Human resting muscle tone (HRMT): narrative introduction and modern concepts

Human resting muscle (myofascial) tone (HRMT) is the passive tonus or tension of skeletal muscle that derives from its intrinsic (EMG-silent) molecular viscoelastic properties. The word tone has been used to convey varying clinical and physiological features that have led to confusion and controversy. HRMT is the vital low-level, passive tension, and resistance to stretch that contributes importantly to maintain postural stability in balanced equilibrium positions. In contrast, co-contraction of muscle is an active neuromotor control that provides greater levels of tonus for increased stabilization. Functionally, HRMT is integrated with other passive fascial and ligamentous tensional networks of the body to form a biotensegrity system. This review aims to achieve better understandings of HRMT and its functional roles. Nature is frugal and man's adaptations to gravitational forces and erect postures seemingly evolved mechanisms in skeletal muscle tissues to economically enhance stability. Normal passive muscle tone helps to maintain relaxed standing body posture with minimally increased energy costs (circa 7% over supine), and often for prolonged durations without fatigue. Available data infer polymorphic variations in normal myofascial tone. However, few quantitative studies have been performed to establish normal frequency distributions of degrees of myofascial tone. Clinical experience indicates that persons with certain symptomatic musculoskeletal conditions may have palpably increased resting muscle firmness or hardness (EMG-silent), such as that of the upper trapezius in tension-type headache, and the lumbodorsal extensors (hartspann) in degenerative lumbar disc disease and ankylosing spondylitis. In summary, resting skeletal muscle tone is an intrinsic viscoelastic tension exhibited within the body's kinematic chains. It functions inseparably from fascial (i.e., myofascial) tissues and ligamentous structures. Thus, HRMT is a passive myofascial property which operates within networks of tensional tissues, i.e., biotensegrity. This passive tension is the CNS-independent component resulting from intrinsic molecular interactions of the actomyosin filaments in sarcomeric units of skeletal muscle and myofibroblast cells. The overarching CNS-activated muscle contractions generate far greater tensions transmitted by fascial elements. Interdisciplinary research on HRMT and its biodynamics promises greater effectiveness of clinical practitioners and productivity of investigators, which warrants priority attention.