Human Resting Muscle Tone (HRMT): Narrative Introduction and Modern Concepts Alfonse T. Masi, MD, DR.P.H. * Professor of Medicine and Epidemiology University of Illinois College of Medicine at Peoria One Illini Drive, Peoria, IL 61656, USA Tel: +1 309 671 8428 Fax: +1 309 671 8513 Email: [email protected] John Charles Hannon, DC Certified Feldenkrais Practitioner 1141 Pacific Suite B San Luis Obispo, CA 93401, USA Tel: +1 805 542 9925 Fax: +1 805 541 2391 Email: [email protected] Key Words: passive tone or tonus; viscoelastic; skeletal muscle; fascia; myofascial, biotensegrity *Correspondence to: Dr. AT Masi 1 Abstract Human resting muscle studies have been performed to establish (myofascial) tone (HRMT) is the passive normal frequency distributions of degrees of tonus or tension of skeletal muscle that myofascial tone. Clinical experience derives from its intrinsic (EMG-silent) indicates that persons with certain molecular viscoelastic properties. The word symptomatic musculoskeletal conditions tone has been used to convey varying may have palpably increased resting muscle clinical and physiological features that have firmness or hardness (EMG-silent), such as led to confusion and controversy. HRMT is that of the upper trapezius in tension-type the vital low-level, passive tension, and headache, and the lumbodorsal extensors resistance to stretch that contributes (hartspann) in degenerative lumbar disc importantly to maintaining postural stability disease and ankylosing spondylitis. in balanced equilibrium positions. In contrast, co-contraction of muscle is an In summary, resting skeletal muscle tone is active neuromotor control that provides an intrinsic viscoelastic tension exhibited greater levels of tonus for increased within the body’s kinematic chains. It stabilization. Functionally, HRMT is functions inseparably from fascial (i.e. integrated with other passive fascial and myofascial) tissues and ligamentous ligamentous tensional networks of the body structures. Thus, HRMT is a passive to form a biotensegrity system. This review myofascial property which operates within aims to achieve better understandings of networks of tensional tissues, i.e., HRMT and its functional roles. biotensegrity. This passive tension is the CNS-independent component resulting from Nature is frugal and Man’s adaptations to intrinsic molecular interactions of the gravitational forces and erect postures actomyosin filaments in sarcomeric units of seemingly evolved mechanisms in skeletal skeletal muscle and myofibroblast cells. muscle tissues to economically enhance The overarching CNS-activated muscle stability. Normal passive muscle tone helps contractions generate far greater tensions maintain relaxed standing body posture with transmitted by fascial elements. minimally increased energy costs (circa 7% Interdisciplinary research on HRMT and its over supine), and often for prolonged biodynamics promises greater effectiveness durations without fatigue. Available data of clinical practitioners and productivity of infer polymorphic variations in normal investigators, which warrants priority myofascial tone. However, few quantitative attention. 2 Introduction application describing the general state of the tissues, such as vascular tone, nerve A systems level understanding of the tone, skin tone, general tone etc. Their human body is more complex than second use exclusively applied to muscle defining characteristics of isolated parts of tonus. They described it, ‘to represent the a cell or the organism. Aristotle result of a purely neural phenomena.’ This perceptively propounded, “the whole is neural focus of muscle tone was intensified greater than the sum of its parts” by the extensive experimental work of (Aristotle). Emergent concepts from Sherrington’s group (1906, 1915, 1919, & systems approach to applied research often 1947; Denny-Brown, 1929a,b). require integration of many fields of thought. We review the mainly neglected Muscle tone has received considerable area of human resting muscle (myofascial) neurophysiological attention over the years, tone (HRMT). The aim is to update but it has mainly been viewed as a current concepts and to stimulate critical manifestation of stretch reflex neuromotor discussion for better understandings in the control (Denny-Brown, 1929a,b; Walsh, future. 1992; Simons & Mense, 1998). Relatively little research has been done on resting muscle tone. In the early 20th century, Sir Life is movement and muscle has been Charles Sherrington (1852 – 1952) studied almost entirely in its activated investigated decorticated and spinal- state transected experimental specimens, rather than intact animals or humans. In his early Following Hippocrates, Claudius Galen, a work, Sherrington (1906) suggested a role second century physiologist, empiricist for skeletal muscle reflex tonus in philosopher and writer, is often considered maintaining postural attitude. Later (1915), the most important contributor to medicine. he wrote ‘a fairly literal meaning attaching He had particular interest in types of body to the term 'tonus' is, of course, mechanical movements. Galen considered muscle tone tension. In this sense it fits well the slight, as belonging to the fourth type of movement steady, enduring tension so characteristic of in which static resistance is generated; an muscles in their state of reflex tonicity.’ example being a shield held against a striking sword (Galen). Such description of Sherrington concluded that muscle tone muscle tone generated in static resistance is resulted from reflex neurogenic mechanisms more accurately described as an action, (Sherrington, 1919 & 1947), based upon his produced by muscle co-contraction and models which interrupted central inhibitory controlled by the nervous system pathways of muscle contraction. Post- (Sherrington, 1919). surgical recovery, the animals displayed generally exaggerated neurogenic reflexes. Tone and tonus [G. tonos, tone, or a tone] Stretch reflex mechanisms were overly- are general terms which have been used for generalized as the cause of all muscle tonus multiple meanings since antiquity, and often (Walsh, 1992; Simons & Mense, 1998). interchangeably. For instance, 100 years Diligent review of Sherrington's writings did ago, Fraenkel and Collins (1903) outlined not reveal a statement referencing the role of two uses. The first was a histological passive (non-reflex), mechanical, 3 viscoelastic tonicity of skeletal muscle. Even today, Sherrington’s experiments have promulgated the common belief that neurogenic reflex mechanisms are responsible for all forms of muscle tone, including resistance to slow passive stretch in fully relaxed normal persons. Many modern textbooks still consider muscle tone as entirely reflex in origin and resulting from a myotatic (a stretching) reflex in the muscle spindles. Table 1 Papers Affirming Human Resting Muscle Tone (Chronological Sampling)* 1934 - Fenn WO, Garvey PH. J Clin Invest 13: 383-97. 1941 - Hoefer PFA. Arch Neurol Psychiat 46: 947-72. 1951 - Clemmesen S. Proc R Soc Med 44: 637-46. 1953 - Ralston HJ, Libet B. Am J Physical Med 32: 85-92. 1960 - Joseph J. Man’s Posture. Electromyographic Studies. Thomas, IL 1962 - Morris JM, Benner G, Lucas DB. J Anat 96: 509-20. 1966 - Stolov WC. Arch Phys Med Rehabil 47: 156-68. 1972 - Donisch EW, Basmajian JV. Am J Anat 133: 25-36. 1986 - Hnik P. Biomed Biochim Acta 45: S139-43. 1992 - Walsh EG. Muscles, Masses and Motion. London, Mac Keith 1992 - Davidoff RA. Neurology 42: 951-63. 1994 - Hagbarth K. Scand J Rehab Med, 30 ( Suppl ): 19-32. 1998 - Simons DG, Mense S. Pain 75: 1-17. 2007 - Loram ID, Maganaris CN, Lakie M. J Physiol 584: 661-75. 2007 - Loram ID, Maganaris CN, Lakie M. J Physiol 584: 677-92. *Full titles in the reference list 4 Notable literature affirming human (MVC) (Mahieu et al., 2007; Gajdosik et al., resting muscle tone (Table 1) 1999). However, when calf muscle tone is measured under even more relaxed static The classical CNS-activated stretch reflex experimental conditions, it is EMG-silent, theory of muscle tone predominates in the and its inherent stiffness can maintain stance literature. However, it overlooks clinical in defined balanced posture (Loram et al., and experimental research supporting resting 2007a,b). muscle tone (EMG-silent) in intact animals and humans, as cited in Table 1 (titles are Importantly, both the subject and the tested included in the reference section). The muscle must be in a state of full relaxation absence of EMG evidence of muscle in evaluating HRMT, since its intrinsic contractile activity of normal resting muscle tension (without extraneous contractions) is (Table 1) has indicated that such clinically only about 1% maximal voluntary recognized tone is caused by intrinsic contractions (Woledge, 2003). The absence viscoelastic properties (Walsh, 1992; of such standardized relaxed testing Simons & Mense, 1998). For example, conditions has led to contradictions in Clemmesen (1951) was a pioneer exponent studies. of passive muscle tone, in both static and movement functions. He specified that Experimental research evidence for EMG-silent passive tone assists and resists passive muscle tone and its categorization active movements, contributes to synergists and antagonists, and counteracts inertia and Space limitation does not permit detailed external forces, including those of gravity. review of experimental evidence for low level tension in isolated, denervated muscle. Since the 1940’s, increased clinical use of Briefly,