that muscle tone may change according to a variety of factors, including stress, febrile state, pain, body position, medical status, medication, CNS arousal, and degree of volitional movement. 7

Muscle tone can be evaluated qualitatively in the following ways:

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Passively as mild (i.e., mild resistance to movement with quick stretch), moderate (i.e., moderate resistance to movement, even without quick stretch), or severe (i.e., resistance great enough to prevent movement of a joint) 22

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Passively or actively as the ability or inability to achieve full joint range of motion

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Actively as the ability to complete functional mobility and volitional movement

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As abnormal decorticate (flexion) or decerebrate (extension) posturing. (Decortication is the result of a hemispheric or internal capsule lesion that results in a disruption of the corticospinal tract. 17 Decerebration is the result of a brain stem lesion and is thus considered a sign of deteriorating neurologic status. 17 A patient may demonstrate one or both of these postures.)

Muscle tone and spasticity can also be evaluated objectively using the following scales:

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Modified Ashworth Scale as described in Table 6-12. This scale has been considered the "gold standard" of measuring muscle tone due to initial studies showing high interrater (0.84) and intrarater (0.83) reliability. 23 However, more recent studies have had less favorable results, showing moderate reliability. 24-26

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Modified Tardieu Scale as described in Table 6-13. The Tardieu Scale was developed by Tardieu in 1954 26a (the patient is in the supine position) and modified by Boyd and Graham in 1999 26b (the patient is in supine, sitting, or standing, depending on the joint tested). This scale measures the quality of muscle reaction to passive stretch at three different velocities. Not only is the muscle reaction quantified (as in the Modified Ashworth Scale), but it also controls for the velocity of the stretch and measures the angle at which the catch, or clonus, occurs. 26b This scale has been shown in recent studies to be a more accurate measure of spasticity than the Modified Ashworth Scale. 27,28

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Hypokinesia and Hyperkinesia

Stanley Fahn , in Textbook of Clinical Neurology (Third Edition), 2007

Tone

Muscle tone represents the resistance to passive movement of a joint. Unlike spasticity, which is characteristic of upper motor neuron disease (see Chapter 15), rigidity, the hallmark of hypokinesia, is increased tone of both flexor and extensor muscles. Often, in parkinsonism, there is a cogwheeling character to the hypertonicity (cogwheel rigidity). Dystonic patients also have increased tone when the dystonia is active, although the tone in uninvolved groups is generally normal. In chorea, the tone is often reduced (hypotonia) and the excess movements take on a puppet‐like quality as they flow from one body part to another. In patients with tics, the tone is normal.

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Adult polysomnography

L.A. Smolley , in Reference Module in Neuroscience and Biobehavioral Psychology, 2021

Electromyography

Muscle tone generally diminishes gradually from wakefulness to the deeper stages of non-REM sleep. During REM sleep most skeletal muscles are atonic with the EMG showing minimal tone, and this sign is particularly important for the staging of REM sleep. Because of their particular location 2  cm below the inferior edge of the mandible and 2   cm to the right and left of the midline, these electrodes need extra attention to prevent them from being dislodged when the patient moves in bed. Double-sided electrode collars and gauze tape are required to secure these electrodes. For bearded patients, collodion or special paste is recommended. Sampling rate must be a minimum of 200   Hz and optimally 500   Hz with a bandwidth of 10–100   Hz.

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The Floppy Infant

Graeme A.M. Nimmo , Ronald D. Cohn , in Swaiman's Pediatric Neurology (Sixth Edition), 2017

Defining Hypotonia

"The floppy infant" is an informal term for generalized hypotonia and is a presenting feature for a wide range of both systemic and neurologic disease. The pediatric neurologist is regularly requested to evaluate an infant with hypotonia, and it poses a particular diagnostic challenge owing to the extensive differential diagnosis. The list of disorders with presenting features, including hypotonia, has expanded rapidly in the genomic era and continues to grow. Although at present most of the conditions have few disease modifying treatments, diagnosis is essential as it provides both families and physicians with prognostic information and screening strategies for associated pathologies. It may also expose certain risks associated with known diagnosis; for example, individuals with RYR1 mutations may be at increased risk for malignant hyperthermia (Brislin and Therous, 2013). Furthermore, many of the diagnoses are genetic and have implications for living family members or future family planning.

Severe hypotonia usually presents in the neonatal period, but milder or slowly progressive pathologies may not come to a physician's attention until the child fails to attain milestones in the latter part of the first or second years. In the neonatal period, the differential diagnosis must include hypoxic-ischemic injury, intraventricular hemorrhage, and systemic disease such as hypoglycemia, sepsis, and heart failure; these diagnoses are not discussed here. For the most part, the remaining conditions presenting in this period have an underlying genetic origin, but acquired causes that present in this age group are also discussed. Chapter 137 provides an approach to the older child with suspected neuromuscular disease.

Muscle Tone

Muscle tone is defined as a skeletal muscle's inherent resistance to passive movement. It is particularly important to differentiate this from muscle strength, which is a muscle's maximum voluntary resistance to movement. Tone is controlled by the peripheral fusimotor system with input from the central nervous system (CNS). The afferent fibers detect muscle spindle stretch and subsequently direct the motor unit system to cause muscle contraction; this is reviewed in detail in Chapter 5. Failure of any component of the motor unit, from the anterior horn cell, motor neuron, neuromuscular junction, or the muscle itself, will result in hypotonia. Supraspinal input from the motor cortex, basal ganglia, striatum, red nucleus, and cerebellum is predominantly inhibitory in its interaction with fusimotor system. In older children and adults, disturbance of inhibitory pathways results in increased excitatory output with hypertonia and hyperreflexia. In contrast, disturbance to these pathways in infancy often presents with decreased muscle tone; but importantly, the reflex arc is the preserved. Hypotonia in infancy, therefore, may be caused by disorders affecting any level of the nervous system.

Some diagnoses may be established directly from history and examination, and confirmation with further testing, often in the form of genetic testing, can be carried out immediately. On the other hand, many of the neuromuscular diagnoses may be more difficult to distinguish based on the initial clinical evaluation alone and require preliminary investigations to better characterize the disease and define a group of diagnoses. The first step in the determination of the cause of hypotonia is to localize the pathology.

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