Increased muscle tone as a cause of muscle pain

Abstract

The traditional concept that reflex tone is the underlying mechanism for tension of a resting muscle is outdated. Sources of muscle tension include mechanisms that are silent in the electromyogram (EMG) (i.e., viscoelastic tone and contracture in the physiological sense) as well as those contractile activities accompanied by EMG signals (voluntary and involuntary contractions, spasm, cramp). Usually, muscle tone is clinically measured as resistance to passive elongation via joint movements. In normal cases at full relaxation, it mainly reflects viscoelastic tone, but may also include extraneous contractile activity. Thixotropy of muscle reflects a decrease of its viscosity and lowered passive resistance by movement. It is a ubiquitous and functionally important phenomenon that is not commonly recognized. Muscle spasm is defined in this chapter as involuntary and long-lasting muscle contraction. It is distinct from spasticity caused by a CNS lesion. The pain-spasm-pain cycle, which is often assumed to be responsible for chronic muscle pain, is a physiologically untenable concept. Tension-type headache (TTH) is a clinical pain condition that can be associated with increased muscle tension in some patients and without evident EMG activity. TTH may also be caused by myofascial trigger points (TrPs). Painful muscle spasm is present in cases of spasmodic torticollis, trismus, nocturnal leg cramps, and stiff-man (preferably, stiff-person) syndrome.