MUSCLE TONE
Jenniffer Voelkl Guevara
September 2017

Subject Author Replies Views Last Message
No Comments

Current definition:

TONE: “State of the muscle”, usually defined by its resting tension, clinically determined by resistance to passive movement (Doggweiler et al., 2016), or also commonly referred to as muscle tension. Muscle tone can also be measured as stiffness which corresponds to the change in resistance or force per unit change in length (Δ force/Δ displacement of the tissue)(Thibault-Gagnon & Morin, 2015)In normally innervated skeletal muscle, tone comprises active and passive components (Simons et al., 1998). Muscle activity resulting from muscular contraction (i.e., electrogenic contraction) is found among the active component contributing to muscle tone; it is created by low-frequency activation of a small number of motor units (Doggweiler et al., 2016).
The passive component is independent of neural activity and reflects the passive physical properties of the viscoelastic tension of the muscle fiber elements and the osmotic pressure of cells (Doggweiler et al., 2016).
A full description of both active and passive components is described below:
PASSIVE – VISCOELASTIC COMPONENT
ACTIVE – CONTRACTILE COMPONENT
Involves:
  • Extensibility of actin-myosin cross-bridges (EMG silent)
  • Noncontractile cytoskeleton proteins such as the desmin and the titin
  • Conjunctive tissues surrounding the entire muscle (epimysium), muscle fascicle (perimysium), and muscle fiber (endomysium).
  • Connective tissues (fascia) linking and covering muscle tissue
An increase in muscle tone may occur due to changes in any of these passive structures, and in the absence of any detectable EMG activity. Both muscle hypertrophy (increase in muscle mass) and muscle atrophy related disuse and immobilization in a shortened position can be associated with increased muscle stiffness due to different physiological adaptations of viscoelastic structures (Gajdosik et al., 2001)
Comprises muscle activity such as:

  • Normal electrogenic contraction (involves resting activity in normally relaxed, voluntarily controlled muscle and also myotatic reflex during stretching), it can be recorded through EMG.
  • Physiological contracture (most commonly defined as a trigger point (TP); i.e. a palpable taut band, that is not accompanied by electrical activity (EMG not detected)
  • Electrogenic spasms (which include unintentional muscle contraction with or without pain amenable to voluntary control) it can be recorded through EMG
(Padoa & Rosenbaum, 2016)
The International Urogynecological Association (IUGA)/ International Continence Society (ICS) joint report on the terminology for the conservative and nonpharmacological management of female pelvic floor dysfunction (Bø et al., 2017) has recently standardized the term tone as follows:

“State of the muscle, usually defined by its resting tension, clinically determined by resistance to passive movement. Muscle tone has two components: the contractile component, created by the low- frequency activation of a small number of motor units, and the viscoelastic component, which is independent of neural activity and reflects the passive physical properties of the elastic tension of the muscle fiber elements and the osmotic pressure of the cells”.

Other Terms:

  • Hypertonicity: is a general increase in muscle tone that can be associated with either elevated contractile activity and/or passive stiffness in the muscle (Simons & Mense, 1998), and may exist in the absence of muscle activity altogether. The ICS Standardization of Terminology in Chronic Pelvic Pain Syndromes has suggestedto use the term “increased tone” when there is a non-neurogenic cause.
It has been suggested that the terms neurogenic hypertonicity and non-neurogenic hypertonicity are recommended to describe the diagnosis and inform management (Bø et al., 2017).
  • Hypotonicity: general decrease in muscle tone that can be associated with either reduced contractile activity and/or passive stiffness in the muscle. As the cause is often unknown, the terms neurogenic hypotonicity and non-neurogenic hypotonicity are recommended (Simons & Mense, 1998), however the ICS Standardization of Terminology Committee suggest to use the term “decreased tone” (Bø et al., 2017)
  • Spasm: Electromyographic (EMG) recording of increased tension with or without shortening of a muscle due to non-voluntary motor nerve activity. Spasm is identified by motor unit potentials that cannot be terminated by voluntary relaxation, and it may or not be painful (Simons & Mense, 1998), authors state that this term has to be distinguished from contracture. (Travell & Simons, 2012).
Spasm might also be defined as: persistent contraction of striated muscle that cannot be released voluntarily. If the contraction is painful, this is usually described as a cramp. Spasms occur at irregular intervals with variable frequency and extent, and over days or weeks may lead to a contracture (Bø et al., 2017)
  • Contracture: Physiological contracture (or rigor) is a state of muscle contractile activity, usually a palpable taut band (also be referred as trigger point), unaccompanied by electrical activity. (Simons & Mense, 1998)

Perspectives:

Muscle tone is evaluated clinically as the resistance provided by a muscle when a pressure/deformation or a stretch is applied to it; it might may be altered in the presence or absence of pain (Bø et al., 2017)
Digital palpation, ultrasound imaging, pressure manometry, dynamometry and EMG evaluation have been described by several authors as methods to assess pelvic floor muscle tone; each method may be able to measure different aspects of tone, such as resting activity, stiffness or elasticity; however there is no standard method of evaluation that encompasses both components of muscle tone.
Assessment Tool
Description
Conclusions
Digital Palpation
Digital palpation assessment of PFMs can be performed by inserting one or two fingers inside the vaginal or rectal cavity. Several scales have been proposed:
Devreese scale: it is a3-point scale (normo-, hypo-, hyper tone used to assess continent and incontinent women.
Dietz’s tone grading scale: ranging from 0 to 5, and it incorporates three components, namely hiatus size, resistance to distension, and pain. It was developed and used in women with incontinence and prolapse.
Among digital palpation flexibility and hiatus diameters has been suggested as a means to assess the maximal PFM length.
The International Continence Society (ICS) standardization group on PFM function and dysfunction recommended assessing the PFM relaxation
ability after a contraction describing the relaxation as present/ absent and partial/complete.
Despite its usefulness in a clinical setting, this assessment technique remains subjective.
There is a lack of evidence regarding validity and reliability when using this method.
These scales only provide a global measurement of PFM tone without stating specific differences between the active and passive components.
Measurements regarding the ability to relax after a contraction might be biased in the case of elevated tone at baseline, in which a patient would be categorized as having a good relaxation if returning to the abnormal precontraction level.
sEMG
Refers to the bioelectrical activity generated by muscle fibers. During voluntary contraction, recruitment of motor units, depolarization of membranes and action potentials are captured.
The number of motor units recruited and their frequency of discharge influence the signal amplitude and, hence, the force output produced.
Neither the passive component (viscoelastic properties) nor a contracture is captured by EMG. Moreover, the presence of crosstalk and artifact has to be taken into account when interpreting sEMG readings.
Ultrasound Imaging
Measures pelvic floor muscle morphology and function via a non-invasive (trans-abdominal or trans-perineal) probe. This method allows good visualization of the bladder neck, urethra, vagina, anorectal junction and levator ani muscle and measurements of organ displacement, making it more reliable for comparison between subjects.
In the mid-sagittal plane, the assessment of organ positioning at rest is attributable in part to PFM tone while organ mobility during contraction is related to PFM contractile properties.
As US imaging only provides an image of the action of the muscle status, it is not considered a direct measure of muscle tone and it cannot be considered as a tool to assess the resistance to stretching of the muscle.
Dynamometry
Measurement of pelvic floor muscle resting and contractile forces using strain gauges mounted on a speculum (a dynamometer), which is inserted into the vagina.
There is a lack of accessibility to these devices tas they are mostly used by their designers and not commercially available;
In addition there are different types of devices and speculums which makes comparison between studies very difficult.
Most of these dynamometers evaluate the summative/global contribution of the active and passive components of tone. With the exception of the Montreal dynamometer, the contribution of the active and passive can be differentiated when combining the dynamometer with EMG measurement (Morin, 2017).
Pressure Manometry or Perineometry
Measurement of resting pressure or pressure rise generated during contraction of the pelvic floor muscles; in millimeters of mercury (mmHg) or centimeters of water (cmH2O) by using a pressure device (a manometer) inserted into the urethra, vagina or anus.
There are no recommendations whether the device needs to be calibrated to 0 prior to insertion; and studies measuring tone in the upright position have resulted in poor reliability. Moreover the size of the probe has to be taken into account and these methods provide a global measurement of tone.
Elastometry
Measures the elasticity of a tissue.
If used in combination with dynamometry, it measures passive forces i.e. the global contribution of both active and passive components of tone.
Based on information from (Padoa & Rosenbaum, 2016) and (Doggweiler et al., 2016)
Controversies:
There is no well-established standard for clinical assessment of muscle tone, and there are no reference or normative values.
Most of the tools available, measure global PFM tone (i.e., summative contribution of active and passive components).

REFERENCES

Bø, K., Frawley, H., Haylen, B. T., Abramov, Y., Almeida, F., Berghmans, B., … Wells, A. (2017). An International Urogynecological Association (IUGA)/International Continence Society (ICS) joint report on the terminology for the conservative and nonpharmacological management of femal pelvic floor dysfunction. International Urogynecology Journal and Pelvic Floor Dysfunction, pp. 1–27. http://doi.org/10.1007/s00192-016-3140-3

Doggweiler, R., Whitmore, K. E., Meijlink, J. M., Drake, M. J., Frawley, H., Nordling, J., … Tomoe, H. (2016). A standard for terminology in chronic pelvic pain syndromes: A report from the chronic pelvic pain working group of the international continence society. Neurourology and Urodynamics. http://doi.org/10.1002/nau.23072

Morin M, Binik YM, Bourbonnais D, et al. Heightened PelvicFloor Muscle Tone and Altered Contractility in Women With Provoked Vestibulodynia. J Sex Med 2017;14:592e600.

Padoa, A., & Rosenbaum, T. (2016). The Overactive Pelvic Floor. The Overactive Pelvic Floor. http://doi.org/10.1007/978-3-319-22150-2

Simons, D. G., & Mense, S. (1998). Understanding and measurement of muscle tone as related to clinical muscle pain. Pain, 75(1), 1–17. http://doi.org/10.1016/S0304-3959(97)00102-4

Thibault-Gagnon, S., & Morin, M. (2015). Active and Passive Components of Pelvic Floor Muscle Tone in Women with Provoked Vestibulodynia: A Perspective Based on a Review of the Literature. Journal of Sexual Medicine, 12(11), 2178–2189. http://doi.org/10.1111/jsm.13028

Travell, J., & Simons, D. (2012). Myofascial Pailn and Dysfunction. The Trigger Point Manual. The Upper Extremities (2nd ed.). Wilkins, Lippincott Williams &.