Muscle tone

Editor: Jenniffer Voelkl Guevara
Last updated: August 2018

Current definition: (1,2)

“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”.

History:

Muscle tone can also be called stiffness which corresponds to the change in resistance or force per unit change in length (Δ force/Δ displacement of the tissue) (3)

In normally innervated skeletal muscle, tone comprises active and passive components (4). 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 (2). 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 (2). 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 (1) 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. (2,5)

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 (4), and may exist in the absence of muscle activity altogether. The ICS Standardization of Terminology in Chronic Pelvic Pain Syndromes has suggested to use the term “increased tone” when there is a non-neurogenic cause. Other authors suggest neurogenic hypertonicity and non-neurogenic hypertonicity (1). • 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 (4), however the ICS Standardization of Terminology Committee suggest to use the term “decreased tone” (1) • 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 (4), authors state that this term is different than contracture. (6). 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 (1) • 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. (4)

Controversy

The terms spam, hypertonicity, overactive, and contracture are often used interchangeably confusing patients and medical professionals searching for treatment strategies. Patients with non contractile increased tone, tested with EMG, have been told there is not PFM dysfunction leaving them to wonder if it is "all in my head". These terms indicate separate and distinct conditions. Although some treatment approaches overlap, there are unique treatments in each case which improve success and function. It is important for medical professionals to be specific about these terms.

Unfortunately, this effort is made more difficult by the lack of research and tools. 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). More research is need in this area.

References:

  1. 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

  2. Doggweiler R, Whitmore KE, Meijlink JM, Drake MJ, Frawley H, Nordling J, et al. A standard for terminology in chronic pelvic pain syndromes: A report from the chronic pelvic pain working group of the international continence society. Neurourol Urodyn [Internet]. 2016 Aug 26 [cited 2016 Oct 16]; Available from: http://www.ncbi.nlm.nih.gov/pubmed/27564065

  3. 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.

  4. 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

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

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

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